Schumm_1963_Sinuosity of Alluvial Rivers on the Great Plains

8/12/2019 Schumm_1963_Sinuosity of Alluvial Rivers on the Great Plains

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S. A. SCHUMM U. S. Geological Survey Denver Colo.

Sinuosity of Alluvial Rivers on the Great Plains

bstract Data on the morphologic and sedimentcharacteristics of stable alluvial rivers of the GreatPlains were collected at 50 cross sections. Thechannel patterns of these rivers were classified intofive types: tortuous, irregular, regular, transitional,and straight. Because no clear demarcation existedbetween each of the types, the pattern of the riverswas described by sinuosity, a ratio of channel lengthto valley length. The sinuosity P) of these riversis related to the shape of the channels expressed asa width-depth ratio (F) and to the percentage ofsilt and clay in the perimeter of the channel (M)as follows:

P = 3.5F- 27P 0.94 M -2 5.

Sinuous streams are characterized by a low width-depth ratio (F), a high percentage of silt-clay in theperimeter of the channel (M), a high percentage ofsilt-clay in the banks (although the banks of straightchannels may also contain large amounts of silt-clay), and a lower gradient than straight channelshaving the same mean discharge. Discharge itself

does not appear to affect the sinuosity of streams.Another possible distinction between straight

and sinuous streams is in the proportions of thecomponents of total sediment load. In a wide,shallow channel much of the sediment transportedis bed-material load. In a narrow, deep channel mostof the sediment transported is wash load.

On the Great Plains both straight and sinuousstreams may flow on the surface of alluvial valleyfills at about the same valley slope. The departureof a stream from a straight course down the alluvialvalley results from changes in both the caliber ofthe sediment load and in the relative proportions o fbed-material load and wash load during the post-Pleistocene alluviation of these valleys. Whenduring this alluviation the proportion of wash loadincreased, most probably by a decrease in bed-material load, the stream adjusted itself by decreas-ing its gradient through the development of asinuous course. Recent changes in stream sinuosityin response to changes in the proportions of bedload and suspended load support this hypothesis.

CONTENTSIntroduction 1089Acknowledgments 1090Description of streams 1090Methods of investigation 1090Channel patterns 1090Properties of sinuous and straight streams . . . 1091Influences of sediment load on sinuosity . . . . 1094Effect of valley history on sinuosity 1096Conclusions 1098References cited 1098

Figure1. Examples of c hannel pattern 10912. Relationship between sinuosity and width-

depth ratio 1092

3. Relationship between sinuosity and silt-clay instream banks 1093

4. Relationship between sinuosity and silt-clay inperimeter of channel 1093

5. Relationship between stream gradient andmean annual discharge 1094

6. Hypothetical cross section of a valley showingchange of shape of stream channels towardtop of alluvium as sediment becomes pro-gressively finer 1097

Table1. Average sediment and channel characteristics . 10922. Data fo r rivers with comparable mean annual

discharge 1095

INTRODUCTION

Rivers are commonly classified according topattern into three major categories: meander-ing, straight, and braided (Leopold and Wol-man, 1957). However, as with most classifica-tions of natural phenomena the grouping isarbitrary and tends to focus attention on the

very sinuous stream, the straight stream, andthe stream that contains islands and to neglectthe transitional patterns. This paper examinesthe sinuosity as well as other characteristicsof some stable alluvial rivers of the GreatPlains. A theory for the development of riverpatterns of varied sinuosity will be presented,based on geology rather than hydraulic theory.

Geological Society of America Bulletin, v. 74, p. 1089-1100, 6 figs., September 1963

1089


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1090 S. A. SCHUMMSINUOSITY OF ALLUVIAL RIVERS ON GREAT PLAINS

ACKNOWLEDGMENTSThis paper refers to one aspect of an in-

vestigation into the effects of sediment charac-teristics on fluvial morphology. R. W. Lichtyof the U. S. Geological Survey assisted duringmost of the field work and made helpful sug-gestions both in the field and during prepara-tion of the report. Lichty, R. F. Hadley, alsoof the U. S. Geological Survey, and Prof. J.Hoover Mackin of the University of Texasread an d criticized the manuscript.

