Reviews and syntheses: Dams, water quality and tropical ......Many aquatic insects are highly...

Biogeosciences 16 1657ndash1671 2019httpsdoiorg105194bg-16-1657-2019copy Author(s) 2019 This work is distributed underthe Creative Commons Attribution 40 License

Reviews and syntheses Dams water quality and tropicalreservoir stratificationRobert Scott Winton12 Elisa Calamita12 and Bernhard Wehrli12

1Institute of Biogeochemistry and Pollutant Dynamics ETH Zurich 8092 Zurich Switzerland2Eawag Swiss Federal Institute of Aquatic Science and Technology 6047 Kastanienbaum Switzerland

Correspondence Robert Scott Winton (robertwintonusysethzch)

Received 13 December 2018 ndash Discussion started 2 January 2019Revised 25 March 2019 ndash Accepted 3 April 2019 ndash Published 23 April 2019

Abstract The impact of large dams is a popular topic inenvironmental science but the importance of altered waterquality as a driver of ecological impacts is often missingfrom such discussions This is partly because informationon the relationship between dams and water quality is rel-atively sparse and fragmentary especially for low-latitudedeveloping countries where dam building is now concen-trated In this paper we review and synthesize informationon the effects of damming on water quality with a specialfocus on low latitudes We find that two ultimate physicalprocesses drive most water quality changes the trapping ofsediments and nutrients and thermal stratification in reser-voirs Since stratification emerges as an important driver andthere is ambiguity in the literature regarding the stratificationbehavior of water bodies in the tropics we synthesize dataand literature on the 54 largest low-latitude reservoirs to as-sess their mixing behavior using three classification schemesDirect observations from literature as well as classificationsbased on climate andor morphometry suggest that mostif not all low-latitude reservoirs will stratify on at least aseasonal basis This finding suggests that low-latitude damshave the potential to discharge cooler anoxic deep waterwhich can degrade downstream ecosystems by altering ther-mal regimes or causing hypoxic stress Many of these reser-voirs are also capable of efficient trapping of sediments andbed load transforming or destroying downstream ecosys-tems such as floodplains and deltas Water quality impactsimposed by stratification and sediment trapping can be mit-igated through a variety of approaches but implementationoften meets physical or financial constraints The impendingconstruction of thousands of planned low-latitude dams willalter water quality throughout tropical and subtropical rivers

These changes and associated environmental impacts needto be better understood by better baseline data and more so-phisticated predictors of reservoir stratification behavior Im-proved environmental impact assessments and dam designshave the potential to mitigate both existing and future poten-tial impacts

1 Introduction

As a global dam construction boom transforms the worldrsquoslow-latitude river systems (Zarfl et al 2014) there is a se-rious concern about how competing demands for water en-ergy and food resources will unfold The challenge createdby dams is not merely that they can limit the availability ofwater to downstream peoples and ecosystems but also thatthe physical and chemical quality of any released water is of-ten altered drastically (Friedl and Wuumlest 2002 Kunz et al2011) Access to sufficient quality of water is a United Na-tions Environment Programme sustainable development goal(UNEP 2016) and yet the potential negative effects of damson water quality are rarely emphasized in overviews of im-pacts of dams (Gibson et al 2017)

Dams are often criticized by ecologists and biogeo-chemists for fragmenting habitats (Anderson et al 2018Winemiller et al 2016) disrupting floodplain hydrologic cy-cles (Kingsford and Thomas 2004 Mumba and Thompson2005 Power et al 1996) and for emitting large amounts ofmethane (Delsontro et al 2011) Such impacts act againstthe promise of carbon-neutral hydropower (Deemer et al2016) In contrast scientists have committed less investiga-tive effort to documenting the potential impacts of dams on

Published by Copernicus Publications on behalf of the European Geosciences Union

1658 R S Winton et al Dams water quality and tropical reservoir stratification

water quality In cases where investigators have synthesizedknowledge of water quality impacts (Friedl and Wuumlest 2002Nilsson and Renofalt 2008 Petts 1986) the conclusions areinevitably biased towards midhigh latitudes where the bulkof case studies and mitigation efforts have occurred

While there is much to be learned from more thor-oughly studied high-latitude rivers given the fundamentalrole played by climate in river and lake functioning it is im-portant to consider how the low-latitude reservoirs may be-have differently For example the process of reservoir strat-ification which plays a crucial role in driving downstreamwater quality impacts is governed to a large degree by lo-cal climate Additionally tropical aquatic systems are moreprone to suffer from oxygen depletion because of lower oxy-gen solubility and faster organic matter decomposition athigher temperatures (Lewis 2000) Latitude also plays animportant role in considering ecological or physiological re-sponses to altered water quality Studies focused on cold-water fish species may have little applicability to warmerrivers in the subtropics and tropics

Low-latitude river systems are experiencing a high rateof new impacts from very large dam projects A review ofthe construction history of very large reservoirs of at least10 km3 below plusmn35 latitude reveals that few projects werelaunched between 1987 and 2000 but in the recent decade(2001ndash2011) low-latitude mega-reservoirs have appeared ata rate of one new project per year (Fig 1) Given that ongo-ing and proposed major dam projects are concentrated at lowlatitudes (Zarfl et al 2014) a specific review of water qualityimpacts of dams and the extent to which they are understoodand manageable in tropical biomes is needed

Large dams exert impacts across many dimension butin this review we largely ignore the important but well-covered impacts of altered hydrologic regimes Instead wefocus on water quality while acknowledging that flow andwater quality issues are often inextricable We also disregardthe important issue of habitat fragmentation and the manyacute impacts on ecosystems and local human populationsarising from dam construction activities (ie displacementand habitat loss due to inundation) These important topicshave been recently reviewed elsewhere (eg Winemiller etal 2016 Anderson et al 2018)

