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River behavior on megafans and potential influences o ndistribution and diversification of aquatic organism s

M. Justin Wilkinson *Earth Sciences and Image Analysis Laboratory, NASA Johnson Space Center, Lockheed Martin Spac eCompany, Mail code C23, 2400 NASA Road 1, Houston, TX 77058, USA .

Larry G . MarshallDepaltment of Paleontology, Mesa Southwest Museum, 53 North Macdonald Street, Mesa, AZ 85201 ,USA.

John G . LundbergDepartment of Ichthyology, The Academy of Natural Sciences, 1900 Benjamin Franklin Parkway ,Philadelphia, PA 19103, USA .

Abstract

Megafans are partial eones of river sediment usually laid down by a single switching river ,

characterized by areas of the order of 103 -105 km2 , smooth plains, and slopes of < 1° . Astronaut handheld

imagery acquired since the early 1980s has permitted the first global geomorphic survey of megafans .

Using examples mainly from South America, and based on stream behaviors common on megafans, eigh t

models are presented that appear to have implications for speciation and dispersal of aquatic organisms .

River behaviors that appear to be significant for the fragmentation and age of aquatic habitats on megafan s

are river switching (models 1 .1—1 .4) and disconnection of the megafan river from the main river of th e

basin (models 2 through 4) . Each model has a habitat fragmentation mode and a habitat combining mode .

In their vicariant mode, models 1 .1 and 1 .2 involve longer periods of time . Models 1 .3 and 1 . 4

involve the relatively instantaneous merging of aquatic populations from neighboring megafan rivers, o r

from neighboring major basins . Models 2, 3 and 4 involve longer-term dynamics with the potential fo r

speciation. We identify levels of diversity related to scales of stream operation: intrafan diversity local

populations and among local populations ; interfan diversity—among populations and species ; interbasin

diversity—species and biotas.

Keywords : River behavior; Megafan; Aquatic organisms; Diversity ; Distribution; Abiotic contro l

*corresponding author

jwilkinl(cí~ems .jsc .nasa .gov

tel 281-483-5159

fax 281-483-209 1

1111 -030

1 . Introduction

Megafans are large, fan-shaped partial eones of river-laid sediment (–7000 to -200,000 km 2 in area)

with radii arbitrarily defined as >100 km . We use the term megafan (following Gohain and Parkash, 1990 ;

Sinha and Friend, 1994) to distinguish these large features from alluvial fans which are commonl y

identified as smaller features (see Blair and McPherson, 1994, and Miall, 1996, for discussion of the scal e

issues) .

Megafans typically develop immediately downstream of a topographic discontinuity, such as th e

Andean mountain front, with the fan apex located at the point where the formative river exits from the

higher country (Fig. la) . Megafans take on either a delta-like shape or a diamond shape depending on th e

existence of a downstream barrier (Wilkinson, 2002, 2003) (Fig. lb) .

Few modere megafans have been described in the geologic literature . Based on remotely sensed data in

astronaut handheld photographs taken over the last thirty years (see Earth Sciences and Image Analysís ,

2004), more than eighty modern or submodern megafans of various types have now been identified in a

global study (Wilkinson, 1996, 2001, in press ; Wilkinson and Cameron, 2002 ; Wilkinson, Cameron and

Burke, 2002) . Megafans occur on all landmasses without existing ice sheets, being particularly wel l

developed in South America (Fig . 2) .

The distribution and geometry of river systems in megafan enviromnents appear to us to hav e

implications for the fragmentation and reconnection of riverine habitats—and by extension therefore ,

implications for understanding the diversification and distribution of freshwater organisms . This paper

presents and discusses eight models based on certain megafan river behaviors in South America .

Models of this type can be thought of as mesoscale models compared with familiar macroscale model s

such as continental separation, mountain building, and land bridge evolution .

The dynamic situations on which the models are based are derived from stream behaviors documente d

on functioning and subrecent megafans worldwide . Specific examples from South America have been

chosen where possible (Fig . 2) since many modern megafans exist on the continent, and since we are mor e

familiar with that continent .

Although the river behaviors discussed below are mostly known from rivers in non-megafan settings ,

the scale of the surface areas of megafan areas alters the significance of the behaviors compared with

usually understood fluvial environments . Our analysis suggests that megafan rivers display behaviors an d

operate at scales of operation that set them apart from both alluvial fans and floodplains, as classically

understood .

We recognize that various plausible theories of sympatric and parapatric speciation and population

divergence exist, but we emphasise the allopatric isolation mode of evolutionary divergence because it s o

clearly fits our models of river behavior .

The classic sequence of processes leading to allopatric divergence and speciation of aquatic organism s

is outlined by Smith et al . (1999) :

—separation of populations and consequent interruption of gene flow by hydrographic or habita t

barriers ;

—accumulation of genetic differences between the separated entities in growth, development ,

physiology, ecology or behavior;

—attainment of some mínimum combination of genetically-based differences sufficient to cause hybrid s

to suffer reduced fltness, should sympatry be established, and continued divergence with our withou t

secondary contact .

The models of drainage dynamics on and between megafans posit a variety of patterns of shifting ,

splitting and merging of river systems, at least some of which systems are connected through larger, lowe r

altitude trunk streams . These landscape processes should provide numerous opportunities for geographi c

range expansion or fragmentation of biological populations, species, and parts of, or whole, aquati c

communities . On relatively short or ecological time scales, megafan drainage flux could impact diversity o r

species richness within habitats (cc-diversity), between habitats ((3-diversity), or regionally among drainag e

basins (y-diversity) .

Biogeographers will be interested to discover if the distributions of some Andean foreland basin o r

piedmont aquatic species, small clades, or communities are attributable to megafan drainage dynamics . O n

longer evolutionary time scales, megafan drainage dynamics could promote population divergence an d

speciation of aquatic organisms . The models are testable using the tools of phylogeography and

phylogenetic systematics.

Limitations of space do not allow us to do more than present the river dynamics as a series of models .

Testing the models against known populations and situations of speciation and migration must await futur e

work . A glossary of terms is provided.

2 . Megafans

Partial tones of river-laid sediment with radii defined arbitrarily as >100 km, have been terme d

megafans—a category of feature argued for by Gohain and Parkash (1990), Sinha and Friend (1994), an d

Wilkinson (1996, 2001) among others—as distinct from the widespread alluvial fan which is usuall y

understood as a smaller feature. The largest megafan yet identified in a global study (Wilkinson 1996 ,

2001) is the Pilomayo megafan which covers more than 200,000 km2 in northern Argentina and western

Paraguay (Iriondo, 1984). Megafans are the dominant mesoscale geomorphic feature in the extensive

aggradational/constructional (as opposed to erosional) landscapes of the Andean foreland, from Venezuel a

to central Argentina . In South America, many rivers draining off the Andes have built up megafan tones ,

as documented by several workers (Iriondo, 1984, 1993 ; Allenby, 1988; Malagnino, 1989 ; Neller et al. ,

1992 ; Clapperton, 1993 ; Comisión Nacional del Río Pilcomáyo, 1994; Dumont and Fournier, 1994 ; Meyer ,

1996) .

Indeed, contiguous megafans cover the vast arca of almost 750,000 km2 in the Bolivian Amazon an d

adjacent plains of Paraguay and northern Argentina immediately to the south (Fig . 2) . Megafans have been

mapped on all unglaciated landmasses, although there are few in North America .

Megafans appear to be features that are orders of magnitude larger areally and apparently of greate r

persistence in the landscape than alluvial fans, at least in the strict definition of Blair and McPherso n

(1994) . Wilkinson (1996, 2001) has argued that these megafans are dissimilar from alluvial fans on the

grounds that a different mix of auto- and allogenic controls typically determine the growth of features o f

different scale. Megafans appear to be features which occupy a different order (group 9) in a recent

hierarchical schema advanced by Miall (1996) . Alluvial fans typically belong to group 8, an order o f

magnitude different in terms of various criteria .

Although the zone of active megafans is extensive in South America today (Fig . 2), major zones o f

deposition on continental surfaces change over time as tectonic environments change . The present

landscapes of western Australia and the High Plains of North America, for example, are true incisiona l

realms . Rivers draining off the Rocky Mountains onto the High Plains formed Neogene megafans but hav e

since incised their course so that they lie today confined in valleys for their entire length to the trunk rive r

(Mississippi) . They are thus incapable of switching behavior of the type that underlies the model s

presented here . Drainage disconnection of the type described in models 2 and 4 may well Nave operated

but cannot have occurred since the rivers began their erosional phase in the last several million years .

Although megafans are known from cratonic and intermontane settings, recent work suggests that

geometries developed in foreland basins—in which the megafan river flows perpendicular to the mountai n

front, as in the case of the Andes foreland, to meet a basin trunk river that flows parallel to the mountain

front (Fig . lb right)—are the most common (Wilkinson, 2001) .

Some major characteristics and geometries of megafans are listed below .

2 .1 Megafans as partial tones of sedimen t

Megafans vary in area from -7000 to –.200,000 km2 (in a population with radii >100 km), arcas that

compare with greater London at the small end, to England and Scotland combined, or half the area o f

Paraguay, at the high end . Megafans are formed mainly by river-laid sediment, with subordinate

componente of windblown and marsh facies and very widespread pedogenic units . Subcones are common

and can be situated on the megafan at any distance from the mountain front, usually related to some minor

constriction of the formative river with the subapex located where the formative river ceases to b e

constricted .

2.2 Dominance by a single formative rive r

A single major river sourced in the upstream highland (orogenic mountains such as the Andes, o r

cratonic uplands such as the Brazilian plateau)supplies sediment to the megafan (Fig . la) . The major rive r

constructs leveed floodplain tracts which are the constituent units by which megafans grow . A minority of

megafans in the global sample have more than one active distributary and more than one formative river ,

but such scenarios are not modeled in this paper .

2.3 Megafan radius and foreland basin width

For megafans aligned transverse to the mountain front, the megafan radius is determined by the width o f

the foreland depression (Fig . lb, left) . More complex geometries, not discussed here, arise in cratoni c

settings where small topographic differences have given rice to megafans . Major rivers on megafans can

reach lengths of hundreds of km.

2 .4 Megafan apexes and subapexes

The apex of the megafan cone lies where the formative river exits the mountain mass (Fig . 1 )

unconfined within valley walls, so that it can potentially occupy any radial position on the conical megafa n

surface . Subapexes of subcones (seé Fig . 18 for a South American example) are controlled by smal l

differences in relief, as, for example, where the formative river passes out of the locally incised sectors, or

where crossing the margin of blocks or arches uplifted by tectonic and epeirogenic forces .

2.5 Switching behavio r

Avulsion by aggradation The formative river builds up its bed and containing levees as much a s

several meters aboye the surrounding floodplain surface . This unstable situation leads to the potential fo r

sudden shifts of the river—especially during floods-finto a new course on lower ground . Such shifts in the

course usually occur in discrete , relatively rapid jumps in a process known as avulsion . Prior river

positions are often visible in the landscape as paleochannels radiating from the fan apex, e .g . as shown in

the examples of the Río Parapetí megafan apexes in Bolivia (Fig . 3) . The Kosi River, in its course between

the Himalayan Mt. front and the Ganges River, is probably the best documented example of a megafa n

river that has shifted position numerous times . Data on avulsion rates are notably scarce (Miall, 1996) .

The Kosi megafan in northern India is, however, the best documented avulsing megafan river. The Kos i

River is known to have shifted progressively westward across the fan surface during the last 270 years

(Holmes, 1965; Wells and Dorr, 1987), avulsing on average every 20 years .

Piracy and stream incision The other major process which appears to operate on megafan surfaces i s

episodic phases of shallow incision into the surface of the fan, with subsequent resedimentatio n

(baclfilling) by the river (see summary in Miall, 1996) . Phases of incision are evident in the terraced

landscapes which characterize the apexes of many South American megafans (Iriondo, 1984) . These hav e

not influenced the broad development of megafans per se . Shallow incision can promote the competitive

expansion of one stream drainage area over another and lead to the capture by the more competitive of th e

less competitive .

By contrast, the aboye mentioned deep river incision of present streams on the High Plains of North

America has rendered the Neogene megafans relict features, with fan surfaces lying tens to hundreds of

meters aboye present river altitudes .