DESCRIPTION OF STREAMSData were collected at 50 cross sections along

a number of western streams. Almost all loca-tions are within the Great Plains province an dare near U. S. Geological Survey gaugingstations. At all cross sections the streams areflowing in channels formed of alluvium; bed-rock is not exposed at any of the cross sections.Aerial photographs show that at some reachesthe streams impinge on the edge of the valley.Bedrock is probably exposed in these reaches,bu t the distance from the measured sectionswas such that any effects of bedrock on mostof the sampled cross sections would be minor.However, at seven locations the stream patternwas controlled by bedrock and possibly bystructure. The data from these seven crosssections will not be used in this discussion.

Evidence from the gauging station recordsand observations made in the field indicatethat the streams are not aggrading or degrad-ing. The data for the 43 sections, therefore,should be representative of stable alluvialstreams that contain less tha n about 10 percent coarse gravel and larger sediment in theperimeter of the channel.

METHODS OF INVESTIGATIONA representative reach of each stream was

selected near a gauging station, and the channelcross section was surveyed. T he gradient of thestream was measured in the field as well asfrom maps and aerial photographs. A compari-son of gradients measured in the field withthose computed from maps and photographsfor the same location indicate that the twomethods yield similar results.

Samples of bank and bed material werecollected at each cross section, and at mostsections a composite sample was collectedalong the perimeter of the channel. A grain-size analysis of the sediment samples was ob-tained by sieving and hydrometer techniques.

Median grain size and the percentage of siltand clay in the samples were obtained fromthe grain-size curves. Per cent silt-clay isdefined as that percentage of the sample passingthe 200-mesh sieve or that portion smaller than0.074 mm.Weighted mean per cent silt-clay, calculatedby giving the bank and bed material a weightequivalent to their exposure in the perimeterof the channel, is related to channel width-depth ratio (Schumm, 1960). The writer ha sbeen criticized for using a weighted mean,which seemed to bias the data; however, thecomposite samples, which included both bedand bank material, were found to contain apercentage of silt and clay close to that ob-tained by calculating a weighted mean fromthe bed and bank samples. Thus the weightedmean per cent silt-clay is truly representativeof the percentage of silt and clay in theperimeter of the stream channel as suggestedpreviously (Schumm, 196la). In this paper theweighted mean per cent silt-clay is used as aparameter descriptive of the sediment formingthe stream channel.

The sinuosity of the rivers near the surveyedcross sections was studied on aerial ph otograp hs.To obtain information on sinuosity a 5-milesegment of the river valley was selected, whichincluded the location of the cross section, andthe length of stream ch annel with in this reachwas measured. The sinuosity, expressed as theratio of stream length to valley length, wascalculated. A straight stream has a sinuosityof 1.0, and th is numbe r increases as the streamdeparts from a straight line. Sinuosity has beenused recently to describe stream patterns byLane (1957), and Leopold and Wolman (1957).

CHANNEL PATTERNSA study of river patterns suggested that the

qualitative classification of straight and mean-dering channels could be expanded to includefive classes. For example, it was apparent thatthere were three types of meanderstortuous,irregular, and regular. There was also a transi-tional channel type between meanders and thestraight c hannels. As Figure 1 shows, the tortu-ous pattern is very irregular. The meanderbends are deformed, and the smoothness typicalof the ideal meander curve is absent. Irregularmeanders are irregular only w ith respect to thesmoothly curved regular meandering pattern.In some cases the irregular pattern seems toconsist of a meander pattern of low amplitudeand wave length superimposed on a larger


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CHANNEL PATTERNS 9

pattern. This condition may be similar to thedouble meanders described by Hjulstrom(1949). He states that the small meanders arefashioned by low perennial flow and the largerby higher flows related perhaps to the meanannual flood. The regular meandering patternneeds little discussion. One should note, how-

A Tortuous

B Irregular

C Regular

E Straight

Figure 1. Examples of channel pattern. A,White River near Whitney, Nebraska P 2.1); B, Solomon River near Niles,Kansas P = 1.7); C, South Loup Rivernear St. Michael, Nebraska P = 1.5); D,North Fork Republican River near Benkle-man, Nebraska P 1.2); E, NiobraraRiver near Hay Springs, Nebraska P= 1.0)

ever, that it is not as regular as one is ordina rilyled to believe. The transitional pattern ischaracterized by very flat curves which tendto repeat like typical meanders. The straightpattern is not truly straight, but the minorbends show no regularity.