In order to understand the severity and ubiquity of wa-ter quality impacts associated with dams it is necessary tounderstand the process of lake stratification which occursbecause density gradients within lake water formed by so-lar heating of the water surface prevent efficient mixing Byisolating deep reservoir water from surface oxygen stratifi-cation facilitates the development of low-oxygen conditionsand a suite of chemical changes that can be passed down-stream To address the outstanding question of whether low-latitude reservoirs are likely to stratify and experience asso-ciated chemical water quality changes we devote a sectionof this study to predicting reservoir stratification This analy-sis includes the largest low-latitude reservoirs and is focused

Figure 1 Construction history of the worldrsquos 54 largest reservoirslocated below plusmn35 latitude Project year of completion data arefrom the International Commission on Large Dams (httpswwwicold-cigbnet last access 1 November 2018) Project start data areapproximate (plusmn1 year) and based on either gray literature sourceor for some more recent dams ie visual inspection of GoogleEarth satellite imagery Grand Ethiopian Renaissance abbreviatedas GER Volume in map legend is in cubic kilometers

on physical and chemical processes within the reservoirs thatmay affect downstream water quality

Finally we review off-the-shelf efforts to manage or mit-igate undesired chemical and ecological effects of dams re-lated to water quality The management of dam operationsto minimize downstream ecological impacts follows the con-cept of environmental flows (eflows) The primary goal ofeflows is to mimic natural hydrologic cycles for downstreamecosystems which are otherwise impaired by conventionaldam-altered hydrology of diminished flood peaks and higherminimum flows Although restoring hydrology is vitally im-portant to ecological functioning it does not necessarilysolve water quality impacts which often require differenttypes of management actions Rather than duplicate the re-cent eflow reconceptualizations (eg Tharme 2003 Richter2009 Olden and Naiman 2010 OrsquoKeeffe 2018) we focusour review on dam management efforts that specifically tar-

Biogeosciences 16 1657ndash1671 2019 wwwbiogeosciencesnet1616572019

R S Winton et al Dams water quality and tropical reservoir stratification 1659

Figure 2 Conceptual summary of the physical and chemical waterquality effects of dams and how they affect aquatic ecology

get water quality which includes both eflow and non-eflowactions

2 Impacts of dams on river water quality

The act of damming and impounding a river imposes a funda-mental physical change upon the river continuum The rivervelocity slows as it approaches the dam wall and the createdreservoir becomes a lacustrine system The physical changeof damming leads to chemical changes within the reservoirwhich alters the physical and chemical water quality whichin turn leads to ecological impacts on downstream rivers andassociated wetlands The best-documented physical chemi-cal and ecological effects of damming on water quality aresummarized in Fig 2 and described in detail in this sec-tion In each subsection we begin with a general overviewand then specifically consider the available evidence for low-latitude systems

21 Stratification-related effects

Stratification ie the separation of reservoir waters into sta-ble layers of differing densities has important consequencesfor river water downstream of dams A key to understandingthe impacts of dams on river water quality is a precise under-standing of the depth of the reservoir thermoclineoxyclinerelative to spillway or turbine intakes At many high-head-storage hydropower dams the turbine intakes are more than10ndash20 m deep to preserve generation capacity even under ex-treme drought conditions For example Kariba Damrsquos in-takes pull water from 20 to 25 m below the typical level ofthe reservoir surface which roughly coincides with the typ-ical depth of the thermocline In more turbid tropical reser-voirs thermocline depth can be much shallower for exam-ple at Murum Reservoir in Indonesia where the epilimnion isjust 4 to 6 m thick Unfortunately the turbine intake depthsare not typically reported in dam databases Furthermore formany reservoirs especially those in the tropics the mixingbehavior and therefore the typical depth of the thermocline

are not well understood Collecting the reservoir depth pro-files necessary to generate this key information may be moredifficult than simply analyzing water chemistry below damsDam tail waters with low oxygen or reduced compoundssuch as hydrogen sulfide or dissolved methane are likely tostem from discharged deep water of a stratified reservoir

211 Changing thermal regimes

Even at low latitudes where seasonal differences are lessthan temperate climates aquatic ecosystems experience wa-ter temperatures that fluctuate according to daily and annualthermal regimes (Olden and Naiman 2010) Hypolimneticreleases of unseasonably cold water represent alterations to anatural regime Although the difference between surface anddeep waters in tropical lakes is typically much less than thoseat higher latitudes (Lewis 1996) the differences are oftenmuch greater than the relatively subtle temperature shifts of3ndash5 C that have been shown to cause serious impacts (Kinget al 1998 Preece and Jones 2002) For example at 17

south of the Equator Lake Kariba seasonally reaches a 6 to7 C difference between surface and deep waters (Magadza2010) The ecological impacts of altered thermal regimeshave been extensively documented across a range of riversystems

Many aquatic insects are highly sensitive to alterations inthermal regime (Eady et al 2013 Ward and Stanford 1982)with specific temperature thresholds required for completionof various life cycle phases (Vannote and Sweeney 1980)Since macroinvertebrates form an important prey base forfish and other larger organisms there will be cascading ef-fects when insect life cycles are disrupted Fish have theirown set of thermal requirements with species often fill-ing specific thermal niches (Coutant 1987) Altered ther-mal regimes can shift species distributions and communitycomposition Development schedules for both fish and in-sects respond to accumulated daily temperatures above orbelow a threshold as well as absolute temperatures (Oldenand Naiman 2010) Fish and insects have both chronic andacute responses to extreme temperatures A systemic meta-analysis of flow regulation on invertebrates and fish popula-tions by Haxton and Findlay (2008) found that hypolimneticreleases tend to reduce abundance of aquatic species regard-less of setting

There exist several case studies from relatively low lat-itudes suggesting that tropical and subtropical rivers aresusceptible to dam-imposed thermal impacts The Murraycod has been severely impacted by cold-water pollutionfrom the Dartmouth Dam in Victoria Australia (Todd etal 2005) and a variety of native fish species were simi-larly impacted by the Keepit Dam in New South Wales Aus-tralia (Preece and Jones 2002) In subtropical China cold-water dam releases have caused fish spawning to be de-layed by several weeks (Zhong and Power 2015) In tropi-cal Brazil Sato et al (2005) tracked disruptions to fish re-

wwwbiogeosciencesnet1616572019 Biogeosciences 16 1657ndash1671 2019


productive success 34 km downstream of the Trecircs MariasDam In tropical South Africa researchers monitoring down-stream temperature-sensitive fish in regulated and unreg-ulated rivers found that warm water flows promoted fishspawning whereas flows of 3 to 5 C cooler hypolimneticwater forced fish to emigrate (King et al 1998)