Both avulsion and stream piracy promote connections between neighboring streams, between

neighboring megafans, and between neighboring river basins .

2 .6 Nested megafans

Depending on the distance between formative streams draining the orogenic highland, megafan s

generated by these streams will be more or less closely spaced along the mountain front . The plains of

southwestern Amazonia in Bolivia, stretching south finto southern Brazil as far as central Argentina ar e

examples of suites of megafans nested against one another. Here contiguous megafan surfaces cover th e

immense area of -750,000 km2 (Figs . 2 and 4), and the Pantanal ofsouthwestern Brazil .

2.7 Hydrology on megafan plain s

A corollary of the extensive, planar morphology ofmegafans is that small vertical topographi c

irregularities of a few meters can cause extensive changes in surface hydrology, a subject of specifi c

interest in this paper . (i) The most obvious way in which lakes appear to form in the South America n

environment is by water accumulation in scoured meanders of abandoned river channels . (ü) Differentia l

compaction on a plain formed by channel sands versus overbank muds probably leads to the highly varie d

micromorphology that has been identified on the medial Pilcomayo megafan (Alvarez, 1998) . (iii) Wind

eroded features are ubiquitous on South American megafans, so that the panoply of features seen in sand

sheets should be expected to play a role in the formation of water bodies . Erosional hollows hundreds of

hectares in size are known from the sand sheets in the southern Chaco, where many depressions intersec t

the water table and thus host small permanent lakes (Iriondo, 1989) . (iv) Sand dunes can impound water .

Due to Pleistocene climatic change, linear dunes are common on megafans (e . g . Fig . 3) and can b e

associated with lakes . Interdunal corridors in the Kalahari Desert of southern Africa have hoste d

permanent water bodies under wetter climates—e .g. Lancaster (1984). (v) Cordini (1947) and Iriond o

(1989) document small depressions (hoyales) that develop as vegetation rots and collapses . (vi) Smal l

topographic irregularities, related to paleosols and small tectonic movements, can act as apexes for subfan s

on the main fan surface .

Many of these dynamics can be encapsulated in the idea of resurfacing of megafans by minor

aggradation and incision . Even when a megafan surface approaches an altitude and associated suite o f

slopes in which the rate of change slows (i .e . approaches some_e_quilibrium morphology), thin zones o f

sedimentation and erosion are presumably common, probably due to autocyclic controls . Stream long

profiles are probably continually regraded in different reaches due to the interplay of auto- and allocycli c

controls . Such adjustments proceed quickly in the relatively unconsolidated sediments of large fans . I n

aggradational mode, where megafan surfaces are actively being built up by the deposition of rive r

sediment, the associated river behavior is avulsion . In erosional mode, as the stream cuts into, or incises ,

the fan surface, piracy by smaller rivers on the megafan surface is probably common . The erosional mode

likewise probably proceeds rapidly once initiated . Shallow incision is widespread on megafans, and may

induce local piracy by energizing the main megafan stream or other streams on the fan surface .

Shallow erosional resurfacing is distinguished from incision in the more usual sense of incision o f

sufficient depth (usually at least several meters) that a river is in effect confined within a valley from whic h

it cannot switch to a new position . This distinction is discussed further in Discussion section 4 .2 below .

3. The Models

3.1 Criteria underlying the models

The main criterion underlying the models is stream behavior . Various stream dynamics, mostly wel l

known from other mesoscale geomorphic environments, have been specifically identified on megafans an d

underlie leven of the models . The eighth model is a combination of three of these dynamics (Table 1) .

Other feasible combinations await attention in the future .

The models explored in this study, selected from a wider set (Wilkinson, 1998, 1999), were chosen fo r

their ubiquity or interest, and to give a flavor of the kind of mesoscale dynamics that occur in many of th e

world's mid-sized rivers, especially as seen on the widespread megafan environments of the planet .

Stream dynamics were chosen as the main criterion for grouping the models because the models appea r

to be a trenchant way of thinking about mesoscale stream dynamics, and their effects on the distribution s

and diversities of riverine populations, species and communities . Further, the models seem appeal to a

broader audience than earth scientists alone (Table 1) . We focus on two specific dynamics, the first relate d

to stream switching (models 1 .1 to 1 .4 in Table 1) ; and the second based on drainage disintegration variou s

types, such as desiccating climates (model 2), growth of stream segments on fans (model 3), and strea m

self-sedimentation (model 4) . Most models have a vicariant mode in which habitats and their biotas ar e

fragmented, indicated by the lower case suffix -v in the text and tablea ; and an opposite mode in which

habitats and their organisms are reconnected, indicated by suffix -c (Table 1) .

The modes can be seen in terms of short and long term diversity effects . The combinatory -c mode is

always a short term phenomenon and promotes rapid dispersion of individual organisms, their population s

or species over the aquatic landscape . The vicariant -v mode implies longer spans of time with potential

effects on community composition and evolutionary divergence .

The models can be seen in terms the scales of geomorphic operation that is implied since scale probably

corresponds to types of biodiversity (Table 2) . Intrafan dynamics imply interaction of separate or separable

populations, species or communities in rivers or lakes on a single fan ; interfan dynamics imply interactio n

of populations, species or communities in streams on two or more neighboring fans ; and interbasin

dynamics imply interaction of populations, species or communities in streams in different major drainag e

basins of regional or subcontinental size .

The geomorphic models can be seen in terms of sub-fan scale habitat (Table 3) . The table illustrates the

point that each model has implications for more than one of these major habitats .

3 .2 River orientation dynamics—Models 1 .1-1 . 4

Four models illustrate the effects, at different scales, of reorientational bahavior on the simple

connection and disconnection of neighboring streams (Table 1) . Reorientation on a single megafan

generates disjunct habitats in the form of lakes and beheaded river segments on a single megafan . By

contrast, reorientation that connects the rivers of entire neighboring megafan river systems, and even

neighboring regional or subcontinental-scale drainage basins, has different impacts .

Model 1.1Isolation and reconnection of lakes by stream switching Stream-sculpted depressions such

as oxbow lakes, can remain in the landscape substantially removed from the functioning surface

hydrographic network when a stream"switches to a distant part of the megafan surface (model 1 .1-v, Fig. 5 a

and b) . Organisms that flourish in the lakes, given sufficient time, may speciate . Ultimately, the megafan

river will switch back to a course resembling the original course, such that the river makes contact with th e

lakes . Populations isolated in the lakes are thereupon introduced into the general stream network (mode l

1 .1-c, Fig . 5c) . Schematically rendered in Fig . 5a-c, the parent green population would encounter th e

daughter reds and blacks that evolved in the isolated lakes, relicts of the former river course .

Megafans in southwestern Amazonia are characterized by numerous lakes . For example, the Río Beni

megafan in northern Bolivia illustrates prior positions of the dominant river in the form of trains of aligne d

lakes (Fig. 6)—the orientations of which, in this case, may be partly controlled by the existence of an activ e

fracture in the basement rocks (Allenby, 1988) .

Other examples of lake-studded surfaces are the lower slopes of the Pilcomayo and Bermejo megafans

of northem Argentina and western Paraguay where numerous lakes are documented, both on remotely

sensed images (Figs . 7 and 8—distal surfaces of the Bermejo and Pilcomayo megafans) and in the meage r

literature on this remote region. Many of the lakes appear to have originated within prior river courses ,

although other mechanisms of lake formation operate (see 2 .7 aboye), some of which are mentioned in the

literature on South American plains (Cordini, 1947 ; Iriondo, 1993) .

The longevity of lakes on megafans is controlled by many factors and is, as yet, unresearched to ou r

knowledge . But various controls can be identified . These include switching rates and stream incision b y

the formative stream, and the growth of new (authochthonous) streams on the megafan surface throug h

time (see Discussion section 4 .2 below) .

Despite the long-term trends and vagaries of lake history on megafans, such as progressive infilling b y

sediment over time (Iriondo, 1993), it seems possible that at least a few of the hundreds of lakes on

megafan surfaces may persist long enough to allow for speciation . Small lakes are a characteristic of many

megafans the world over (Wilkinson, 1996, 1998) so that processes of lake formation and the concomitan t

isolation of aquatic populations may well be common .

A testable hypothesis is that lakes which have persisted for longer periods of time—for example, thos e

situated furthest from the active megafan river (or occupying the oldest parts of the megafan surface, wher e

this can be determined)—should manifest greater degrees of speciation than younger lakes situated in th e

active floodplain of a functioning megafan river . Another testable hypothesis is that there ought to b e

greater numbers of species in megafan rivers than in rivers of comparable size that do not flow acros s

megafans .

Model 1 .2—Abandoned river courses and reoccupied courses In a process similar to that described in

model 1 .1, the entire lower course of a megafan river may act as a vicariant habitat for some species, i f

three requirements are fulfilled : (i) if switching diverts the megafan river at the upstream end, (ü) if

enough discharge remains in the lower course (from local streams and groundwater discharge) to suppor t

riverine habitats, and (iii) the trunk river of the basin at the downstream end acts as a barrier to migration o f

aquatic organisms.

Consider an original course x —y (Fig . 9a) diverted at A into a new course x—z (Fig . 9b) . Consider also

the trunk river is a hostile environment for some organisms . Such organisms in the abandoned sector

(sector y, Fig . 9b) may be separated effectively from the rest of the population which occupy the new

course of the megafan river (model 1 .2-v) . The hostile environment of the regional trunk river separatin g

the two megafan reaches x —z and y is indicated by the double-headed arrow (Fig . 9b) . Presumably th e

effectiveness of the barrier reach is increased as the distance is increased .

If the megafan river later reverts to its original course, mother and daughter populations are throw n

together in sector y (Fig . 9c) .

Schematically, Fig . 9a shows a green parent species distributed throughout sector x —y. Red colors in

sector y suggest speciation of the population that becomes isolated after the river switches course (Fig . 9b) .

Greens reinvade sector y if the river switches back to its original x —y configuration (Fig . 9c) .

This model is based on the geometries of the Pastaza-Corrientes river system in southeastern Colombia .

The Pastaza River (in effect sector x—z in Fig . 9b) presently flows south-southwest from the town o f

Andoas (A) . The Río Corrientes has been identified as an earlier course of the Río Pastaza (Neller et al . ,

1992), oriented southeast from Andoas and corresponding to sector y in Fig . 9b . The Río Pastaza thus

formerly drained finto the trunk river (Marañon) at the present Corrientes confluence . This former

confluence is at least 250 km distant from its present confluence with the Río Marañon—and nearly twic e

this distance measured along the meandering course of the Marañon .

The isolating effect on vulnerable species would be the same as that illustrated in model 1 .1-v (Fig . 5b) .

The events modeled here can be considered in greater time depth . If, for example, sector z itself is a n

earlier course of the megafan river, it may contain a new species, evolved for the same reasons of

vicariance as apply in the newly isolated sector y . With the switch shown in Fig . 9b, the diverging

population in sector z would suddenly rejoin the population in sector x, aboye the subapex (switch point) a t

A . This is a habitat reconnection in the style of model 1 .2-c, and implies the associated biologica l

dynamics .

A single river switching event can thus both divide a population as well as reconnect populations .

Schematically, the green parent population becomes disjunct, but rejoins daughter yellows in sector z (Fig .

9b) .

A testable conclusion is that beheaded courses of megafan streams should show signs of speciation du e

to vicariance, at least for those species that cannot negotiate environments in the trunk river .

Model 1 .3-Connection between megafan rivers Streams flowing independently to the trunk river o n

neighboring megafans or alluvial fans (Fig . 10a) can occupy the common topographic depression betwee n

the fans . In these cases populations in separate megafan streams—again, especially those for which the

trunk river is inhospitable—can suddenly occupy the same environment when both rivers flow finto th e

marginal depression shared by both fans (model 1 .3-c, Fig . 10b) . This scenario for connection has a lowe r

probability than the vicariant mode since it requires both rivers to be oriented towards one anothe r

simultaneously. Thereafter the combined population finds itself split apart when the megafan streams onc e

again diverge away from each other at the next avulsion event (model 1 .3-v, Fig. 10c)

The simplest interfan dynamic is illustrated schematically in Fig . 10 . Separate species occupy two

neighboring megafan rivers (Fig . 10a) . The red and green species meet when the megafan streams bot h

avulse into the common marginal depression (model 1 .3-c—Fig . 10b). Both populations are divided when

the rivers diverge again (model 1 .3-v—Fig . 10c) ; but here, both populations occupy both streams, in

distinction to the situation in Fig . 10a .