In spite of this detailed descriptive classifi-cation of channel patterns, it was difficult toassign some of the channels to a specific patternbecause there are transitional types. This sug-gests that a continuum of channel patter ns doesexist, as Leopold and Wolman (1957, p. 63)suggested.

PROPERTIES OF SINUOUSAND STRAIGHT STREAMS

Each of the streams was classified accordingto the five types of pattern. Table 1 gives theaverage channel and sediment characteristicsfor each pattern. It is apparent that certainproperties of the rivers are associated with eachpatte rn. As sinuosity decreases from the tortu-ous to straight channels, the width-depth ratioof the channel increases (Mackin, 1956), thepercentage of silt-clay in the banks decreases,the per cent silt-clay in the perimeter of thechannel decreases, and mean annual dischargeincreases. The other variables, median grainsize of channel sediment, gradient of thestream, and gradient of the valley, show noprogressive change with sinuosity.

The average data for six straight channelsthat contained islands are also listed. Exceptfor a larger mean ann ual discharge, the charac-ter of the channels with islands is very similarto that of the straight channels. Islands werenot present at any of the sections where datawere collected in the field.

The relationships suggested by the averagevalues of Table 1 are shown on Figures 2, 3,and 4. The relationship between channelwidth-depth ratio and sinuosity is shown onFigure 2. The regression line was fittedgraphically to these data and to the data ofFigure 4 by the method described by Searcy(1960). The relation between width-depthratio F) and sinuosity (P) is as follows:

P=3.5F- 2 7 (1 )Relatively wide and shallow channels tend tobe straight, whereas relatively narrow anddeep channels depart from a straight course.

On Figure 3 the relationship between silt-clay in the banks of the streams is plottedagainst sinuosity. The banks of sinuous channelsare composed of fine sediments and have a highpercentage of silt-clay; however, the bank ma-terial of straight channels varies greatly, andsome straight and transitional channels havebank sediments containing 50-38 pe r centsilt-clay. Thus, although sinuous channelsapparently must have resistant banks, thepresence of large amounts of silt-clay in thebanks is no guarantee that the channel will besinuous.

The relationship between weighted meansilt-clay (M) in the perimeter of the channeland sinuosity (P) is better developed than thatfor bank material alone and indicates that as


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TABLE 1. AVERAGE SEDIMENT AND CHANNEL CHARACTERISTICS OF CHANNEL PATTERNS

Channel

patterntortuousirregularregulartransitionalstraightstraight (islands)

Numberof

channels10947

116

Sinuosity2.31.81.71.31.11.1

Streamslope

ft/ft.00095.00062.00077.00154.00145.00148

Valleyslopeft ft

.00223

.00116

.00132

.00193

.00175

.00170

Width-depth

ratio5.2

19.025.556.043.052.0

Weightedmeans

silt-clay

per cent43.414.08.84.93.44.1

Silt-claybank

per cent898272544145

Mediangrainsize

mm.42.71.74.45.35.37

Meanannual

discharge

cfs

70149209255370421

the silt-clay content of the channel as a wholeincreases the sinuosity of the channel increases(Fig. 4). The relationship is described by theequation

P = .94 M-25

(2)The scatter of the data in Figures 2, 3, and

4 can partly be explained by considering thevariability of sinuosity along one stream. Forexample, along one of the streams studied, thesinuosity has decreased recently from 1.9 to1.6 by meander cutoffs. Thus, along any riverthe sinuosity may vary somewhat with timedepending on the formation of new bends orcutoffs. However, over a long period sinuosityshould average that value indicated by theregression lines of Figures 2 and 4.

Because of the increased length of channelper unit length of valley a sinuous stream isgenerally associated with a low gradient and astraight stream with a high gradient. Slope has

been shown to be a distinguishing charact eristicbetween braided and meandering streams(Lane, 1957; Leopold and Wolman, 1957, p.59). Figure 5 shows that for a given annualdischarge the less sinuous streams generallyhave the steepest gradient.