212 Hypoxia

Stratification tends to lead to the deoxygenation of deepreservoir water because of heterotrophic consumption anda lack of resupply from oxic surface layers When dam in-takes are deeper than the oxycline hypoxic water can bepassed downstream where it is suspected to cause signifi-cant ecological harm Oxygen concentrations below 35 to5 mg Lminus1 typically trigger escape behavior in higher organ-isms whereas only well-adapted organisms survive below2 mg Lminus1 (Spoor 1990) A study of 19 dams in the southeast-ern United States found that 15 routinely released water withless than 5 mg Lminus1 of dissolved oxygen and 7 released waterwith less than 2 mg Lminus1 of dissolved oxygen (Higgins andBrock 1999) Hypoxic releases from these dams often lastedfor months and the hypoxic water was detectable in somecases for dozens of kilometers downstream Below the HumeDam in Australia researchers found that oxygen concentra-tions reached an annual minimum of less than 50 satura-tion (well under 5 mg Lminus1) while other un-impacted refer-ence streams always had 100 oxygen saturation (Walkeret al 1978) Researchers recently observed similar hypoxicconditions below the Bakun Dam in Malaysia with lessthan 5 mg Lminus1 recorded for more than 150 km downstream(Wera et al 2019) Although observations and experimentshave demonstrated the powerful stress that hypoxia exertson many fish species (Coble 1982 Spoor 1990) there existfew well-documented field studies of dam-induced hypoxiadisrupting downstream ecosystems This is partly becauseit can be difficult to distinguish the relative importance ofdissolved oxygen and other correlated chemical and physi-cal parameters (Hill 1968) Hypolimnetic dam releases con-taining low oxygen will necessarily also be colder than sur-face waters and they may contain toxic levels of ammoniaand hydrogen sulfide so it was not clear which factor wasthe main driver for the loss of benthic macroinvertebrate di-versity documented below a dam of the Guadalupe River inTexas (Young et al 1976)

Regardless regulators in the southern US found the threatof hypoxia to be sufficiently serious to mandate that dam tailwaters maintain a minimum dissolved oxygen content of 4to 6 mg Lminus1 depending on temperature (Higgins and Brock1999) These dams in the Tennessee Valley are on the north-ern fringe of the subtropics (sim 35ndash36 N) but are relativelywarm compared with other reservoirs of the United StatesSince oxygen is less soluble in warmer water and gas trans-fer is driven by the difference between equilibrium and actualconcentrations it follows that low-oxygen stretches down-

stream of low-latitude dams will suffer from slower oxygenrecovery

In addition to the direct impact imposed by hypoxic reser-voir water when it is discharged downstream anoxic bottomwaters will also trigger a suite of anaerobic redox processeswithin reservoir sediments that exert additional alterations towater quality Therefore anoxia can also exert indirect chem-ical changes and associated ecological impacts Here we dis-cuss two particularly prevalent processes phosphorus remo-bilization and the generation of soluble reduced compounds

213 Phosphorus remobilization and eutrophication

Phosphorus (P) is an important macronutrient Its scarcityor limited bioavailability to primary producers often limitsproductivity of aquatic systems Conversely the addition ofdissolved P to aquatic ecosystems often stimulates eutroph-ication leading to blooms of algae phytoplankton or float-ing macrophytes on water surfaces (Carpenter et al 1998Smith 2003) Typically eutrophication will occur when Pis imported into a system from some external source but inthe case of lakes and reservoirs internal P loading from sed-iments can also be important Most P in the aquatic envi-ronment is bound to sediment particles where it is relativelyunavailable for uptake by biota but anoxic bottom waters oflakes greatly accelerate internal P loading (Nurnberg 1984)Iron oxide particles are strong absorbers of dissolved butunder anoxic conditions the iron serves as an electron ac-ceptor and is reduced to a soluble ferrous form During ironreduction iron-bound P also becomes soluble and is releasedinto solution where it can build up in hypolimnetic watersWater rich in P is then either discharged through turbinesor mixed with surface waters during periods of destratifica-tion Therefore sudden increases in bioavailable P can stim-ulate algal and other aquatic plant growth in the reservoirepilimnion Theoretically discharging of P-rich deep watercould cause similar blooms in downstream river reaches butwe are not aware of any direct observations of this phe-nomenon Typically nutrient releases in low-latitude con-texts are thought of as beneficial to downstream ecosystemsbecause they would counteract the oligotrophication imposedby the dam through sediment trapping (Kunz et al 2011)

Although dams seem to typically lead to overall reduc-tions in downstream nutrient delivery (see ldquoOligotrophica-tionrdquo Sect 222) the phenomenon of within-reservoir eu-trophication because of internal P loading has been exten-sively documented in lakes and reservoirs worldwide In theabsence of major anthropogenic nutrient inputs the eutrophi-cation is typically ephemeral and is abated after several yearsfollowing reservoir creation A well-known tropical exampleis Lake Kariba the worldrsquos largest reservoir by volume Formany years after flooding a 10 to 15 of the lake surfacewas covered by Kariba weed (Salvinia molesta) a floatingmacrophyte Limnologists attributed these blooms to decom-posing organic matter and also gradual P release from inun-



dated soils exposed to an anoxic hypolimnion (Marshall andJunor 1981)