The neighboring Ariari and Güejar river fans lie against the eastern Andean mountain front -400 k m

south of Columbia's capital city, Bogotá . The upper Río Ariari presently flows southeast, where it meet s

the Río Güejar (Fig . 11) . These rivérs then flow as one river along the depression between the fans . Th e

Río Güejar appears to llave avulsed into its present more easterly course sine acquisition of the (aeria l

photographic?) data on which a recent world atlas map of the crea (Times World Atlas, 1994, plate 119) ,

was based . Although its date is unknown, the data probably were acquired within the last four decades .

When the Río Ariari diverges from the interfan depression and flows east again across its fan at some

time on the future (upper red arrow Fig . 11), its population, in the sector of the river where switchin g

occurs, will be separated from those in the Güejar drainage .

Model 1 .3 is thus similar to model 1 .2 in its biologic effects, except for the significant difference wit h

respect to scale, and hence type, of diversity . Species associations on any single megafan, without a

neighboring fan, as happens in some landscapes, are controlled partly by the history of the megafan rive r

(intrafan diversity aboye) . By contrast, two separate populations are mixed when neighboring megafa n

rivers avulse together. Diversity on the combined megafans could be described as interfan diversity, a typ e

which achieves added significance when more than two megafan rivers interact, as discussed in model 5

below .

A testable hypothesis, of interfan order, that flows from this hydrographic pattern, is that species fro m

river habitats on the megafans should be closely related, not only in the Ariari and Güejar River drainages ,

but also in other neighboring rivers whose connections are mediated by megafan dynamics .

Examples of potential connections between megafan rivers are instructive . Iriondo (1974, 1984) has

documented a lateral shift of 300 km of the megafan course of the Río Bermejo, a major river in northern

Argentina . This reach now lies 30-50 km north of the course it occupied about one hundred years ago . The

new course lies closer to, if not within the wide, ill-defined depression between the Bermejo and th e

neighboring Pilcomayo megafan to the north . The Bermejo riverine habitats are now more prone to direc t

connection with the Pilcomayo than before (see Fig . 20) .

The Kosi River in northern India flowed in its eastern marginal depression before 1736 AD and toda y

flows in its western marginal depression . Its actual or potential connection with the Mahanda-Tist a

megafan to the east thus ended in 1736, and its potential connection with the Gandak River on its wester n

margin is far more probable, now thaf it lies hundreds of km nearer the Gandak.

Model 1 .4—Megafans on basin divides—alternating connection between major basins Megafan river s

situated on the divide between major drainage basins drain alternately into one then the other major basi n

(Fig . 12) .

The wider environment of the aboye mentioned Ariari and Güejar rivers illustrates this interbasin

perspective. At some time in the future, when the Ariari and Güejar rivers diverge away from thei r

common course in the interfan depression (red arrows, Figs . 11 and 13), runoff from these rivers will flow

by very different paths to the sea : the Río Ariari in this orientation will join the northeast-flowing Met a

River whereas the Río Güejar will continue to feed the regional Río Guaviare which flows east (Fig . 13) .

Thus, a small autogenic switch of river course—on a fan critically situated on a major drainage divide —

can divert water alternately into the major northeast-flowing Meta drainage and the east-flowing Guaviare

drainage . The Río Meta meets the Río Orinoco -900 km downstream from the Ariari fan apex . Via th e

Río Guaviare the distance from this apex to the same point on the Río Orinoco is -4000 km (Fig . 13) .

River dynamics on the Río Ariari fan thus separate organisms aboye and below the switching point b y

2000 km . Any aquatic organisms that find stream environments inhospitable in this great distance will b e

effectively isolated from one another.

Major continental basins are also affected by this dynamic . Most importantly for the present topic is the

Río Parapetí megafan that is situated on the southern divide of the Amazon watershed, with its apex agains t

the Andes mountain front in Bolivia. At present, the Río Parapetí is oriented towards the north, and feeds

water into the Amazon system via the Río Mamore . However, during the wet season water flows via the

Río Timané into the Paraguay system (Iriondo, 1993). Drainage on the Parapetí megafan therefore connect s

both the Amazon and Paraná basins hydrologically .

Examples of similar dynamics from megafans on divides can be quoted in other parts of the world . The

Yamuna River, an upper tributary of the Ganges, has laid down an enormous fan of material locate d

securely on the present lowland divide between the Ganges and Indus river basins . Alter exiting from the

Himalayan Mountains, the Yamuna River today flows southeast . In the past, however, the Yamuna River

flowed southwestwards into the Indus River basin (Pal et al ., 1980) . Assuming that switching behavior has

always been active on the Yamuna megafan, one can postulate that the Yamuna River has flowe d

alternately into the Ganges and Indus basins .

Similarly, the Okavango River megafan in southwestern Africa (Botswana) occupies the continental

divide between the Zambezi River system and the inland drainage of the Makgadikgadi basin in central

Botswana. As with the Parapetí, the Okavango megafan also constitutes a "leaky divide," with th e

Makgwegana spillway leading drainage between the two basins depending on the region of highest rainfal l

(Debenham, 1952 ; Stanistreet and McCarthy, 1993) .

Course switching by rivers on megafans provides a dynamic for the interchange of organisms betwee n

major basins, and mediates interbasin species diversity.

3 .3 Models based on drainage net disintegration and reintegration on megafans—Models 2, 3 and 4

River dynamics that underlie models 2 and 3 are climatic change and the growth of streams on

abandoned fan surfaces, both well-known processes and not restricted to rivers on megafans. Model 4

concerns the process of riverbed blockage by self-sedimentation, a dynamic only recently brought to the

attention of geomorphologists . The only known example of this process exists on a megafan, but probably

has operated in non-fan rivers. River behaviors represented in models 2, 3, and 4 illustrate what we refer t o

as intrafan diversity.

Model 2—Drainage net disintegration and reintegration due to climatic change Megafan rivers that

reach the trunk drainage under sufficiently moist climates can be severed from the trunk as climates

become increasingly arid (Fig . 14a and b) . This process of drainage disintegration leads to inland, o r

interior, drainage in which river flow does not reach the sea . It is a well documented phenomenon in ari d

parts of the world (e .g . Cooke et al ., 1993) . It is commonly assumed that separated continental basins ar e

the only explanation for what is known as interior (endorheic) drainage. However, this kind of climaticall y

controlled stream network disintegration does not require topographic basins that are cut off from the sea .

Basins that slope continuously to the sea may act as inland drainage basins hydrologically if the availabl e

slope is insufficient to maintain river flow . Discharge reduced on account of climatic drying is the most

common cause . Many cases are known of systems apparently open to the sea in terms of topographic slop e

that nevertheless fail to reach the sea.

Evaporation is aspect of climatic control of discharge well illustrated by the Okavango River delta in

northwest Botswana . Under its present regime, the Okavango River loses 95 percent of its annual discharg e

in the Okavango "river delta," a megafan located in the semiarid Kalahari Desert . The Okavango' s

connection to the regional drainage is intermittent .

Trends to aridity in semiarid regions typically cause the active, flowing sectors of rivers to rivers t o

shorten, regressing back towards the region of higher rainfall—in the case of the Chaco plains, towards th e

Andes mountains (model 2-v, Figs . 14b, 15 ; see also the Río Itiyura, Fig . 18) . Examples of megafan rivers

being cut off by aridity are the Río Dulce that flows finto the inland Mar Chiquita basin and neighborin g

salars in central Argentina (Iriondo, 1993) .

With the onset of a moist climatic trend, the megafan river can be reconnected to the regional rive r

system, and populations isolated in the megafan river are able to rejoin those in the trunk river (Fig. 14c) .

In the most straightforward scenario, populations are isolated for periods as long as major (Pleistocene )

sclimatic cycles, although many variables in individual drainage basins presumably modify this 10 yr

periodicity (Model 2-c), especially since the correspondence between glacial phases and tropical rive r

discharge is known to be complex . Schematically, the newly differentiated reds in the megafan river (Fig .

14b) rejoin the original green stock in the main trunk river (Fig . 14c) .

The obvious test of such a model lies in examining the degree of divergence in presently severe d

megafan rivers vis-á-vis their trunk rivers . Finer-grained tests suggest themselves : is the climatically

induced periodicity that drives the separation and rejoining of megafan and trunk rivers detectable as a

pulse of speciation? Detailed geomorphic-paleoclimatic histories, of which more are becoming available ,

may explain the vicariant origins of some species .

Model 3—Drainage reconnection by autochthonous stream development In wetter climates, new

drainage networks are known to evolve on those surfaces of megafans that have been abandoned by th e

main megafan river . The type examples come from the Ganges basin (Gohain and Parkash, 1990) . Local

streams which rise on the megafan súrface (Fig . 16a-c)—rather than in the highland upstream—are know n

as autochthonous streams . As these develop their networks headward (towards the highland and apex of th e

megafan), they are presumably able to connect any disjunct depressions that may lie in fine with th e

headward growth .

Autochthonous streams are known to expand their networks progressively on newly deglaciate d

landscapes. Landscapes that have been deglaciated longest therefore display the best integrated drainag e

systems .

The same process undoubtedly takes place on South American megafan surfaces . The significance of

the process in this model is that water-filled depressions are reintegrated into the regional surfac e

hydrographic net without the agency of the avulsing megafan stream . The major effect on populations

would be to reintroduce lake-adapted species into the integrated river system (new red and black species i n

Fig. 16b rejoin the regional river system occupied by the parent green group, Fig . 16c) .

The cycle envisaged for this model is thus a vicariant phase (model 3-v), the same phase of lak e

formation in the relict course of a river as described in model 1 .1-v; and a phase of reintegration of th e

lakes by the development new stream segments .

Model 4—Drainage net disintegration by stream self blockage Other mechanisms such as stream self

blockage also serve to disconnect the megafan river from the trunk river (Fig . 17) . The Río Pilcomayo i s

the only example of a river known that is presently sedimenting its channel to such a degree that i t

completely blocks its channel, causing discharge to overtop the levees and spill out as vast swamplands o n

either side of the alluvial ridge . By this process, locally termed plugging or colmatation, the Pilcomayo i s

now completely cut off from its downstream trunk river, the Río Paraguay (Fig . 18) .

Self blockage has wider implications for rivers . As the channel has filled progressively from th e

downstream end, the blockage point has migrated upstream in the course of four decades, a process termed

end point recession. The end point of the Río Pilcomayo is now located 550 km from the trunk drainag e

(and further, measured along the course of the river) . Even though discharge of this river has not decreased ,

the end point of the river is receding upstream rapidly (8-10 km yr 1 ) towards the apex of the megafa n

system (Meyer, 1996). The reasons for this behavior by the Pilcomayo remain unclear (Meyer, 1996) . I t

seems possible that causes may relate to reduced stream slope in the Andean foreland due to tectoni c

activity .

The panorama of the Andean foreland basin (Fig . 4) illustrates how far the end point of the Rí o

Pilcomayo has receded: it has retreated west (left to right, Fig . 4) across much of the width of the foreland

basin (indicated by the arrow, Fig . 4), from near the Río Paraguay (far left) to a point near the Ande s

Mountain front (dashed river course, right) . Fig . 18 shows a more detailed view of the end point sector .

The net effects of such behavior on sediment dispersal and aquatic organisms are similar to those tha t

occur in rivers undergoing increasing aridity (model 2-v aboye) . A newly speciated red population in the

megafan stream (Fig . 17b) rejoins the original greens in the trunk river (Fig . 17c) .

Unlike rivers dying of reduced discharge, the flanking swamplands in the case of self-blocking river s

(Fig. 17b) provide swamp habitats for numerous organisms—in a region that will not have experience d

regular flooding of this kind perhaps for thousands of years .

Species loss is related to river changes as well . It is now known that destruction of midfan riverin e

habitats can have an opposite and adverse effect on species diversity . The loss of ten species in the apex

reaches of the Río Pilcomayo in southem Bolivia has been documented and has been explained on the basi s

of the loss of breeding habitats in the midfan, reaches of the river that no longer exist (Dlouhy, pers . com .) .

the Río Pilcomayo suggests that present decades may be witnessing the beginning of dislocation of thes e

rivers from the regional river network .