As Figure 5 shows, the more sinuous streamsare no t among the rivers with highest annualdischarge, which suggests, as do the data ofTable 1, that discharge may have an inverseeffect on sinuosity. However, an analysis ofvariance reveals that a significant relationshipdoes not exist between sinuosity and meanannual discharge, because the variation of dis-charge for a given sinuosity is great. Thatdischarge only affects the dimensions of themeanders can be demonstrated by noting thatthe sinuosity of the Mississippi River betweenMellwood, Arkansas, and Lake Providence,Louisiana, is 2.1. Average discharge betweenthese two points is on the order of half a million

WIDTH-DEPTH RATIOFigure 2. Relationship between sinuosity and width-depth ratio. Standard error

is 0.064 log units. Correlation coefficient is .89.


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PROPERTIES OF SINUOUS AND STRAIGHT STREAMS 1093

10

1.0

.

M

A

0 20 40 60 80 OOSILT-CLAY IN B NKS IN PER ENT

Figure 3. Relationship between sinuosity and silt-clay in stream banks

cfs. Table 2 shows that Red Willow Creek,with a sinuosity also of 2.1, has an averagedischarge of only 42 cfs. Discharge thereforeseems to have little effect on the sinuosityof rivers. However, a change in discharge maycause a modification of sinuosity through itseffect on the type of sediment load transportedthrough the channel. Table 2 presents the datafrom four pairs of rivers. In each pair the dis-

charge is comparable but the sinuosity differs.The data are representative of the type ofrivers studied and illustrate for individ ual cases

the differences in the other properties of riversas sinuosity varies.

One additional reason for the lower dis-charges of the sinuous rivers is that data werecollected only for rivers that could be wadedinto. For a given discharge the wide shallowstreams could be waded into, whereas thenarrow deep stream s could not.

A review of the d ata indic ates that a sinuous

channel has the following properties: lowwidth-depth ratio; high percentage of siltand clay in the perimeter of the channel; high

5.0

3

o

z

>. 2.0

Q _

SILT CLAY IN PERCENT M )Figure 4. Relationship between sinuosity and silt-clay in perimeter of channel.

Standard error is 0.059 log units. Correlation coefficient is 0.91.


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percentage of silt-clay in the banks, althoughbanks of straight channels may also containlarge amounts of silt-clay; a lower gradientthan straight channels with the same meandischarge.

INFLUENCES OF SEDIMENTLOAD ON SINUOSITY

The relationships presented heretofo re affordthe basis for an explanation of variations insinuosity of the Great Plains streams. A transi-

arnount and character of sediment load trans-ported by these streams may be the dominantfactor determining channel sinuosity.

The sediment forming the perimeter of thesechannels should be representative of the type

of load transported through the channels.Median grain size, however, appears not to berelated to sinuosity. Indeed, the data presentedin Table 2 show that the median grain size ma ybe larger for the more sinuous streams.

The one parameter of sediment character

001

.0001

4

EXPLANATION

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1 0

1 5

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1 4

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2,0

+

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T

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INFLUENCES OF SEDIMENT LOAD ON SINUOSITY 1095

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related to the average percentage of total sedi-ment load transported as bed load.

No data exist to support the suggestion thatas the percentage of sediment coarser than0.074 mm in the perimeter of a channel in-creases the proportion of total load transportedas bed load increases. However, some supportfor this hypothesis is given by the relationshipbetween pe r cent silt-clay in the. channelperimeter and channel shape (equation 3), forothers have noted that wide shallow rivers arecharacterized by high bed-load transport.Leopold and Maddock (1953, p. 29) concludetha t with other factors constant an increase inwidth is associated w ith an increase in bed-loadtransport. They give quotes from Lane (1937,p. 138), Griffith (1927, p. 246), and M ackin(1948, p. 484) supporting the view that to be

stable a channel transporting large amounts ofbed load must have a relatively wide and shal-low cross section. In addition, flume experi-ments reveal that sinuosity is not conduciveto efficient bed-load transport, for bed load isone third less in a 180-degree bend than in astraight channel, and in a meandering channelthe bed load per unit width is 20 per cent lessthan in a straight channel (Shulits, 1959).Sundborg (1956, p. 203-204) has suggestedthat as bed load decreases a channel becomesnarrow and deeper and tends to meander. Th ewriter believes that this is true for the Great

Plains rivers.It is necessa ry to clarify the use of the termsbed load and suspended load. Bed load is notsimply the sediment creeping or rolling alongthe stream bed. According to Einstein andothers (1940, p. 632) . . . the bed-load consists of material moving assurface-creep and material moving in suspension,both of which can be expressed as a rate related tothe stream discharge. The wash load, on the otherhand, moves almost entirely in suspension and bearsno relation to ditcharge. Primarily the conceptionis that bed-load and suspended load do not supple-

ment each other, as a certain grain may easily bein suspension and still be considered bed-load."