Indeed some characteristics of tropical lakes seem tomake them especially susceptible to P regeneration from thehypolimnion The great depth to which mixing occurs (of-ten 50 m or more) during destratification a product of themild thermal density gradient between surface and deep wa-ter provides more opportunity to transport deep P back tothe surface (Kilham and Kilham 1990) This has led limnol-ogists to conclude that deep tropical water bodies are moreprone to eutrophication compared with their temperate coun-terparts (Lewis 2000) There is of course variability withintropical lakes Those with larger catchment areas tend toreceive more sediments and nutrients from their inflowingrivers and are also more prone to eutrophication (Straskrabaet al 1993) These findings together suggest that thermallystratified low-latitude reservoirs run a high risk of experi-encing problems of eutrophication because of internal P re-mobilization

214 Reduced compounds

Another ecological stressor imposed by hypoxic reservoirwater is a high concentration of reduced compounds such ashydrogen sulfide (H2S) and reduced iron which limit the ca-pacity of the downstream river to cope with pollutants Suf-ficient dissolved oxygen is not only necessary for the sup-port of most forms of aquatic life but it is also essentialto maintaining oxidative self-purification processes withinrivers (Friedl and Wuumlest 2002 Petts 1986) Reduced com-pounds limit the oxidative capacity of river water by act-ing as a sink for free dissolved oxygen The occurrence ofH2S has been documented in some cases in the tail watersof dams but the co-occurrence of this stressor with low tem-peratures and hypoxia make it difficult to attribute the ex-tent to which it causes direct ecological harm (Young et al1976) Researchers investigating fish mortality below GreensFerry Dam in Arkansas USA found H2S concentrations of01 mg Lminus1 (Grizzle 1981) well above the recognized lethalconcentrations of 0013 to 0045 mg Lminus1 of H2S for fishbased on toxicological studies (Smith et al 1976) Lethalconcentrations of ammonia for fish are 075 to 34 mg Lminus1

of unionized NH3 (Thurston et al 1983) though we are un-aware of specific cases where these thresholds have been sur-passed because of dams At the very least the presence ofreduced compounds at elevated concentrations indicates thatan aquatic system is experiencing severe stresses which ifsustained will be lethal to most macroscopic biota

22 Sediment trapping

Dams are highly efficient at retaining sediments (Donald etal 2015 Garnier et al 2005 Kunz et al 2011) As riversapproach reservoirs the flow velocity slows and loses the po-tential to slide and bounce along sand and gravel while lost

turbulence allows finer sediments to fall out of suspensionBlockage of sediments and coarse material drives two relatedimpact pathways The first is physical stemming from theloss of river sediments and bedload that are critical to main-taining the structure of downstream ecosystems (Kondolf1997) The second is chemical the loss of sediment-boundnutrients causes oligotrophication of downstream ecosys-tems including floodplains and deltas (Van Cappellen andMaavara 2016)

221 Altered habitat

The most proximate impact of sediment starvation is the en-hancement of erosion downstream of dams from outflowscausing channel incision that can degrade within-channelhabitats for macroinvertebrates and fish (Kondolf 1997)Impacts also reach adjacent and distant ecosystems suchas floodplains and deltas which almost universally dependupon rivers to deliver sediments and nutrients to main-tain habitat quality and productivity In addition to sedi-mentnutrient trapping dams also dampen seasonal hydro-logic peaks reducing overbank flooding of downstream riverreaches The combination of these two dam effects leads toa major reduction in the delivery of nutrients to floodplainswhich represents a fundamental disruption of the flood pulseaffecting the ecological functioning of floodplains (Junk etal 1989)

River deltas also rely on sediment delivered by floods anddamming has led to widespread loss of delta habitats (Giosanet al 2014) Sediment delivery to the Mekong Delta has al-ready been halved and could drop to 4 of baseline if allplanned dams for the catchment are constructed (Kondolfet al 2014a) Elsewhere in the tropics dam constructionhas been associated with the loss of mangrove habitat suchas at the Volta estuary in Ghana (Rubin et al 1999) Themorphology of the lower Zambezirsquos floodplains and deltawere dramatically transformed by reduced sediment loadsassociated with the Cahora Bassa megadam in Mozambique(Davies et al 2001) With diminished sediment delivery andenhanced erosion from rising sea levels the future of manycoastal deltas is precarious as most of the worldrsquos mediumand large deltas are not accumulating sediment fast enoughto stay above water over the coming century (Giosan et al2014)

222 Oligotrophication

Although the densely populated and industrialized water-sheds of the world typically suffer from eutrophication dam-induced oligotrophication through sediment and nutrienttrapping can also severely alter the ecological functioning ofrivers and their floodplains deltas and coastal waters Glob-ally 12 to 17 of global river phosphorus load is trappedbehind dams (Maavara et al 2015) but in specific loca-tions trapping efficiency can be greater than 90 such as



at Kariba Dam on the Zambezi River (Kunz et al 2011)and the Aswan Dam on the Nile (Giosan et al 2014) Suchextreme losses of sediments and nutrients can cause seriousacute impacts to downstream ecosystems though examplesare relatively scarce because predam baseline data are notoften available

Most of the best-documented examples of impacts stem-ming from oligotrophication are from temperate catchmentswith important and carefully monitored fisheries For exam-ple damming led to the collapse of a valuable salmon fisheryin Kootenay Lake British Columbia Canada through olig-otrophication (Ashley et al 1997) The fishery was eventu-ally restored through artificial nutrient additions Oligotroph-ication may impose similar ecological impacts in tropicalcontexts such as in southern Brazil where an increase inwater clarity following the closure of the Eng Seacutergio MottaDam (Porto Primavera Dam) was associated with a shift infish communities (Granzotti et al 2018) Impacts have beenperhaps most dramatic in the eastern Mediterranean follow-ing the closure of the Aswan Dam The Lake Nasser reser-voir following its closure in 1969 began capturing the total-ity of the Nilersquos famously sediment-rich floodwaters includ-ing some 130 million tons of sediment that had previouslyreached the Mediterranean Sea In the subsequent years therewas a 95 drop in phytoplankton biomass and an 80 dropin fish landings (Halim 1991) With dams driving rivers to-ward oligotrophy and land-use changes such as deforestationand agricultural intensification causing eutrophication glob-ally most rivers face some significant change to trophic state