An example of the explanatory power of the models in combination, is evident from this application t o

an undescribed species of small endemic banjo catfish (Aspredinidae) recently discovered in the proxima l

megafan sector of the Río Pilcomayo in Bolivia (John Friel, pers . com .) . The geographically mos t

proximate relatives to this species live in the Río Madeira system of the Amazon basin (others occur farthe r

north in the Pastaza and Napo drainages). A plausible explanation for both the distribution and divergent e

of these small fishes may lie in three megafan-related dynamics . First, migration of an ancestral populatio n

across the basin divide, via the connecting megafan of the Río Parapetí, from, or to, the Amazon basi n

(model 1 .4) . The Río Parapetí actively connects the basins today . Second, migration between the Parapetí

megafan and the neighboring Pilcomayo basin immediately to the south, may have occurred by means o f

river switching on the megafans (rather than via the trunk river, the Río Paraguay) (model 1 .3) . Third,

population isolation may have occurred by means of river recession, climatically or tectonically induce d

(models 2 or 4 respectively) ; or it may have been induced through long-continued non-connection between

two megafan rivers such as the Parapetí and the Pilcomayo (model 1 .3-v).

In more general tercos, a preliminary search reveals that several freshwater fishes appear to b e

distributed in rivers in the megafan zone east of the Andes Mountains, but not in large trunk rivers .

Otocinclus huoarani appears to be restricted to the megafan zone of western Amazonia (Ecuador) an d

central Colombia (Schaefer, 1997), Xyliphius lepturus to eastern Ecuador (Friel, 1994), and

Pterobunocephalus depressus to western Amazonia (Ecuador) and southwestern Amazonia (Peru and S W

Brazil) (Friel, 1994) . Otocinclus vittatus is distributed in the megafan zone in a long swath of country fro m

central Colombia through eastern Peru to central Bolivia, with a subpopulation in the megafan country o f

the Pantanal of southern Brazil (Schaefer, 1997) . These distributions suggest that megafan rivers may have

played a part in the dispersal of these organisms, an hypothesis for further investigation .

4 . Discussio n

4.1 Diversity and types of river connections

Intrafan, interfan, inter-basin dynamics clearly operate on megafans, with significance both for

vicariance and distribution of aquatic organisms . Intrafan and interfan dynamics appear to be of greate r

relevance in terms of providing isolating habitats . Interfan dynamics facilitate the merging of populations ,

and the subsequent splitting of the mixed population. Interbasin dynamics mediate the mixing o f

populations in the sector of the divide megafan common to two basins .

All these dynamics can be expected to have operated in the Amazon basin, especially at times whe n

when rivers operated in the depositional/aggradational mode (see 4 .3 below) . Similar general trends hav e

undoubtedly occurred on other continents . Periods of more changeable climate will likewise hav e

increased the probability of drainage network break-up, with tributaries becoming disconnected from their

trunk streams . Other effects such as stream self blockage and autochthonous stream growth are probabl y

less important in terms of the number of streams they affect, and they probably operate at least partl y

independently of climate .

4.2 Rates of biological diversification and the persistence of lakes and abandoned river reache s

Are the time scales of megafan drainage dynamics appropriate for promoting population divergence an d

speciation? We demonstrate that megafan dynamics indeed overlap time spans proposed for successfu l

speciation . We compare the geological data with what is known of biological diversification rates .

One of the more complex and less constrained aspects of the proposed models concerns the age of

different parts of megafan surfaces, which can be thought of as a mosaic local surfaces of different of ages .

The age of a local surface gives information on the likely temporal duration of lakes and beheaded rive r

sectors at those points (especially models 1 .1 and 1 .2) .

Since the main agent of change on a megafan surface is the switching river, any controls of the rate or

location of switching affect the persistence of lakes on the fan surface—with less or restricted switchin g

related directly to the persistence of lakes . Other factors have been mentioned as controlling th e

persistence of lakes, such as (i) their location aboye active faults, (ü) sediment compaction, (iii) initiall y

larger size, and (iv) long-term positive changes in water table level related to tectonic movements or

climatic shifts . We suggest that at least a few lakes of the hundreds that can exist on larger megafans, may

be optimally located to remain wet in the long term .

We therefore examine below the most active general control, namely river course switching, which i s

the main agent that sculpts the megafan surface . The river switching phenomenon is discussed in terms o f

switching frequency and incision .

Switching frequency Avulsion rates are poorly known (Miau, 1996) and data are mainly derived fro m

confined, non-distributary systems. The process is regarded as autogenic (see discussion in Miall, 1996 )

and thus ought to apply generally in large fan environments (Table 4) . The data suggest that rates can be a s

low as 10 3 years . Dates recently acquired from fossil water in an aquifer underlying western Paraguay

indicate that the Río Pilcomayo itself has occupied its present general position, near the southern margin o f

this vast fan, for the last 35,000 years (Geyh, no date) . At the fast end of the spectrum, the well dated

courses of the Kosi River on the Ganges plain show that eleven major avulsions have occurred since 173 6

(Holmes, 1965), giving an average avulsion rate of -24 years .

At minimum it can be concluded that avulsions operate in known rivers with rates that vary between 10 1

and 10 3 years. These rates imply that some parts of megafan surfaces may not be influenced by the main

megafan river for hundreds, thousands or even tens of thousands of years .

Incision by the megafan river Even the rapidly avulsing Kosi River in the Himalayan foreland, has no t

resculpted the entire surface of its megafan . Rather, the river has avulsed into slightly incised pre-existin g

depressions, leaving higher-lying intervening surfaces ("terraces") unscathed by the devastating floods that

characterize the sudden avulsion events (Gohain and Parkash, 1990) .

The phenomenon of megafan resurfacing, is defined as minor aggradation and incision—minor in terms

of depth and length of the river reaches affected . Resurfacing is probably in constant play on activ e

megafans . Of different significance is incision beyond a critical depth : at some threshold, incision andlor

erosional widening of the incised zone with time, prevents subsequent phases of minor aggradation fro m

refilling the depression to the leve] of the megafan surface .

The megafan river then lies, in effect, in a confined valley .

This in turn means that the river is incapable of avulsing to any point on the megafan surface . What

does this mean for the upper (aggradational) surface of the megafan? It becomes subject to significantly

less energetic landforming processes—aeolian accumulation and deflation, and pedogenesis (in the tropic s

and subtropics) .

Water levels in lakes and beheaded streams hundreds of km away on the megafan surface may b e

unaffected by the incision, at least in some cases at some times .

The upper surfaces of the Ogallala sandstones of the High Plains of North America, represent an almost

pristine set of megafan surfaces (Gustavson, 1996), that has persisted into the Holocene . This means that

the megafan surfaces have not been subjected to the effects of their formative rivers (Platte, Arkansas ,

Canadian) for about the last 8 million years (Gustavson, 1996) .

Lakes and beheaded streams on relict megafan surfaces, and their biotas, may thus remain undisturbed

for periods of time arguably as long as a few millions of years (models 1 .1-v and 1 .2-v) .

Any drainage rearrangement that permanently fragments an ancestral population or species (i .e . pure

vicariance) sets the stage for speciation. However, drainage rearrangements that divide and later reconnec t

water bodies will provide the opportunity for biological diversification only if time of separation i s

sufficient for development of re-enforceable reproductive isolating mechanisms . Speciation is ofte n

considered to be a long term process. Haldane (1957) suggested that 10 5 generations of gradual genetic

divergence between large isolated populations would be needed for speciation . Molecular-based estimates

of allopatric speciation in fishes suggest rates ranging from 10 4 -10 6 years (McCune & Lovejoy, 1998) .

Direct fossil evidence of freshwater fishes in South and North America indicates undetectable or ver y

minor morphological change in several modern species and species groups for 10 6 -10' yrs (Lundberg,

1998 ; Lundberg and Aguilera, 2003). Similarly, vicariance biogeographic analysis of eastern North

American fishes suggests the origin of modern species since fragmentation of a widespread ancestral faun a

is probably as long ago as the Pliocene (Mayden, 1988) .

Nevertheless, theoretical models of divergence with or without complete genetic isolation, but includin g

small population size or strongly divergent selection pressures, indicate that rapid differentiation to

reproductive isolation is possible . More to the point, empirical evidence is growing for rapid population

divergence, or even speciation, in freshwater or coastal fishes in the time range 10 1 to 104 yr (Greenwood ,

1965 ; Owen et al ., 1990 ; McCune, 1995 ; Beheregaray and Sunnucks, 2001 ; Kristjánsson et al ., 2002 ;

Reusch, 2001).

We conclude that lakes and beheaded streams on relict megafan surfaces, and their aquatic populations ,

may thus remain undisturbed for periods of time arguably as long as a few millions of years (models 1 .1-v

and 1 .2-v), overlapping the range of separation times for sister populations isolated by drainag e

disconnection-reconnection events .

4.3 Megafan altitude and diversity

There is a notable dearth of published detail on longitudinal zonation of fish communities in large

tropical river systems that head in high mountains . Nevertheless, general patterns have long bee n

recognized and partly documented (e .g . Pearson, 1924; Eigenmann and Allen, 1942 ; Ibarra and Stewart,

1989 ; Galacatos et al ., 1996 ; Cox Fernandes, 1999) . South American riverine fish faunas are most specie s

rich, and perhaps also most stable, at low altitude (e .g . lowland tropical Amazon)—below about 200 m

where gradients are everywhere very gentle and wide floodplains and riparian forests experience extensive

seasonal inundation . Species richness declines upriver, especially along the upper piedmont and montan e

reaches . Tumover of species composition ((3 diversity) is also more evident in the uplands . For example ,

Ibarra and Stewart (1989) found a raid decline of lowland fish species between about 250 - 300 m in th e

Napo and Aguarico rivers, whereas upland species become increasingly common aboye 250 m . In light of

this general perception of fish distribution along rivers, it is of interest that 12 of the 18 megafans for whic h

topographic data are available (central Argentina through Bolivia to western Amazonia)—including all th e

larger, more complex megafans—straddle the 200 m contour, or reach within 20 m of this contour .

Considering that megafans exhibit some of the flattest surfaces and lowest slopes on the planet, the

concentration of most megafans in the sample (66%) within a narrow altitudinal zone is striking .

Have river dynamics on megafans played any part in generating this zone of higher species richnes s

change and turnover rates ?

4.4 Amazon Paleogeographies

There are reasons to believe that the dynamics described aboye may have operated in the past . It i s

generally believed that rivers in what is today the upper Amazon basin were oriented towards the north fo r

much of the last 70 million years or more, as part of an expanded Orinoco basin (see for example ,

discussions in (Potter, 1997 ; Lundberg et al ., 1998) . The orientation of the paleodrainage about 20 Ma i s

shown in Fig . 21 .

We examine the models in three geomorphic settings in South America .

Foreland basin. It is reasonable to suppose (i) that megafans have probably characterized the forelan d

during phases of continental deposition, and (ii) that the same river dynamics as occur today have operated

in the past—and perhaps for the last 67 million years—in the more mature foreland rivers on the east sid e

of the Andes Mountains (Potter, 1997, and Lundberg et al ., 1998) . With the modem landscape as a guide ,

we may suppose that within-fan dynamics and interfan dynamics have probably operated in the wester n

Amazon, and south to northern Patagonia, and northward to the llanos .

Amazon trough . We can take this reasoning further. We have recently achieved greater understanding

of the nature of the topography in the Amazon trough . The existence of a paleo-megafan has been

postulated recently by (Wilkinson and Harper, in press ; Latrubesse, 2002) to explain the distributary patter n

of sinuous features observed in astronaut photography in southwestern Amazonia . The distributary patterns

appear to be non-functioning traces of the Río Aripuaná in the southern Río Madeira basin, and as such ,

probably indicate a paleo-megafan .

The apex of this putative megafan is situated at the topographic discontinuity between a hilly cratoni c

hinterland and the vast Amazon lowlands . This pattern fits expectations derived from the global survey

(Wilkinson, 1996, 2002, 2003 ; see also section 1 .4 aboye), namely that any megafans are most likely to be

found anchored at the topographic break (steeper slopes and low cliffs) between the cratonic uplands wher e

rivers are confined in bedrock valleys, and lowlands where rivers are broadly unconfined by valley slopes.