The term bed load as used above is synony-mous with the term bed-material load, whichis defined as that part of the sediment loadwhich consists of grain sizes represented in thebed (Einstein, 1950, p. 4). However, thisdefinition has been modified to mean sedimentsignificantly represented in the bed in orderto exclude the small percentage of silt and clayfound in most bed material (Einstein, 1950,p. 6, 7). As discharge is increased in a river, the


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1096 S. A. SCHUMM-SINUOSITY OF ALLUVIAL RIVERS ON GREAT PLAINS

amount of bed material transported by sus-pension increases. Nevertheless, bed-materialload differs from wash load, that part of thesediment load not significantly represented inthe bed (Einstein, 1950, p. 7), because at low

flows bed-material load is stationary or moveson the bed; wash load, however, is always insuspension as it is washed through the channel.In the Great Plains rivers bed-material load iscomposed of sand, whereas wash load is com-posed of silt and clay. To avoid confusion theterms bed-material load and wash load will beused in the following discussion.

An opportunity to observe the effect ofchanges in the proportion of wash load and bed-material load along a river is afforded by theSmoky Hill-Kansas River system. In westernKansas the sinuosity of the Smoky Hill River

is abou t 1.2, the per cent silt-clay forming thechannel is about 5 per cent, and width-depthratio is about 85 . Near the junctions of theSaline and Solomon rivers with the Smoky Hillthe sinuosity increases to about 2.5, pe r centsilt-clay is 20, and w idth-depth rati o decreasesto about 10. The Saline and Solomon riversintroduce large amounts of fine sediments orwash load into the Smoky Hill River. Below thejunction of the Republican River with theSmoky Hill River, sinuosity decreases pro-gressively to 1.1 at Topeka; per cent silt-claydecreases to about 3, and width-depth ratio

increases to about 45. The Republican Riverhas only about 4 per cent silt-clay in its channelabove the junction and carries a large propor-tion of bed-material load into the KansasRiver. These changes in sediment load down-stream along the Smoky Hill-Kansas Riverillustrate the importance of the proportionsof bed-material load and wash load to riversinuosity.

EFFECT OF VALLEY HISTORYON SINUOSITY

In spite of the previous explanation of river

sinuosity as a function of the components oftotal sediment load, another major problemremains. If the shape, gradient, and dimensionsof a channel adjust to discharge and sedimentload, why is the valley gradient too steep forthe discharge and load of sinuous streams?Sinuosity reflects not only the ratio of channellength to valley length bu t also the ratio ofvalley slope to channel slope. The data onsinuosity plotted on Figures 2, 3, and 4 revealthat in some cases valley gradient is 2.5 timesas great as stream gradient, whereas in other

instances valley and stream gradients are almostidentical. If tectonic factors have not modifiedthe slope of a valley, the gradient of the alluvialsurface should be at just that slope required forthe movement of the water-sediment mixture

through the valley. The fact that the surfaceof the alluvium is too steep in many cases re-requires an explanation. That explanation de -pends on an understanding of the changes instream regimen following the Pleistocene.

Most rivers of the Great Plains flow on theupper surface of alluvium which fills valleyscut into bedrock. The deep valleys and theassociated alluvium are a result of the changesin base level, climate, and runoff during andfollowing the Pleistocene. A classic account ofthe formation of such a valley and its filling byalluvium is Fisk's (1944; 1947, p. 19-22)

description of the post-Pleistocene changes inthe Mississippi River valley between Cairo,Illinois, and the Gulf of Mexico. The lowermostpart of the alluvium in this valley is composedof coarse sands and gravels which were trans-ported on the steep slope of the incised river.As sea level rose with the melting of the icesheets, gradient was decreased, and the coarsersediment was deposited. As sea level con tinu edto rise, progressively finer sediment was de-posited, u nti l finally the river was trans porti ngonly the finest fraction of its previous load.According to Fisk (1944) the river was braided