223 Elemental ratios

The attention to phosphorus and nitrogen can obscure the im-portance of other nutrients and their ratios The element sili-con (Si) which is also efficiently sequestered within reser-voirs is an essential nutrient for certain types of phyto-plankton The simultaneous eutrophication and damming ofmany watersheds has led to decreases in Si to nitrogen ra-tios which tends to favor nonsiliceous species over diatoms(Turner et al 1998) In the river Danube efficient trap-ping of Si in reservoirs over several decades led to a shiftin Black Sea phytoplankton communities (Humborg et al1997) coinciding with a crash in an important and produc-tive fishery (Tolmazin 1985) Turner et al (1998) documenta similar phenomenon at lower latitude in the MississippiDelta Decreases in Si loading led to a drop in the abun-dance of copepods and diatoms relative to the total meso-zooplankton population in the Gulf of Mexico (Turner et al1998) These community shifts may have important impli-cations for coastal and estuarine fish communities and theemergence of potential harmful algal blooms

23 Reversibility and propagation of impacts

One way to think of the scope of dam impacts on water qual-ity is in terms of how reversible perturbations to each variablemay be Sediment trapped by a dam may be irreversibly lostfrom a river and even unregulated downstream tributaries areunlikely to compensate Temperature and oxygen impacts ofdams in contrast will happen gradually through exchangewith the atmosphere as the river flows The speed of recov-ery will depend upon river depth surface areas turbulenceand other factors that may provide input to predictive mod-els (Langbein and Durum 1967) Field data from subtropicalAustralia and tropical Malaysia suggest that in practice hy-poxia can extend to dozens or hundreds of kilometers down-stream of dam walls (Walker et al 1978 Wera et al 2019)Where reaeration measures are incorporated into dam opera-tions hypoxia can be mitigated immediately or within a fewkilometers as was the case in Tennessee USA (Higgins andBrock 1999) A study in Colorado USA found that thermaleffects could be detected for hundreds of kilometers down-stream (Holden and Stalnaker 1975) Regardless of the typeof impact it is clear that downstream tributaries play an im-portant role in returning rivers to conditions that are moreldquonaturalrdquo by providing a source of sediment and flow of moreappropriate water quality Water quality impacts of dams aretherefore likely to increase and become less reversible whenchains of dams are built along the same river channel or onmultiple tributaries of a catchment network

3 How prevalent is stratification of low-latitudereservoirs

Because the chemical changes in hypoxia and altered ther-mal regimes both stem from the physical process of reservoirstratification understanding a reservoirrsquos mixing behavior isan important first step toward predicting the likelihood of wa-ter quality impacts Unfortunately there exists ambiguity andmisinformation in the literature about the mixing behavior oflow-latitude reservoirs To resolve the potential confusionwe review literature on the stratification behavior of tropicalwater bodies and then conduct an analysis of stratificationbehavior of the 54 most-voluminous low-latitude reservoirs

31 Stratification in the tropics

For at least one authority on tropical limnology the funda-mental stratification behavior of tropical lakes and reservoirsis clear Lewis (2000) states that ldquoTropical lakes are fun-damentally warm monomictic rdquo with only the shallowestfailing to stratify at least seasonally and that periods of de-stratification are typically predictable events coinciding withcool rainy andor windy seasons Yet there exists confu-sion in the literature For example the World Commission onLarge Damsrsquo technical report states that stratification in low-latitude reservoirs is ldquouncommonrdquo (McCartney et al 2000)



The authors provide no source supporting this statement butthe conclusion likely stems from the 70-year-old landmarklake classification system (Hutchinson and Loffler 1956)which based on very limited field data from equatorial re-gions gives the impression that tropical lakes are predomi-nately either oligomictic (mixing irregularly) or polymictic(mixing many times per year) The idea that low-latitude wa-ter bodies are fundamentally unpredictable or aseasonal aswell as Hutchinson and Lofflerrsquos (1956) approach of classi-fying lakes without morphometric information critical to un-derstanding lake stability (Boehrer and Schultze 2008) hasbeen criticized repeatedly over subsequent decades as addi-tional tropical lake studies have been published (Lewis 19832000 1973 1996) And yet the original misleading clas-sification diagram continues to be faithfully reproduced incontemporary limnology text books (Bengtsson et al 2012Wetzel 2001) Since much of the water quality challengesassociated with damming develop from the thermal andorchemical stratification of reservoirs we take a critical lookat the issue of whether low-latitude reservoirs are likely tostratify predictably for long periods

32 The largest low-latitude reservoirs

To assess the prevalence of prolonged reservoir stratificationperiods that could affect water quality we reviewed and syn-thesized information on the 54 most-voluminous low-latitudereservoirs Through literature searches we found descrip-tions of mixing behavior for 32 of the 54 Authors describednearly all as ldquomonomicticrdquo (having a single well-mixed sea-son punctuated by a season of stratification) One of thesereservoirs was described as meromictic (having a deep layerthat does not typically intermix with surface waters) (Zhanget al 2015) The review indicates that 30 reservoirs stratifyregularly for seasons of several months and thus could ex-perience the associated chemical and ecological water qual-ity issues such as thermal alterations and hypoxia The twoexceptions are Brazilian reservoirs Trecircs Irmatildeos and IlhaSolteira described by Padisak et al (2000) to be ldquomostlypolymiticrdquo (mixing many times per year) On further investi-gation this classification does not appear to be based on di-rect observations but is a rather general statement of regionalreservoir mixing behavior (see Supplement)

We compared our binary stratification classification basedon available literature to the results of applying reservoir datato three existing stratification classification schemes Firstwe consider the classic Hutchinson and Loffler (1956) clas-sification diagram based on altitude and latitude Secondwe plot the data onto a revised classification diagram fortropical lakes proposed by Lewis (2000) based on reservoirmorphometry Finally we apply the concept of densimetricFroude number which can be used to predict reservoir strati-fication behavior (Parker et al 1975) based on morphometryand discharge