The sedimentary body is developed on the downhill side of the topographic margin, namely within th e

Amazon trough. The association of the putative megafan with the point where a larger river (Aripuaná )

crosses the topographic breaks further supports the megafan interpretation . The global study shows that

megafans exist in settings other than foreland basins, especially in cratonic geological settings, with many

megafans documented in the cratonic basins of Africa and Australia (Wilkinson, Cameron and Burke ,

2002) .

The recognition of a megafan at one point on the flanks of the Amazon trough allowed Wilkinson an d

Harper, in press) to predict that other megafans should be found at similar topographic locations elsewher e

in the Amazon trough. Wilkinson and Harper (in press) thus suggested a zone of likely megafa n

development along the northern margin of the Brazilian craton and the southern margin of the Guyana

craton (Fig. 21) . Interestingly, Latrubesse (2002) has recently documented the existence of such a

sedimentary body immediately west of, and adjacent to, the Aripuaná fan (Fig . 21) .

We conclude that long periods of intra- and interfan dynamics probably occurred on the trough margins ,

wherever megafans existed, in the way envisaged for the South American foreland adjacent to the Andea n

chain.

Interbasin connections . We discuss three megafans on two critica] paleo-divides, the divide betwee n

the Amazon-Orinoco and the Paraná, and the divides between the western and eastern drainages of th e

Amazon trough before their integration into a single drainage system. These locales are indicated by

rectangles in Fig . 21 .

(i) Paleo-divide between the Orinoco-upper Amazon and Paraná basins . The divide between the two

great paleo-river systems of the Orinoco-upper Amazon and Paraná was probably often occupied by a

megafan, as it is today by the Parapetí megafan, since a major cone of river sediment is the likel y

topographic high point in a foreland depression .

We propose, therefore, that the topographic divide between tributaries flowing to the Orinoco-uppe r

Amazon basin and the proto-Paraná basin—when it was structured as a megafan—was breached repeatedl y

by the stream operating on such a critically located megafan .

Any similarities between the freshwater fauna in the two basins must surely take account of this likel y

paleodrainage dynamic .

(ü) Paleo-divide between the upper Amazon and lower Amazon basins . As recently as - 8 Ma, th e

Amazon trough was drained by two major rivers, one flowing west into the upper Orinoco, and one flowin g

east into the Atlantic at the present mouth (Fig . 21) (Potter, 1997 ; Lundberg et al ., 1998) . We propose that

megafans were probably also located on the divide between these proto-Amazon drainages (rectangles, Fig .

21), at least during some periods in the past . Megafans may have been deposited by rivers flowing into th e

trough from either, or both, flanks (north and/or south) of the trough . We have shown that megafans often

occupy modern drainage divides between major basins. If such megafans ever existed, it seems probable

that the eastern and western basins of the Amazon trough were connected hydrologically by the switchin g

behavior of the megafan rivers . Habitat connections of model 1 .4 type (Figs . 12-13) would then hav e

operated in the Amazon trough, at great distances from the Andean foreland .

Examples of megafan rivers connecting basins in other parts of the world suggest that these ideas hav e

applicability to issues of the distribution of freshwater organisms beyond the Amazon basin .

5 . Conclusion s

Well-known stream processes—stream course switching, and drainage-net dismemberment due to

aridity—and less well-known processes (self-sedimentation), achieve special significance in the unconfme d

and aggradational settings of large continental fluvial landforms known as megafans . Switching behavio r

translocates the formative megafan stream to widely separated positions and may allow not only

neighboring megafans to communicate hydrologically, but also neighboring river basins . Since megafan s

attain great size, portions of a megafan surface remain entirely undisturbed by fluvial processes for lon g

periods . All these characteristics potentially impact speciation and migration in aquatic populations that

inhabit rivers on large fans .

Remotely sensed imagery, especially in the form of the variable-scaled astronaut handheld imagery ,

with views of many parts of the Earth between 55°N and 55° S, has provided the impetus and basis for th e

first worldwide systematic study of megafans, a study which is ongoing, and from which the rive r

dynamics in this study are derived .

Stream behaviors that potentially impact speciation and migration in aquatic populations in megafa n

rivers, are described in the form of eight models . Other impacts could be modeled as more is learned o f

these rivers.

Salient characteristics of megafan rivers and their environments, that underlie the models, are the

following . (i) The main megafan stream, in the aggradational phase of fan building, is highly mobile, a

behavior termed switching behavior (by avulsion andpiracy on low declivity fan surfaces—see Glossary

appendix for definitions) . These behaviors can provide situations both of a vicariant and mixing variety fo r

freshwater organisms . (ii) The vast size that can be attained by megafans has implications for the longevit y

of megafan subenvironments such as lakes and beheaded rivers. The probable longevity of at least some of

these landscape features overlaps with temporal ranges suggested as necessary for successful speciation, a s

developed in a wide and developing literature . (iii) The severing of megafan streams from their trunk river s

is widespread. (iv) The ubiquity of megafans, not only in South America but worldwide, suggests that th e

dynamics identified may have wide applicability .

All the models have a habitat fragmentation mode and a habitat connection (or reconnection) mode . The

first set of models (models 1 .1 to 1 .4) is based on the river switching phenomenon (Table 1, Fig . 22) .

Isolation of populations in relict lakes (model 1 .1), especially on distant or slightly raised "terrace

surfaces," is the subject of model 1 .1 (Fig. 22a) . Isolation in distal "beheaded" reaches of the megafan river

is the subject of model 1 .2 (Fig . 22b) . Because megafans are generally characterized by single

throughflowing rivers, most points on a megafan surface are far removed from the active river and hence,

quiescent geologically. The isolated arcas can be very large (a figure of 1 .2 million km2 of modern fan

surface is estimated for South America alone) . The great size of megafan surfaces allows parts to b e

unaffected by fluvial activity for periods of time in the order of 10 3 -10' years, on the evidence of th e

Pilcomayo basin groundwater ages and Ogallala megafan surface-age evidence . The probability o f

divergence and speciation thus also tends to rise .

Switching behavior also serves to connect or reconnect disjunct populations with the wider

hydrographic network . Since megafans often develop adjacent to one another, switching behavior ca n

connect megafan rivers and the populations they host ; subsequent avulsion divides the mixed populatio n

(model 1 .3—Fig . 22c) . On megafans that occupy divides between major basins, switching implies rive r

drainage alternately into one then the other basin . This introduces the possibility of population mixing, a t

least for those aquatic populations that occupy the megafan sector of the drainage net (model 1 .4—Fig .

22d) .

So many of the megafan streams in South America aré severed from their trunk rivers that we suggest

that this may be a normal situation on megafan streams, Stream network disruption of this kind develops i n

settings with very low inherent declivity, with probable episodic tectonic influence of river slope, and

especially during semiarid climatic phases . Although severing of this type is by no means confmed t o

megafan rivers, it is included here as the basis of three models because so many of the drainage networks i n

South America, and in other continents, are dislocated in this way.

Climatic boundaries have shifted across the Amazon basin (Clapperton, 1993) . Seasonally or

permanently drier drainages, or entire basins, have probably communicated with the Amazon basin by th e

dynamics described here .

Climate change can cause dismemberment and subsequent reintegration of stream networks, particularl y

on megafans where high infiltration capacity is a characteristic (model 2—Fig . 22e) . Closed system

behaviors, in which rivers do not drain into the sea but cease flowing far from the coast, have majo r

implications for continental sedimentation, stream profiles and surface and groundwater flow. In wetter

climatic phases, authochthonous stream networks can develop, which can potentially reestablish strea m

connections, for example, with disjunct waterbodies (model 3—Fig. 22f) .

Switching rates on floodplains, in delta settings and on megafans, although the data are few, vary from

decades to tens of thousands of years . For basins connected geomorphically by megafan systems, aquati c

populations in one can conceivably migrate between basins as often as the avulsion mechanism allows .

Stream self-blockage too can sever the megafan river from the trunk river (model 4—Fig . 22e), with

fragmentation and connection effects similar to those described in model 2 .

Avulsion and aridity mechanisms can work together. Thus it seems likely that a set of neighborin g

megafan streams may be cut off froni'the basin trunk river due to aridity or other causes, whil e

communicating repeatedly with one another . The streams on neighboring fans then act as a center of

speciation (model 5—Fig . 22g) .

The models address questions of both distribution and divergence . With respect to geographic

distributions, all stream behavior models involve change in aquatic habitat extent and connectivity at thre e

levels, the intra-fan, inter-fan and interbasin . The models describe how megafan river behavior can promot e

or restict dispersion over the aquatic landscapes of individual organisms, their populations and species .

Changes in range or geographic distribution might in turn affect both community composition and th e

potential for evolutionary divergence (or fusion/hybridization/introgression) .

The most salient perspectives of this overview are the following . With respect to diversification at the

species level and below, the fragmentation (-v) models, at the population level, encompass allopatric

divergence—be this phylogeographic structuration, intraspecific divergence, or full speciation—and

extirpation . For the habitat connection models (-c), at the level of fully developed species, the results are

ecological and biotic with an increase of local (alpha) diversity . However, for closely related species or

divergent populations within a species that are brought into new sympatry, hybridization processes may b e

initited, and these could allow introgression, lineage fusion, or the reverse reinforcement/strengthening of

incipient barriers to reproductive isolating mechanisms .

At the level of regional biotas (faunas, floras), the effect of the vicariance or fragmentation models i s

disruption of original biotas that may be subsampled either randomly, or by the historical accident o f

species' habitat preferences. In the combining models at the biota level, the effect is new admixtures of

species to produce complex or composite biota. Occasionally a species introduced by there processes coul d

become invasive, with damaging effects on native species via competition or predation .

We suggest that there exist three orders of diversity related to the dynamics of river activity on

megafans (Table 2) . There is a type of diversity controlled mainly by river dynamics of the single major

megafan river. Intrafan diversity relates to diversity within local populations and among local populations .

Interactions between fans underlie another kind of biological diversity which we terco interfan diversity.

This level relates to diversity among populations and species . Interbasin diversity refers to diversity

controlled by the action of megafan riGers on major basin divides, and relates to species and biotas .

Intrafan, interfan, inter-basin dynamics can be, expected to have operated along both margins of the

Amazon trough where breaks of slope, similar to those which occur today, have probably been in existence ,

to greater or lesser degree for tens of millions of years, when rivers have operated in th e

depositional/aggradational mode.

The models presented aboye have many implications for speciation of aquatic populations, not only fo r

isolating populations in megafan streams but also for the interchange of populations between majo r

continental river basins . These implications deserve further attention since it is likely that geomorphi c

dynamics may assist in interpretation of faunal distributions . It is also likely that known fauna l

distributions will suggest prior river connections not obvious in the modem and submodern landscapes.

A number of potential influences on biological entities emerge from the river behavior models ,

some ecological (individuals/populations/communities) and some evolutionary (taxic - populations ,

species) . All are potentially of interest and importance. None will be easy to document or demonstrate, bu t

hypotheses have been suggested by which to test the nature of the models .

Acknowledgements

It is a pleasure to acknowledge stimulating discussion with Martín Iriondo , Carlos Ramonell, Pau l

Potter, Frank Wesselingh, and Hanna Tuomisto, and my colleagues Cindy Evans, David Amsbury, Bil l

Muehlberger and Juan Somoano . Martín Iriondo, Carlos Ramonell, Andrea Parboni Arquati, Fernand o

Gomez and Patricia Brogdon-Gomez, and Luis Meyer Jou provided generous hospitality and logistical hel p

on various trips to Argentina and Paraguay . I thank Kamlesh Lulla, sometime Chief of the Office of Earth

Sciences at NASA's Lyndon B . Johnson Space Center, for encouragement to pursue this work, and fo r

helping refine some ideas . Astronaut crews have taken numerous provocative photographs of the planee s

surface, photographs which provided the inspiration and data for this work . Margaret Marker and Julie

Robinson gave valuable comments on an interim draft .