during deposition of the coarser sediments. It isnow meandering over the greater part of itscourse, and its load is predominantly silt, clay,and sand. Probably the same sequence of eventsoccurred in the valleys of the Great Plains, forlogs of wells, which were drilled into thealluvium filling these valleys, show an upwarddecrease in sediment size from gravel and coarsesand at the base ot the deposit to either finesand or silt, clay, and fine sand at the presentvalley surface. Great variability of sedimenttype occurs in these fills, but all availa ble infor-mation shows a decrease in size of the alluvium

toward the surface of the fill (Wenzel andWaite, 1941, p. 19; Williams, 1944, p. 38 andFig. 3; Latta, 1949, PI. 1; Davis and Carlson,1952, p . 229, PI. 3; Leonard, 1952, p. 47, PI. 3 ;Bjorklund and Brown, 1957, p. 190, wellB10-48-1 lac). This relationship i s shown in thegeneralized sketch of Figure 6.

Depending on the geology of a drainagebasin, the load of a river might change fromgravel and sand to sand alone or from silt,clay, sand, and gravel to predomin antly silt andclay during aggradation. Thus in streams drain -


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EFFECT OF VALLEY HISTORY ON SINUOSITY 1097

ing areas underlain by sandstone, the caliberof the load decreased, but the proportion ofbed-material load to wash load probablychanged little. Streams draining areas of sand-stone and shale underwent not only a decreasein the caliber of sediment load but also a re-

duction in the ratio of bed-material load towash load (Fig. 6). As a result the streams drain-ing areas of mixed sediments, after depositionof the coarser fraction of the sediment load,were flowing on alluvium with a gradient inexcess of that required for transport of thepredominantly wash load. A reduction ofgradient by degradation could be only partly

Figure 6. Hypothetical cross section of avalley showing change in shape of streamchannels toward top of alluvium as sedimentbecomes progressively finer. Shape of chan-nel is related to sinuosity (Fig. 2).

effective, for with incision the stream encoun-tered the coarser sediments transported duringa previous regime, and the development of anarmor of coarse sediment could preven t furt herdegradation . The formation of a sinuous courseappears to have been the only alternative. Anincrease in sinuosity would not only decreasethe gradient of a stream, but it would also in-crease the frictional resistance to flow withinthe channel (Dryden and others, 1956, p.482). Streams draining areas of relativelycoarse or sandy sediments were less affectedby the change in caliber of sediment load andcontinued to flow on a gradient which is todayessentially that of the valley itself, because theyhave been tra nspo rting relatively large amounts

of bed-material load throughout their history.An increase in sinuosity and the accompany-ing decrease in stream gradient reflect theneed to dissipate the energy, in excess of thatexpended in friction and sediment transport,that became available as the proportions ofbed load and suspended load changed. Themaintenance of a straight c hannel during valleyaggradation resulted from the need to utilizeall the stream's energy in overcoming frictionalresistance to flow and in the transport of sedi-ment through th e channel.

Undoubtedly during valley alluviation boththe dimensions and shape of the channelschanged in response to a decrease in dischargeand changed sediment-load charac teristics. AsFigure 2 indicates, an increase in sinuositywould be accompanied by a decrease in width-

depth ratio. Channels which are still straightunderwent only a decrease in width and depthas discharge decreased.

If the preceding scheme is correct then recentchanges in the sediment load of rivers mightbe expected to have similar effects. For ex-ample, when aggradation occurs in ephemeralstreams, the coarser sediment is deposited, andthe fines continue downstream. In streamstransporting predominantly sand the down-stream effects are minor, but in streams trans-porting a mixed load of silt-clay, sand, andgravel, the lower reaches of the streams tendto narrow (Schumm, 196lb, p. 47, 63). Thisis probably the first phase of conversion to ameandering stream.

Many modern channels have changed frommeandering to straight. For example, a deple-tion of vegetational cover on hillslopes hascaused an influx of coarse sediment into thechannels of some New Zealand rivers. Theresult is a change from a narrow meanderingchannel to a wide straight one (Grant, 1950).It is not improbable that an improvement invegetative cover would decrease the proportionof bed-material load, which in turn wouldcause a reversion of the channel to its formercondition.