The Hutchinson and Loffler (1956) classification is meantto be applied to ldquodeeprdquo lakes and therefore is not useful fordiscriminating between stratifying and nonstratifying reser-voirs based on depth It does suggest that all sufficientlydeep reservoirs (except those above 3500 m altitude) shouldstratify Most of the reservoirs in our data set fall into anldquooligomicticrdquo zone indicating irregular mixing (Fig 3) whenavailable literature suggests that most would be better de-scribed as monomictic with a predictable season of deepermixing This finding reaffirms one of the long-running crit-icisms of this classification scheme its overemphasis onoligomixis (Lewis 1983)

We found that the Lewis (2000) classification system fortropical lakes correctly identified most of the reservoirs in ourdata set as monomictic however six relatively shallow reser-voirs known to exhibit seasonal stratification were misclassi-fied into polymictic categories (Fig 4a) Five of these sixlie within a zone labeled ldquodiscontinuous polymicticrdquo whichrefers to lakes that do not mix on a daily basis but mixdeeply more often than once per year The literature suggeststhat these lakes would be better described as ldquomonomic-ticrdquo We should note that Lewisrsquos (2000) goals in generatingthis diagram were to improve upon the Hutchinson and Lof-fler (1956) diagram for low-latitude regions and to develop aclassification system that could be applied to shallow lakesLewis (2000) does not mathematically define the boundariesof difference and describes them as ldquoapproximaterdquo based onhis expert knowledge so it is not terribly surprising that thereappears to be some misclassifications

In a final stratification assessment we compared theknown mixing behavior from literature to calculations ofdensimetric Froude number (Fr) (see Supplement) for the 35reservoirs for which discharge and surface area data are avail-able (Fig 4b) We calculated Fr using two different values fordepth first using mean depth by dividing reservoir volumeby area and second using dam wall height as a proxy formaximum depth While some authors have suggested that ei-ther value for depth can be used (Ledec and Quintero 2003)our analysis suggests that this choice can have a strong im-pact on Fr calculations and interpretation The ratio betweenmean and maximum depth within our data set ranges from01 to 04 with a mean of 023 This means that all Fr val-ues could be recalculated to be roughly one-quarter of theirvalue based on mean depth This is inconsequential for reser-voirs with small Fr but for those with large Fr it can lead toa shift across the classification thresholds of 03 and 1 Forexample two reservoirs in our data set exceeded the thresh-old of Fr= 1 to indicate nonstratifying behavior when us-ing mean depth but they drop down into the weakly strati-fying category when maximum depth is used instead Nineother reservoirs shift from weakly to strongly stratifying Sowhich value for Fr better reflects reality It is worth consid-ering that reservoirs are typically quite long and limnologistsoften break them down into subbasins separating shallowerarms closer to river inflows from deeper zones close to the



Figure 3 The 54 most-voluminous low-latitude reservoirs overlaid onto a lake classification diagram (redrawn from Hutchinson and Loffler1956)

dam wall Use of maximum depth for Fr calculations prob-ably better reflects stratification behavior at the dam wallwhereas average depth may better indicate behavior in shal-lower subbasins that are less likely to stratify strongly Sincethe deepest part of a reservoir is at the dam wall and becausestratification in this zone is the most relevant to downstreamwater quality it is probably most appropriate to use maxi-mum depth in Fr calculations The two reservoirs describedas polymictic by Tundisi (1990) and Padisak et al (2000) fallinto the intermediate category of weakly stratifying (whenusing mean depth) but three others within this zone are re-ported to exhibit strong stratification (Deus et al 2013 Na-liato et al 2009 Selge and Gunkel 2013) Better candidatesfor nonstratifying members of this reservoir data set are Ya-cyretaacute and Eng Seacutergio Motta but unfortunately we couldfind no description of their mixing behavior in the literatureA field study with depth profiles of these reservoirs coulddispel this ambiguity and determine whether all of the largestlow-latitude reservoirs stratify on a seasonal basis

Overall this exercise of calculating Fr for large low-latitude reservoirs seems to indicate that most if not all arelikely to stratify This is an important realization because itpoints to a significant probability for downstream water qual-ity problems associated with deep-water releases Further-more the bulk of evidence suggests that these reservoirs mixduring a predictable season and not irregularly throughoutthe year or across years indicating that under normal condi-tions these reservoirs should be able to stratify continuouslyfor periods of at least a few months

4 Managing water quality impacts of dams

41 Environmental flows

The most developed and implemented approach (or collec-tion of approaches reviewed by Tharme 2003) for the mit-igation of dam impacts is the environmental flow (eflow)which seeks to adjust dam releases to mimic natural hydro-

logic patterns On a seasonal scale large dams often homoge-nize the downstream discharge regime An eflow approach toreservoir management could implement a simulated flood pe-riod by releasing some reservoir storage waters during the ap-propriate season Although the eflow approach has tradition-ally focused on the mitigation of ecological problems stem-ming from disrupted hydrologic regimes there is a grow-ing realization that water quality (variables such as watertemperature pollutants nutrients organic matter sedimentsdissolved oxygen) must be incorporated into the framework(Olden and Naiman 2010 Rolls et al 2013) There is al-ready some evidence that eflows successfully improve waterquality in practice In the Tennessee valley the incorpora-tion of eflows into dam management improved downstreamdissolved oxygen (DO) and macroinvertebrate richness (Bed-nariacutek et al 2017) Eflows have also been celebrated for pre-venting cyanobacteria blooms that had once plagued an es-tuary in Portugal (Chiacutecharo et al 2006) These examples il-lustrate the potential for eflows to solve some water qualityimpacts created by dams

Unfortunately eflows alone will be insufficient in manycontexts For one flow regulation cannot address the issueof sediment and nutrient trapping without some sort of cou-pling to a sediment flushing strategy Second the issues ofoxygen and thermal pollution often persist under eflow sce-narios when there is reservoir stratification Even if eflowseffectively simulate the natural hydrologic regime there is noreason why this should solve water quality problems as longas the water intake position is below the depth of the reservoirthermocline and residence time is not significantly changedThe solution to hypoxic cold water is not simply more of itbut rather engineers must modify intakes to draw a more de-sirable water source or destratification must be achieved Toaddress the problems of aeration and cold-water pollutiondam managers turn to outflow modification strategies or de-stratification