Table 1Megafan river behavior—models of vicariance and habitat connection or reconnectio n

MODELS BASED ON RIVER BEHAVIORS

FIGURE POTENTIAL BIOLOGICAL EFFECT Sv—habitat disconnection; c—habitat (re)connection

Stream reorientation—relict lakes, relict streams; connection/disconnection between megafan streams and major basin s

Models 1 .1 and 1 .2—Stream reorientation on individual megafans (intrafan diversity )1 .1-v

lake formation—disconnection of water body from

5-8

range fragmentation, population divergence ,drainage net

speciation, extirpation1 .1-c

lake reintegration into drainage net

5

range expansion, population mixing, introgression ,reinforced genetic isolation, competitio n

9

range fragmentation, population divergence ,speciation, extirpation

9

range expansion, population mixing, introgression ,reinforced genetic isolation, competitio n

range fragmentation, population divergence ,speciation, extirpation

1 .3-c

connection between megafan streams

10-11

range expansion, population & fauna] mixin g

(interbasin diversity)

12, 13

range expansion, faunal mixing

(intrafan & interfan diversity )

range fragmentation, extirpation populationdivergence, speciation ,range expansion, population mixing, introgression ,reinforced genetic isolation, competitio n

16

range expansion, population mixing, introgression,reinforced genetic isolation, competition

Model 4—Self sedimentation (and associated river end-point retreat)4-v

fan river–trunk river separation

17, 18

range fragmentation, population divergence ,speciation, extirpation

4-c

fan river–trunk river reconnection

17

range expansion, population &faunal mixing ,introgression, reinforced genetic isolation,competitio n

Combinations of river behaviors and effect s

Model 5—Fan complexes disjunct from the trunk river (interfan diversity)range fragmentation, population divergence ,speciation, extirpationrange expansion, population & fauna lmixing, introgression, reinforced geneti cisolation, competition

5-v fans connected; fan and trunk rivers disconnected 19, 2 0

5-c fans connected; fan and trunk rivers connected 19, 20

•

the opposite 3-v mode is supplied by 1 .1-v, 1 .2-v or 1 .3-v .

1 .2-v

abandonment of course (river beheading)

1 .2-c

reoccupation of course

Model 1 .3–Connection between megafan rivers (interfan diversity )1 .3-v

disconnection of megafan streams

10-1 1

Model 1 .4—Connection between trunk rivers of major basin s1 .4

switching behavior connects major basin s

Drainage net disintegration and reintegration on megafan s

Model 2—Climatic chang e2-v

dry trend: river end-point retreat

2-c

wet trend : river end-point advance anddrainage reconnectio n

Model 3—Autochthonous stream evolution3-c*

autochthonous stream evolution

14-1 5

1 4

Table 2Classification of geomorphic models by scale of hydrologic connectivity and biological scal e

Geomorphic models

Relevant biological scal eby scale of activity

Intrafan dynamics

Population and locally isolated populations

a diversityModel 1 . 1Model 1 . 2Model 2

Model 3Model 4

Interfan dynamics

Isolated populations and species

a and 13 diversityModel 1 .3Model 5

Interbasin dynamics

Species and regional biotas

y diversity

Model 1 .4

Table 3Classification of geomorphic models by reach-scale habitat connection/disconnection (allmodels appear in more than one major group )

Fan apex river reachesModel 1 .1

switching occurs at fan apexModel 1 .4

special case of 1 .1 in which one megafan river flows into alternatemajor basins (near-apex connection)

Model 4

habitats are destroyed as river end point retreats to ape x

Midfan and/or distal fan river reache sModel 1 .1

switching occurs at fan apex with lake formation downstreamModel 1 .4

special case of 1 .1 in which one megafan river flows into alternatemajor basins (midfan connection)

Model 1 .2

switching occurs in midfan locations with lake formatio ndownstream

Model 1 .3

megafan rivers connect/divergeModel 2

aridity destroys reaches of megafan rive rModel 3

midfan lakes joinedModel 4

habitats are destroyed as river end point retreats upstream

Distal fan/trunk river relationship sModel 1 .1

megafan stream enters trunk at different pointModel 1 .2

megafan stream enters trunk at different pointModel 1 .3

megafan stream enters trunk at different point

Note : Model 5 combines effects of models 1 .3 and 2

Table 4Large fans and avulsion periodicity

Distributary system Switchin gType periodicity

(yrs)

Rivers on large fan s

Kosi, N India 20 . 5

Siwalik rivers, N India 200 0Mississippi, USA 10 0

40 0Okavango, Botswana 5 0Parapetí, Bolivia <1400

Pilcomayo, Argentina/Paraguay

>35;000

Bermejo, Argentina

Confined river tracts

>100

mean for major rivers 500-1000

alluvial stratigraphymodels

111-1780

Old Red Sandstone, UK

Delta distributaries

8000

Mississippi lobes, USA 75 0

100 0

Rhine-Meuse, Netherlands 100 0Huang He, China 213

twelve avulsions in 246 yrs (Holmes 1965, Wells and Dorr1987a)assuming one river (Behrensmeyer & Tauxe 1982 )(Fisk 1952)(Saucier 1974 )two avulsions in 103 yrs (McCarthy et al . 1992)present river course cuts aeolian units dated older than 140 0BP. (Iriondo 1993 )present tract unchanged in >35,000 yr (Geyh et al ., no date)

one avulsion in 100 years (Iriondo 1984 )

(Leeder 1978)

(Bridge & Leeder 1979)

(Alíen & Williams 1982)

seven lobes since -5300 BP (Kolb & van Lopik 1966 )5 avulsions since 5000 BP, occurring at -5500-5000 BP ,-3800-4000 BP, 2500-2000 BP, 1000-800 BP, --50 yearsago (Atchafalaya) (Coleman 1988 )(Tórngvist 1994 )twelve major avulsions in 2550 yrs (Hsieh 1973 )

Glossary

Alluvial fan In this work alluvial fans are distinguished from larger features known as largefluvial fans, or megafans (see Megafan)

Avulsion The major component of the river behavior termed river course switching (seeswitching below) . Avulsion is the process by which rivers switch from one course to another,[usually on a river floodplain, but also on megafans (see below) and alluvial fans (see aboye)] .Avulsion occurs when a river builds a channel and levee system aboye the level of thesurrounding plain . This is an unstable situation that leads to the river breaking out of the levees ,and establishing a new course across the plain .

Colmatation (also River, self sedimentation) A Spanish earth science term (colmatado — blocked)used in Argentina and Paraguay for the sedimentation and complete blockage by a river of it sown bed. Water and sediment then spill over the levees . Such blockage is permanent .

Gutter — see Interfan depression

Interfan depression Concave zone, or depression, where neighboring convex megafan surfacesmeet . Rivers from either megafan can flow in the gutter depression . Also termed a suture .

Megafan Large, fan-shaped partial cone of river-laid sediment (—7000 to -200,000 km2 in area) withradius arbitrarily defined as >100 km, with low slopes, characterized by flat plains .

Mesoscale landform features and models According to hierarchical thinking, features that ar esmaller than macroscale features such as entire sedimentary basins and larger than river channe lbelts. Megafans and sets of megafans lie squarely within this medium scale of landform . Themodels treated are termed mesocale models because they treat this level of landform aggregation .

Orogenic belt A broad zone of mountain building often formed by the collision of tectoni cplates . Typical examples are the Andes and Himalayan Mountains .

Resurfacing Minor aggradation and incision by ivers, especially the main river on megafans—minor i nterms of depth and length of the reaches affected . Resurfacing is probably in constant play on activ emegafans .

Rivers-- avulsion—see Avulsion aboye—beheading When a river switches course, the old course is said to be abandoned or

beheaded because the runoff of the river is diverted to the new course. The original course lose srunoff but may not dry up if the climate is sufficiently wet to maintain some surface flow .

—piracy . The process by which a more energetic river captures another river in the cours eof natural headward extension (erosion) of the river course . On plains of low declivity, such asmegafans, piracy can occur repeatedly due to shallow erosional incision into a megafan surface .The term switching is used in this work to encompass both avulsion and piracy .

—self sedimentation (also colmatation) . Translation of a Spanish earth science term(colmatado — blocked) used in Argentina and Paraguay . It refers to the sedimentation and completeblockage by a river of its own bed. Water and sediment then spill over the levees . Such blockageis permanent.

—switching— see Switching below

Stream piracy — see Rivers-piracy

Switching Processes by which rivers are diverted to new courses, either as result of avulsion(see Rivers-avulsion) or piracy (see Rivers-piracy) .

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TSrngvist, T . E ., 1994 . Middle and late Holocene avulsion history of the River Rhine (Rhine-Meuse delta ,

Netherlands) . Geology 22, 711-714 .

Wells, N . A ., Dorr, J . A ., 1987 . Shifting of the Kosi River, northern India. Geology 15, 204-207 .

Whitehouse, F . W., 1944 . The natural drainage of some very flat monsoonal lands . Australian Geographer

4(7), 183-196 .

Wilkinson, M . J ., 1996 . Large subaerial distributary systems : global distribution and implications . 33 pp .

[Unpublished ]

Wilkinson, M. J ., 1998 . River behavior on large fluvial distributary systems and putative dynamics for fis h

speciation in South America . 48 pp . [Unpublished]

Wilkinson, M. J ., 1999 . Fluvial dynamics in the Andean foreland : Megafans and mechanisms for

biodiversity in aquatic biota . Geological Society of America, GSA Fall Meeting Abstracts, 25-2 8

October 1999, Denver, Colorado .

Figure 5

Model 1.1—Lakes on megafan surfaces The megafan surface is shaded and th e

active megafan river shown in blue ; paleocourses of the megafan river are shown as a pattern of

discontinuous, radial lines . a—The megafan stream switches to a new course, leaving remnant lakes in th e

relict floodplain tract . Green represents the original population of a species . b—Colors represent differen t

populations or incipient species evolving in three separated lakes on the abandoned course of the rive r

(model 1 .1-v) . c—Two of the three lakes are reconnected with the regional river network when th e

megafan river avulses back finto parts of the prior course (model 1 .1-c) . The red and black daughter species

enter the river system .

Wilkinson, M. J ., 2001 . Where large fans form : Interim report of a global survey . Fluvial Sedimentology

2001, 7 `h International Conference on Fluvial Sedimentology, Program and Abstracts, University o f

Nebraska-Lincoln, Lincoln, Nebraska (USA), 6-10 August 2001, p . 282 . (University of Nebraska-

Lincoln, Institute of Agriculture and Natural Resources, Conservation and Survey Division, Open-fil e

Report 60) . <http ://www.unl .edu/geosciences/ICFS/ICFS .html> (29 Nov . 2003 )

Wilkinson, M. J ., 2002 . Modern river and sedimentation patterns in Africa--subbasin-scale models derive d

from astronaut photography, Poster, Petroleum Exploration Society of Great Britain and Housto n

Geological Society, First Annual International Symposium, 17-18 September 2002 .

Wilkinson, M. J., 2003 . A geomorphic classification of sedimentary subbasin typesin the South America n

foreland — tectonic and drainage pattern controls . Alluvial Fans 2003, Conference Abstracts, Sorbas ,

Almería, Spain, 8-13 June, 2003 . <http ://alluvialfans .net/Abstracts .htm> (29 Nov . 2003) .

Wilkinson, M. J., 2004 . Meso- and macroscale African landscapes as seen by astronauts from low Eart h

orbit . Keynote address, Southern African Association of Geomorphologists, Biennial Conference ,

Knysna, South Africa, 4-6 April 2004 .

Wilkinson, M. J ., in press . Global distribution of large fluvial fans . NASA Tech Briefs Online, No . MSC-

23424 <http ://www.nasatech .com/Briefs/ps .html> (30 Mar . 2004) .

Wilkinson, M. J ., Cameron, N . R., 2002. Global geomorphic survey of large modern fans : distribution and

exploration implications, Program and Abstracts, AAPG Annual Meeting, Houston, Texas, 11-1 5

March 2002 .

Wilkinson, M .J ., Cameron, N .R ., Burke, K ., 2002 . Global geomorphic survey of large modern subaerial

fans. Houston Geological Society, HGS Bulletin 44(7), 11-13 .

Wilkinson, M . J ., Harper, R ., (in press) Paleostream patterns in the Aripuaná-Manicoré lowland, Madeir a

River watershed : A central Amazon Basin megafan?

....m

....m av.a.rn >.~nn~

.. .