Another example of the conversion of ameandering river to a straight course is fu r-nished by the Cimarron River in SouthwesternKansas. Prior to 1914 the Cimmaron in Kansasflowed in a narrow, deep, m eandering channe l,but during and following a major flood in 1914,the valley was gutted , and the unde rlyingsands were exposed. The banks collapsed, anda wide, sandy, essentially straight channel wasformed (McLaughlin, 1947). The type of sedi-

ment load transported through this cha nne lhas undoubtedly changed greatly as the chan nelchanged. It seems probable that no t muchbed-material load could have moved throughthe pre-1914 narrow, meandering channel.

Apparent exceptions to the conc lusions givenheretofore are plentiful. For example, streamsdraining the mou ntain meadows of South Park,Colorado, are meandering, yet their beds arecomposed of cobbles. A possible explan ation isthat the coarse sediment acts as an armor overwhich the stream meanders. Under the present


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regimen these coarse sediments are not moved;therefore, the predominant sediment load istransported in suspension. In addition, it ap-pears probable that, although a stream istransporting fine sediments, the valley gradientmay be so gentle as to inhibit meandering.Examples of this may be the Mississippi Riverbelow New Orleans (Fisk, 1947) and theIllinois River (Ru bey , 1952). Both rivers flowon surfaces which slope very gently down-stream, and both rivers are essentially straightalthough they transport fine sediments.

Some knowledge of the geologic history of avalley is important for an understanding of themodern river. Indeed, it may be critical in somecases. For example, if the lower course of theIllinois River had not been drowned by therapid deposition in the Mississippi River valleyfollowing the Pleistocene (Rubey, 1952) itsvalley gradient would be steeper, and themodern Illinois River would meander. Modernrivers need to be considered no t solely withrespect to the present regimen of the streambut also with regard to geological perspective,for it appears that the valley gradient may bean independent variable influencing the presentpattern of the alluvial river.

CONCLUSIONSSinaous streams on the Great Plains are

characterized by relatively narrow and deepchannels, a higher percentage of silt-clay

forming the perimeter of the channel, and agentler gradient for the same discharge thanless sinuous streams. The bank m aterials ofsinuous streams are cohesive, bu t this propertyis not restricted to the most winding rivers.Mean annual discharge does not influence thesinuosity of these rivers.

The sinuosity of stable alluvial streams,which transport predominantly sand, silt, andclay, appears to be determined by the propor-tions of w ash load and bed-material load trans-ported. A relatively wide and shallow channelis associated with the movement of a highproportion of bed-material load, whereas anarrow deep channel is associated with thetransport of a sediment load composed pre-dominantly of fine material, wash load.

The flowing of sinuous and straight streamson valley fills of the same gradient may beexplained by the change in caliber of the sedi-ment load and in the ratio of bed-material loadto wash load transported by these streams dur -ing post-Pleistocene valley filling. Recentchanges of stream sinuosity and shape can beexplained by a change in the type of sedimentintroduced into the channel. Th e beg inning ofmeandering in the m ature stage of the Davisiancycle of erosion may also be attributed to achange in the ratio of bed-material load towash load transported by the streams as therelief of a region is lowered.

REFERENCES CITED

Bjorklund, L. J., and Brown, R. F., 1957, Geology and ground-water resources of the lower South PlatteRiver valley between Hardin, Colorado, and Paxton, Nebraska: U. S. Geol. Survey Water-SupplyPaper 1378, 431 p.

Davis, S. N., and Carlson, W. A., 1952, Geology and ground-wat er resources of the Kansas River valleybetween Lawrence and Topeka, Kansas: Kansas Geol. Survey Bull. 96, p. 201-276

Dryden, H. L., Murnaghan, F. D., and Bateman, H., 1956, Hydrodynamics (Reprint): New York,Dover Publications, 634 p.

Einstein, H. A., 1950, The bed-load funct ion for sediment transportation in open channel flows: U. S.Dept. Agriculture Tech. Bull. 1026, 70 p.