Figure 4 Reservoir morphometry and stratification behavior(a) Relationships between area and depth for 40 of the 54 worldrsquosmost-voluminous reservoirs located below plusmn35 latitude Data arefrom the International Commission on Large Dams (httpswwwicold-cigbnet last access 1 August 2018) (we excluded 14 reser-voirs because of missing surface area data) Stratification behaviorclassification is synthesized from literature circle symbols indicatethat the reservoir has an extended predictable season of stratifica-tion andor mixes deeply no more than once per year triangle sym-bols refer to two Brazilian reservoirs that authorities suggest arelikely to be polymictic but for which no direct observations ex-ist (see Tundisi 1990 Padisak 2000) for reservoirs indicated bycross symbols no published information on stratification behaviorappears in literature searches Dashed lines and classification la-bels are approximations proposed by Lewis (2000) (b) Reservoirssorted by densimetric Froude number which is a function of reser-voir depth length volume and discharge (Parker et al 1975) Thevertical dashed lines at Fr= 1 and Fr= 03 indicate the expectedboundaries between strongly weakly and nonstratifying reservoirs(Orlob 1983) Small dots represent Froude numbers if maximumdepth (height of dam wall) is used instead of mean depth as sug-gested by Ledec and Quintero 2003 Discharge data is from theGlobal Runoff Data Centre (httpswwwbafgdeGRDC last ac-cess 1 August 2018) five dams were excluded because of missingdischarge data

42 Aeration

Hypoxia of reservoir tail waters is a common problem im-posed by dams As a result various management methodsfor controlling dissolved oxygen content in outflows existranging in cost-effectiveness depending on the characteris-tics of the dam in question (reviewed by Beutel and Horne1999) Options include turbine venting turbine air injectionsurface water pumps oxygen injection and aerating weirsAs the issue of dam-induced hypoxia has been recognizedfor many decades most modern dams incorporate some sortof oxygenation design elements Where rules strictly regulate

dissolved oxygen in the Tennessee Valley United States hy-dropower operators continuously monitor DO in large damoutflows DO levels are managed by hydropower plant per-sonnel specializing in water quality aeration and reservoiroperations (Higgins and Brock 1999) However dams donot always function as designed especially older construc-tions in regions with less regulation and oversight and insuch cases retrofits or adjusted management strategies maybe effective For example Kunz et al (2013) suggest that hy-poxic releases from Zambiarsquos Itezhi-Tezhi Dam (built in the1970s) could be mitigated by releasing a mixture of hypolim-netic and epilimnetic waters This proposed action could helpprotect the valuable fisheries of the downstream Kafue Flatsfloodplain

43 Thermal buffering

The problem of cold-water pollution much like hypoxia isdriven by reservoir stratification and thus can be addressed bysimilar management strategies (Olden and Naiman 2010)Most common multilevel intakes are designed so that out-flows are derived from an appropriate mixture of epi- andhypolimnetic waters to meet a desirable downstream temper-ature threshold (Price and Meyer 1992) A remaining chal-lenge is a lack of understanding of what the thermal require-ments are for a given river system Rivers-Moore et al (2013)propose a method for generating temperature thresholds forSouth African rivers based on time series data from dozensof monitoring stations Unfortunately basic monitoring datafor many regions of the tropics is sparse and quite fragmen-tary which further complicates the establishment of ecologi-cal thermal requirements

44 Sediment manipulation

Sediment trapping by dams is not only a water quality prob-lem as we have discussed but also a challenge for dam man-agement because it causes a loss of reservoir capacity overtime Thus in order to maintain generation capacity manymanagers of hydropower dams implement sediment strate-gies which include the flushing of sediments through spill-ways or sediment bypass systems A recent review of sed-iment management practices at hydropower reservoirs pro-vides a summary of techniques in use and evaluates their ad-vantages and limitations including operations and cost con-siderations as well as ecological impacts (Hauer et al 2018)Sediment management can be implemented at the catchmentscale within the reservoir itself or at the dam wall Sedimentbypass systems are regarded as the most comprehensive solu-tion but may be expensive or infeasible because of reservoirdimensions or cause more ecological harm than they allevi-ate (Graf et al 2016 Sutherland et al 2002)

Typically sediment flushing is practiced in an episodicmanner creating a regime of sediment famine punctuated byintense gluts that are not so much a feast but rather bury



downstream ecosystems alive (Kondolf et al 2014b) Envi-ronmentally optimized sediment flushes show potential forminimizing risks where these are feasible but are rare inpractice A limitation is that not all dams are designed toallow for sediment flushes or reservoir characteristics im-posed by local geomorphology render them impractical Fur-thermore no amount of flushing is able to transport coarserbedload material (ie gravel or larger) downstream To com-pensate for lost bedload and sediments some managers havemade the expensive effort of depositing loose gravel pilesonto river margins so that they can gradually be incorporatedinto the downstream sediment pool as sediment-starved wa-ter inevitably cuts into banks (Kondolf et al 2014b)

An alternative to sediment management at the site of thedam is the restoration of sediment-starved floodplain or deltawetlands but this process is likely to be prohibitively expen-sive in most if not all cases Restoration of drowning Missis-sippi River Delta in Louisiana United States are estimatedto cost USD 05 to 15 billion per year for 50 years (Giosanet al 2014)