.w,... .. . rw

Figure 1

Megafan features andgeometries a—Megafan sediment cones generally appear fa n

shaped in planform (shaded area) . Characteristic features are the single active river and the radial pattern

ofpaleocourses centered on afan apex. The apex is located where the river exits the mountain mas s

(orogenic belt—patterned area left margin) . Fan margins are indicated by dashed unes . The fan eones meet

in a feature known as the interfan depression . The trunk river winds down the far right side of the pane l

(this diagram organization is generally followed in subseqúent model figures) . bGeometries of megafan

axial river (left) and basin trunk river (right) —T/T Transverse/Transverse, and T/L

Transverse/Longitudinal (alter Miall, 1996). T/L pattern characterizes a aboye, in which the trunk river o f

the foreland basin flows parallel to the mountain front . The T/L pattern is the basis of models developed i n

this paper .

Figure 2

Distribution of megafans

Foreland basin and Amazon trough megafans o f

South America (alter Iriondo, 1987, 1993 ; Clapperton, 1993 ; Baker, 1986; Neller et al ., 1992 ; Wilkinson,

Cameron, and Burke, 2002; Wilkinson and Harper, in press).

`a.aya,

orar,

Figure 3

Apex of the Río Paraapetí megafan Near-vertical view of the apex region of the Río

Parapetí megafan located at the foot of the Andes Mountains (outside left margin), showing the mode m

course curving north. Several paleocourses radiate eastano southeast towards the Paraguay-Paraná basin .

North-south aligned patches crossing the paleocourses reprgsent a partly vegetated dune field . The Río

Parapetí sometimes delivers water to the Paraná basin and zsometimes to the Amazon basin. NASA photo

STS73-747-91, center point 25 .8°S 59 .8°W., 30 October, 1995 . Image courtesy of Earth Sciences an d

Image Analysis Laboratory, NASA Johnson Space Center .

,,1

Figure 4

Andean foreland in Paraguay and northern Argentina

South-looking panorama

of western Paraguay and northern Argentina showing the Andean foreland basin situated between th e

Andes Mountains (right) and the Río Paraguay (left) . The sector of the basin shown in this view is entirel y

dominated by megafans, mainly of the Pilcomayo, Parapetí and Grande rivers . The foreland basin is 55 0

km wide in northem Argentina/western Paraguay along the double-headed arrow . River courses in the

mountains cannot be detected in this panorama, but show up well on the plains of the foreland : the Parapetí

and Grande rivers bend north (towards the bottom of the view) . The active sector of the Río Pilcomay o

appears top right . The Pilcomayo drains into the Paraguay. The Río Grande ultimately flows to the

Amazon River, whereas the Parapetí sometimes delivers water to the Paraná basin and sometimes to th e

Amazon . See Figure 18 for detail of the active sector of the lowland Río Pilcomayo, and Figure 3 for detai l

of the apex of the Parapetí megafan . NASA photo STS57-81-54, center point 21°S 59 .5°W., June 1993 (no

date) . Image courtesy of Earth Sciences and Image Analysis Laboratory, NASA Johnson Space Center .

Figure 5

Model1 .1—Lakes on megafan surfaces The megafan surface ís shaded and th e

active megafan river shown in blue; paleocourses of the megafan river are shown as a pattern of

discontinuous, radial Unes. a—The megafan stream switches tu a new course, leaving remnant lakes in th e

relict.floodplain tract . Green represents the original population of a species_ b Colors represent different

populations or incipient species evolving in three separated lakes on the abandoned course of the rive r

(model 1 .1-v) . c—Two of the three lakes are reconnected wíth the regional river network when th e

megafan river avulses back finto paras of the prior course (model 1 .1-c) . The red and black daughter species

enter the river system .

Figure 6 Model 1.1-v--Lakes on megafan surfaces Northeast-looking view from the Bolivian

Andes Mountains (under cloud in foreground), shows the modem, highly contorted trace of the Río Beni

leading north from an apex against the mountain front (bottom) . Clouds cast long shadows across the Beni

lowlands . The Beni megafan occupies the right half of the view (shaded) . Prior positions of the Ben i

floodplain are indicated by arrows radiating from the apex, one cocarse located along a modem foreste d

tract with floodplain soils (two left arrows on megafan), one following a fine of rectangular lakes (so-calle d

lagos cuadrados—middle megafan arrow), and a third following a geological lineament related to a

putative subsurface fault zone (right megafan arrow) (interpretation in part follows FAO-UNESCO, 197 7

and Allenby, 1988) . Scale fines in this oblique view show approximate scale in the foreground (bottom )

and background (top) of the view . NASA photo STS51F-42-87, center point 14.5°S 67 .5°W., 5 August,

1985 . Image courtesy of Earth Sciences and Image Analysis Laboratory, NASA Johnson Space Center.

Figure 7

Model 1.1-v-Lakes en megafan sur,faces Numcrous lakes appearon this dctailed ,

south-lookíng, infrared image of the lower Río Bermejo megafan in northern Argentina. The Rio Bermej o

course snakes along the top left of the view. Very light and very dark patches are numerous small lakes ,

same with irregular and contorted outlines (see interpretive diagram), and some of which are set far from

the active river tract. The arcaof the view is -360 km2, whereasthe crea of the entire Bermejo megafan i s

-116,000 km2 . Dashed unes show probable prior courses of the Río Bermejo . NASA image STS69-733-83

(part), cerner point 25°S 61°W . 17 September, 1995 . Image courtesy of EarthSciences and Image Analysi s

Laboratory, NASA Johnson Spaee Center,

Figure-8 l>iudel 1.1-v-Lakes pan m gofan surfaccs Numerous lakes uppear an this southwuest-

looking,infrared image of the interfan depressionbetweenthe lower Bermejo and Pilcomayo megafans in

northernmost Argentina . The discontinuous Arroyo Monte Lindo is one of several small streams that

occupythc broad zone whcre-thcBermejo and .Pilcomayomegafansmcct. Eventhis dctailcd view suggests

the immensity of the megafan surface on which lake depressions can be preserved, compared with narro w

tract of the active axial river . Sinoke from a fine dóminates the bottom of the view. The area of the megafan

surface shown- in the view is -350 km', whcrcas the area of the entire Pilcomayo megafan is 210 ;QQQ km2

(Triando, 1993), NASA image STS69-733-83(part), center point 25 .3°S 59 .3°W, 17 September, 1995.

Image courtesy of Earth Sciences and Image Analysis Laboratory, NASA Johnson Space Center .

Figure 9

Model 1.2 Abandoned river reaches as isolated habitats Fan margins are indicate d

by dashed fines . The trunk river and megafan streanhiie broa lly transverse to the mountaín front so tha t

switching of-the =gafan atream shifts thepoint ufcuntluence great distantes . a—A megafan river (xy)

lcaddsdischargcfrom thcmountains (lcft) to thc trunk river; green indica-tes aparcnt specics in thc megafan

river . b—The megafan river switches atpoint A díverting díscharge into a new course (or finto an ol d

disused course), sector z. Such geometries permit megafan rivers to shift conflüences hundreds of km

along the trunk river of the basin (arrow) . Red colors suggest the straightfonvard population divergence or

speciation.frotn the greenparent stock i-n_disjunctsector y (=del 1,2-v) . (The switching point, A, liesata.

subapex far from the mountain front, to represent the type example of this configuration, the Río Pastaza o f

Peru—see text.) c—if the river switches back to its first configuration, the original green population

mingles with the new reds (model 1 .2-c).

O..ther-scenarios.mentioned .in the text suggesta differentse0uence of population joining_an d

spiitting, with sector z designated as a prior course of the megafan river : if the yellow population is a

daughter population of the greens (by the same dynamic of river switching in b aboye), then the yellows

enter sector x alter the river reorientation shown in model 1 .2-c. Yellows are then part of the specie s

association that encounters the newer red daughter population when the river shifts back into in the

connection modc, tothc-configuration shawn inFig . 9c. Both switchcs thus act in taco ways, bothas

vícariant and connection events for different species .

Figure 10

Mode11.3—Connections between megafans

a—Riverson ncighborin g

megafans A, B and C flow independently to the trunk river . b—Rivers on the neighboring megafans A an d

B flow together finto the conunon topographic low point between the conical fan surfaces, bringing together

their respective giren and red populations or species (model 1 .3-4 e—When the netighboringrivers A and

B.. once again diverge away from one another, the mixed faunabecomes disiunct (model 1 3-v)—with th e

potential for speciation in the wake of any dynamics that may have occurred during interaction in the 1 .3-c

phase . (Megafan river C is shown oriented to a new course within the suture depression, in which positio n

Itere is a higher probability of its encountering river B if it switches south .)

Figure 11

Model L3—Connections between megafans

Ariari and Güejar river fans ,

eastern Andes foothils, Colombia, show ing convergent stream orientations (May 1991) . Discontinuous

lines on the fine diagra a show the numerous traces of past river courses of the Río Ariari on the Aliad fan

and the Río Güejar on the Güejar fan. The Ariari and Güejar rivers flow together (Mark arrows) into the

common depression between the fans, and drain southeast. Red arrows show divergent prior courses . The

course of the Güejar, as mapped in existing atíases, shows drainage in a southeasterly direction (lower red

arrow). NASA photo STS37-15 .1-32,center point 3 .5°S 73 .5°W, 5 April, 1991 . Image courtesy of Earth

Sciences and Image Atialysis Laboratory, NASA Johnson Space Center .

Figure 12

M©del L4—Connectians-between -river basins

Amegafan -(shaded) situatcd-onthc

divide between major drainage basins holds a uniqueposition in the fluvial landscape : the megafan river

can alternately drain into one then the other major, hígher-order basin. a—The megafan river (A, with red

species) flows northeast into a-major basin (ay. b--The river switches to a more southerly-course ;

introducing the red_ species into the southern basin . C . Thisrepresents the connectionmode of the model

(model 1 .4-c) expanding species ranges and combining faunas, but also represents a break up of the reds '

habitat(model 1 .4-v). The green color in the southern river represents a different species (or association j

with a-similar niche and hence vulnerable to competition .

75°W

2°N

72°W

200 km

2°N--

Figure-13

Morlel-1.4—Connections between majorbasins

Regional drainage-map showing

wider hydrographicconnectionsof the Ariari and Güejar rivers (A and G) . Curved arrows .indicate th e

range of reorientation available to these rivers on their respective fans . The Ariari and Güejar today flo w

together andlead southeast into the Río Guaviare . When the Ariari is oriented due east (red arrow) i t

becomes a tributary of the Río Meta. Both the Guaviare and Meta are major tributaries-of the Río Orinoco ,

SouthAmerica'sthir-d largestriver . A switch by the Ariari results in .2000 km separation between

populations in the apea region of the river. The inset expands the ares of the fans and shows the outlíne o f

Fig . 11 .

Figure 14

Model2 Dynarnics on -a single fan—streambehaviarsrelated to-aridiy Fan margin s

are indicated by dashed Fines, a—A megafan stream reaches the trunk drainage(with a Breen species

cornrnon to both). b—Aridity causes sufficient reduction of flow in the megafan river to sever the megafa n

river from the trunk river (cither at an imprcciscpoint, as is commonly the case ; or at a spccifieend point

such as a tectonic depression, asin.the case of the Río Parapetí Fig. 3) . Grecia species isolated on the

megafan undergo divergence and speciation in the disjunct megafan river (model 2-v) . e—Increasingly wet

climates cause the river end point to migrate distally until it rejoins the trunk river . Species that have

evolved on the megafan (red) may then-ente-r-the-main-trunk-stream, joining the parcnt species (green )

(mo_del2-c) . Arrowsshow directions ofmigration, greens recolonising the megafan stream and red s

migrating into the trunk river .

M AEs T

aCONVENIO N° ASR /

B7-3100/99/13 6

XA -- 01d~

Figure -13

Mod l2-v=Dynamics on a single fan—aridity

The RíoBoyuibe of southern

Bolivia fiows from the subandean mountainranges (1eft) into the Chaco plains where it ends, dissipatíng

into the sedirent of its fan. (Another example .istheRío .Itiyura(a in Fig. 18),which alsoends some

distance from its trunk stream.) The past extension of the river appears as a faint continuation of the Río

Boyuibe eastwards (right arrow) from the present end=point marsh. NASA photo STS86-723-25, center

point 20 .3°S 63°W, 4 October, 1997. Image courtesy of Earth Sciences and Image Analysis Laboratory ,

NASA Johnson Space Center.