Einstein, H. A., Anderson, A. G., and Johnson, J. W., 1940, A distinction between bed-load and sus-pended load in natural streams: Am. Geophys. Union Trans., v. 21, p. 628-633

Fisk, H. N., 1944, Geological investigation of the al luvial valley of the Lower Mississippi River: Vicksburg,Miss., Miss. River Comm., 78 p.1947, Fine-grained a l luvial deposits and their effects on Mississippi Rive r activ ity : Vicksburg, Miss.,

Wa t e r w a y s Exp. Sta., 82 p.Grant, A. P., 1950, Soil conservation in New Zealand: New Zealand Inst. Eng. Proc., v. 36, p. 269-301Griffith, W. M., 1927, A theory of silt and scour: Inst. Civil Eng. Proc., v. 223, p. 243-314Hjulstrom, F., 1949, Climatic changes and river patterns: Geografiska Annaler, v. 31, p. 83-89Lane, E . W., 1937, Stable channels in erodible materials: Am. Soc. Civil Eng. Trans., v. 102, p. 123-194

1957, A study of the shape of channels formed by natural streams flowing in erodible material: U. S.Army Corps of Engineers (Omaha), Missouri River Div. Sediment Series No. 9, 106 p.


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Latta, B. F., 1949, Ground-water conditions in the Smoky Hill Valley in Saline, Dickinson and Gearycounties, Kansas: Kansas Geol. Survey Bull. 84, 152 p.

Leonard, A. R., 1952, Geology and ground-water resources of the North Fork Solomon River in Mitchell,Osborne, Smith and Phillips counties, Kansas: Kansas Geol. Survey Bull. 98, 250 p.

Leopold, L. B., and M addock, T., Jr., 1953, The hydraulic geometry of stream channels and some physio-graphic implications: U. S. Geol. Survey Prof. Paper 252, 57 p.

Leopold. L. B., and Wolman, M. G., 1957, River channel patterns: braided, meandering, and straight:U. S. Geol. Survey Prof. Paper 282-B1960, River meanders: Geol. Soc. America Bull., v. 71, p. 769-794

Mackin, J . H., 1948, Concept of the graded river: Geol. Soc. America Bull., v. 59, p. 463-5121956, Cause of braiding by a graded river (Abstract): Geol. Soc. America Bull., v. 67, p. 1717-1718

McLaughlin, T. G., 1947, Accelerated channel erosion in the Cimarron valley of southwestern Kansas:Jour. Geology, v. 55, p. 76-93

Rubey, W . W., 1952, Geology and mineral resources of the Hardin and Brussels quadrangles (in Illinois):U. S. Geol. Survey Prof. Paper 218, 179 p.

Schumm, S. A., 1960, The shape of alluvial channels in relation to sediment type: U. S. Geol. SurveyProf. Paper 352-B, p. 17-30

1961a, The effects of sediment type on the shape and stratification of some modern fluvial depositsAreply: Am. Jour. Sci., v. 259, p. 234-239

1961b, Effect of sediment characteristics on erosion and deposition in ephemeral-stream channels:U. S. Geol. Survey Prof. Paper 352-C, p. 31-70

1962, Dimensions of some stable a l luvial channels, p. B26-B27 n Geological Survey Research, 1961:U. S. Geol. Survey Prof. Paper 424-B, 344 p.

Searcy, J. K., 1960, Graphical correlation of gaging-station records: U. S. Geol. Survey Water-SupplyPaper 1541-C, p. 67-100

Shulits, S., 1959, Stability of talweg in natural channels (Abstract): Geol. Soc. America Bul l . , v. 70,p. 1675

Sundborg, A., 1956, The River Klaralven, a study of fluvial processes: Geografiska Annaler, v. 38, p.127-316

Wenzel, L . K., and Waite, H. A., 1941, Ground water in Keith County, Nebraska: U. S. Geol. SurveyWater-Supply Paper 848, 68 p.

Williams, C. C., 1944, Ground-water conditions in the Neosho River valley in the vicinity of Parsons,Kansas: Kansas Geol. Survey Bull. 52, p. 29-80

MANUSCRIPT RECEIVED BY THE SOCIETY, JULY 31, 1962PUBLICATION AUTHORIZED BY THE DIRECTOR, U. S. GEOLOGICAL SURVEY

Schumm_1963_Sinuosity of Alluvial Rivers on the Great Plains

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