Figure 16

11odel 3Dynamies on a single fan—autochthonous stream evolutionThe fan i s

shaded with margins indicated by dashed fines . -a—A-megafan stream switches coursc-(arrow) leaving a

series of lakeson theabandoned floodplain (a Breen population now occupiesthelakes as wellthe

megafan river, evolving into the red and black populations in the lakes) . b—Divergence or speciation ,

indicatedas the redandblack populations, may take place in the lakes (model 3-v—as in model 1 .1-v) . In

welter climates, drainage-networks can evolve-on thosc-megafan surfaces that-have-been abandoned by the

main river. This process allows disjunctdepressions to be progressively connected and reintegrated int o

the system of regional rivers . c—The new stream, extending itself headward, reaches the lake with the

black population, and subsequently, guided by the original drainnage depression, also reaches-the lake with

the red population. The-new red and black species maythen be int oduced hito the wider river networ k

(model 3-c) .

hundreds of kmretreat of chokepoint in a few

decadeschoke fpoirtt

—°A

ftanking swamp_lands migrate up-stream as chokepoint migrates

Figure 17

Modelo Dynamicson a singlefan—behauiorsrelated to self blackageFan margins

are indicated by dashed unes . a—A megafan stream is connected to its trunk drainage (with a gree n

population conunon to both) . b—When a megafan river plugs the channelwíth the sediment it carnes, i t

can cutitsclf off effcctivcly from the trunk river ofthc-basin (model 4-v) . ()verbanl discharge then oceurs

finto lower lying tracts beyond the.levees to form flanking swamplands . The red color suggests speciation in

the disjunct megafan river . c—When the river reconnects with the trunk drainage, potential dispersal take s

place, as shown by the arrows, with the new species introduced into the regional river system (model 4 -

c)—as in modcls 2-c and 3-c .

( ( ( ( ( ( ( ( ( ( ( ( (

(

(

( (

(

( ( (

(

(

(

(

(

Figure 18 Model-4-v—Dynamics on a single fan—behaviors related to self blockage This south-wcst-

looking view shows the blocked sectors of both the Río Pilcomayo (center) and the neighboring Río Itiyura

on the fan to the south—with its end poínt (a, top left). Until the 1940s, the Pilcomayo flowed southeast

along the entire-sector shown inthis view (right-to left), most of the 65Q-km distance-to-thc-Río Paraguay ,

which itfailed to reach . Since then the river end point has rapidly retreated west (left to right inthis view)

towards the Andes Mountains . Wider contexts of this view are shown in Figs . 4 and 20 . Arrows indicate

present and pass flow directions of the Pilcomayo and the Itiyura rivers . The end point of the blocked Rí o

Pilcomayo was a diffusc reach (b) in this photograph-taken in January 1990 . -In the mid-1990s, tak-c off

canals were constructed upstream Olear b') to prevent sedimentation in the bed of the river, and thereby

maintaining the flow of the river at least to this point. The river threatens to cut through its levee and

avulse into new courses at two critica/ points (e, e') : ifthese switches oecur, the river will foilow prior

flood courses marked by dashed arrows (details taken from Cordini, 1947; Comisión Nacional del Rí o

Pilcomayo, 1994 ; and Meyer, 1996). The Río Itiyura fails to reach the Río Pilcomayo : its end point lies in

a medial position on íts fan, the margin of which follows the line c–f–f . [The margin of the smaller fa n

appears to control the position of one of the iritical p-oints (e) . This critical point is thus a fan subapex, a s

indicated-bythe dry courses of die Río Pilcomayo diverging from this point .] Shadcd rectanglcs are

agricultura) developments. Scale lines ín thís oblique view show approxímate scale in the foreground

(bottom) and background (top) of the view . NASA photo STS32-88-93, cerner point 22 .5°S62°W, 1 9

January, 1990 . Image courtesy of Earth Sciences and Image Analysis Laboratory, NASA Johnson Spac e

Center .

>

)

1

Figure 19

Model5—Gombinationofmodels—megafanisland biogeography Margins of

megafans are índicated by dashed lines . a—Megafan rivers flow independently to joín the trunk river (wit h

a green population or species inhabiting allfour). b—the megafan streams are cut off from the trunk

drainage by various mechanisms (see°text and models 2 and 4 above) : If the-megafan streams intcraet

independently for long .enough, to allow divergente by selection or drift, the new organisms .(the red .

population) may spread throughout the rivers of neighboring fans . The interacting rivers become an island

of speciation, indicated as a green shaded area. c—When one or more of the severed megafan stream s

rejoinsthe sea-connected drainage net (arraws), any new species in the megafan streams will entcrth c

hasin-widefreshwater population associations .

Figure 20

Model5—Combinaiion ofmodels Vertical view ofalarge area of the westem Chaco

and central Andes Mountains showing the upper zones and apcxes of the Salado, Bermejo and Pileomayo ,

Parapetí, and Grande river fans . Near the mountain frant the.small Itiyura alluvial fan with .a .radiusof

-100 km, lies between the Bermejo and Pilcomayo megafans . These megafans meet along a poorly define d

common margin furthcr from thc mountain front . The Itiyura and Pileomayo rivers both fail as activ e

rivers, oniy teeris of km from the-mountain frant, due-to aridity and self blockagu reespeetively . Neither are

connected ta the_ regional .drainage_taday (detail .in

1R) . . The. Ría Bermeja, hy contrast, remains_

connected to the wider basin via its trunk river, the Paraguay (outside right margin) . NASA photo STS61 -

100-35 (part), center point 23°S 64°W, 12 December, 1993 . Image courtesy of Earth Sciences and Imag e

Analy-sis Laboratory, NASA Johnson Space Center .

recentl ydiscoveredpaleo-megafans

Marin e® Andes and magmatic ares▪ Rivers, cakes & wetiands

Marine & fresh water flux■ S,hleIds & massifs

Direction of flow

divide betweeneast- and west-flowing sectorsof the Amazonbasin

Figure 21

Megafan geomorphic : environments in South American paleogeography A series of

modem megafans (named after their formative rivers) occupies the tectonic feature known as the forelan d

depression along the eastern flank of the Andean Móuntain chaiii . Two recently documentedmegafans are

shown in a different tectonic environment ; on the south flank of the Amazon trough . Modem megafans are

overlayed on amap showing major drainage orientations in theearly-middleMiocene, .20-11 . millionyears

ago (after Lundberg et al ., 1998) . At this period the upper Amazon basin drained into the Orinoco basin ,

and the lower Amazon basin drained directly to the Atlantic Ocean (arrows) . The entire length of the

Amazon-facing margins of both the Guyana and Brazilian cratons (tones outlined by red dashes—se e

sourre references in Fig .2) .have probably provided topographic breaks (small,cliffs or.steeper slopes

dividing confined upland valleys from unconfined downstream valleys) where megafan apexes may hav e

been preferentially anchored at various times in the past (map adapted from Lundberg et al ., 1998 ;

Wilkinson and Harper, in press; Latrubesse, 2002) .

Figure 22 Summary of ~deis Eightmodels of megafan river behavior with possible evolutionar y

importance for aquatic organisms are sununarized as single panels (derived from three-panel diagrams—

seo text for full discussion of each) . The modas are based on documentedriver switching dynamic s(Models 1 .1-1 .4, and 4) and river network hreakdown and reconnection dynamics in river networks

(Models 2, 3 and 4). Various feasible combinations ofmodels can be made, one of which is developedas

Moda All models shown here involve- within-basin population dynamics exeept for interbasi ndynamics in Moda 1 .4 . Interplay of the various dynamics isu.ndoubtedly more complex in the real world .

We attempt to reveal and isolate some of the most basic stream and evolutionary dynamics that appear t obe operating on and between fan rivers .

a Model 1.1—River switching (ingle-fan stream •dynamics) Speeiation -in reliet =lakesdiseonnected from regional river nebror_k (vicariance and _later reconnection leading to increased q -

diversity; extirpations of small isolates would decrease a-diversi tty). A megafan river hosting a Bree n

aquatic population or species avulsedto a new position (stage 1), leaving relict populations in lakes along

the original course grecns in thc relict lakes diverge- or -ven speeiate- into red, back and yello w

populations(ctage 2—shown here) ; the megafan river switches back to aposition near. its original course,

intersecting with one or more of the lakes (stage 3), thereby introducing newly divergent populations int o

thc wider river network .

b Model 1 .2—River-switching, with inhospitable trnnk river (stream =dynamics on a single fan )

Divergente or .speciationin fan river diseonnected from regional river network (range expansionandvicariance leading to increased a-diversity) . As in model 1 .1, the megafan river shifts, introducing it s

green population finto the new river (stage 1), leaving the relict population in the abandonedriver sector t o

diverge or speciate into a red population (stage 2—shown here) ; the trunk river (shown highiy sinuous) is

assumed to be= inhospitable= to the greens, aring as a barrier . to migration ; the megafan river may switch

back to its original course, introducing the reds into the main megafan river (stage 3) .

c Morid. 1.3-River switching on neighboring largo fans (stream dynamics on múltipl e

(neighboring) fans) -Habita' -eoalescence with naigration/invasion(increasein-a- and f3-diversity) . Rivers

onneighboring megafansimultaneously switch towards each other, leading to the mixing of green andredpopulations in the common river below the confluence (stage 1—shown here) . At the subsequent switchingevent (stage 2), a mixed population exists in each river.

d Model L4—River switching_ on large fan situated on divide between major basin (strea mdynamics onasingle fan, with -interbasin effects)Potencial expansionof species ranges and domase iny-diversity as faunas from previously separate basins are mixed A megafan occupies the divide, or

watershed, between major river basins A andB .The river on such a watershed megafan alternately deliver s

water into basin A to the north and basin B to the south . The red population in the megafanriver is thereby

introducedinto thegreens'basin (stage 1 ---shown hese). At the next switching event, any organisms fro m

basin B that invade the megafan sector of the drainage can be delivered hito basin A when the next avulsion

occurs (stage 2) .

e

Models 2 & 4=Breakdown of river network (due to aridity or river self bloekage ; streamdynamics on . a- single_ fan) Genetic divergence infan river disconnected from regional river network

(increased -a diversit). Active flow in a megafan river becomes detached from the trunk river of the basi n

(stage 1), either when discharge is reduced, most commonly by climatic change ; or when regional

topographic slopes are reduced (see text) . The green population (in the megafan river and the trunk river )

is thus divided. With time, the megafan population diverges or speciates (as reds—stage 2, shown here) .

When the megafan river later reconnects with the trunk river, the reds are introduced finto the trunk rive r

(stage 3).

f Model 3-River switching and new stream growth (single-fan stream dynamics with growt h

of -additional fan river) Speciation in relict lakes disconnected from -the -regional river networ k

(vicariance lending -to increased a-diversity) . As in model 1 .1, a megafan-river -switches from its origina l

course, carrying its green population with it (stage 1), leaving lakes with viable populations in its relic t

course . The disjinct green populations diverge or speciate (as reds and blacks—stage 2). The reds and

biacks are subsequently reconnected with the wider drainage net when a small local river develop s

headward up the fan from the trunk river (posssibly along the course of the original river), until it activel y

connects the lake or lakes with the wider river network (stage 3—shown here) .

g

Modél -S-Drainage net breakdown and multiple fan dynamics (combined model )

Genetic divergence in mountain footzone where severa! fanrivers be-come disconnected from regional

network bnt neighboring fan rivers maintain connection (vicariance lending-to-increased a- an d

decreasing,&diaersity). Combination of models 1 .3 and 2 (or 4)_ As one of many plausible combinations o f

stream dynamics, this multi-fan model shows the dissociation of megafan rivers, as during an arid climatic

phase, from the trunk river (stage 1—short arrows). The consequent splitting of the green population

allows divergence or speciation inone megafan-river (red population, circled stagc 2). The reds are

subsequently fed along the piedmont from river to river (curved arrows—stage 3, shown here), at points i n

time when rivers on neighboring fans meet one another in the interfan depression . The new reds migrate

progressively clown the shaded piedmontzone .