5 Further research needs

51 More data from low latitudes

It is telling that in this review focused on low latitudes weoften had to cite case studies from the temperate zone Forexample we were able to locate one study describing eco-logical impacts stemming from dam-driven oligotrophicationat low latitudes (Granzotti et al 2018) The simple fact isthat most of the tropics and subtropics lie far from the mostactive research centers and there has been a correspondinggap in limnological investigations Europe and the UnitedStates have 15 to 4 measurement stations for water qualityper 10 000 km2 of river basin on average Monitoring den-sity is 100 times smaller in Africa (UNEP 2016) Our reviewfound that of the 54 most-voluminous low-latitude reservoirs22 (41 ) have yet to be the subject of a basic limnologi-cal study to classify their mixing behavior Further efforts tomonitor river water quality and study aquatic ecology in reg-ulated low-latitude catchments are needed to elucidate theblind spots that this review has identified

52 Studies of small reservoirs

Compared with larger dams the ecological impact of smallhydropower dam systems have been poorly documented Al-though small dams are likely to have smaller local impactsthan large dams the scaling of impacts is not necessarilyproportional That is social and environmental impacts re-lated to power generation may be greater for small dams thanlarge reservoirs (Fencl et al 2015) Generalizations aboutsmall hydroelectric systems are difficult because they comein so many different forms and designs For example nondi-version run-of-river systems will trap far less sediment than

large dams and those that do not create a deep reservoir arenot subject to stratification-related effects Thus it is tempt-ing to conclude that small-scale hydro will have minimal wa-ter quality impacts but without a systematic assessment it isimpossible to make a fair comparison with large-scale hy-dropower (Premalatha et al 2014) Our analysis is biasedtowards large systems for the practical reason that larger sys-tems are much more likely to be described in databases andstudied by limnologists Future studies on the environmentalimpacts of small hydropower systems would be valuable

53 Better predictions of reservoir stratificationbehavior

Our predictions of reservoir stratification behavior basedupon morphometric and hydrologic data while helpful forunderstanding broad patterns of behavior are not terriblyuseful for understanding water quality impacts of a specificplanned dam It would be much more useful to be able to re-liably predict the depth of the thermocline which could becompared with the depth of water intakes to assess the likeli-hood of discharging hypolimnetic water downstream Exist-ing modeling approaches to predicting mixing behavior fallinto two categories mathematically complex deterministic orprocess-based models and simpler statistical or semiempiri-cal models Deterministic models holistically simulate manyaspects of lake functioning including the capability to pre-dict changes in water quality driven by biogeochemical pro-cesses Researchers have used such tools to quantify im-pacts of reservoirs on downstream ecosystems (Kunz et al2013 Weber et al 2017) but they require a large amountof in situ observational data which is often lacking for low-latitude reservoirs This data dependence also makes themunsuitable for simulating hypothetical reservoirs that are in aplanning stage and thus they cannot inform dam environmen-tal impact statements A promising semiempirical approachwas recently published proposing a lsquogeneralized scalingrsquo forpredicting mixing depth based on lake length water trans-parency and MoninndashObukhov length which is a function ofradiation and wind (Kirillin and Shatwell 2016) This modelwas tuned for and validated against a data set consisting ofmostly temperate zone lakes so it is unclear how well it canbe applied to low-latitude systems If this or another semi-empirical model can be refined to make predictions aboutthe stratification behavior of hypothetical reservoirs beingplanned it could provide valuable information about poten-tial risks of water quality impacts on ecosystems of futuredams

6 Conclusions

We have found that damming threatens the water quality ofriver systems throughout the worldrsquos lower latitudes a factthat is not always recognized in broader critiques of large



dam projects Water quality impacts may propagate for hun-dreds of kilometers downstream of dams and therefore maybe a cryptic source of environmental degradation destroyingecosystem services provided to riparian communities Unfor-tunately a lack of predam data on low-latitude river chem-istry and ecology makes it a challenge to objectively quantifysuch impacts Building the capacity of developing countriesat low latitudes to monitor water quality of their river systemsshould be a priority

Seasonal stratification of low-latitude reservoirs is ubiqui-tous and is expected to occur in essentially any large tropicalreservoir This highlights the risk for low-latitude reservoirsto discharge cooler and anoxic hypolimnetic waters to down-stream rivers depending on the depth of the thermocline rela-tive to turbine intakes Of course in the absence of a random-ized sampling study it is difficult to assess whether the anec-dotes we have identified are outliers or part of a more generalwidespread pattern Further research could investigate howcommon these problems are and assess the geographic or de-sign factors that contribute toward their occurrence

It is difficult to assess which of the water quality impactsare most damaging for two reasons First dams impose manyimpacts simultaneously and it is often difficult to disentanglewhich imposed water quality change is driving an ecologi-cal response or whether multiple stressors are acting syn-ergistically Second to compare the relative importance ofimpacts requires a calculation of value which as we havelearned from the field of ecological economics (Costanza etal 1997) will inevitably be controversial It does appear thatwater quality effects which can render river reaches unin-habitable because of anoxia and contribute to loss of flood-plain and delta wetlands through sediment trapping exert agreater environmental impact than dam disruption to connec-tivity which only directly affects migratory species

The mitigation of water quality impacts imposed by damshas been successful in places but its implementation is de-pendent on environmental regulation and associated fundingmechanisms both of which are often limited in low-latitudesettings Environmental impact assessments and follow-upmonitoring should be required for all large dams The fea-sibility of management actions depends upon the dam de-sign and local geomorphology Thus solutions are typicallycustom-tailored to the context of a specific dam We expectthat as the dam boom progresses simultaneous competingwater uses will exacerbate the degradation of water qualityin low-latitude river systems Further limnological studies ofdata-poor regions combined with the development and vali-dations of water quality models will greatly increase our ca-pacity to identify and mitigate this looming water resourcechallenge

Data availability All data used to produce Figs 1 3and 4 are available in the ETH Zurich Research Collection(httpsdoiorg103929ethz-b-000310656 Winton et al 2018)

The data on reservoir size and morphometry are available in theWorld Register of Dams database maintained by the InternationalCommission on Large Dams which can be accessed (for a fee) athttpswwwicold-cigborg (International Commission on LargeDams 2018) The data on discharge is available in the GlobalRunoff Data Centre 56068 Koblenz Germany accessible athttpswwwbafgdeGRDC (Bundesantalt fuumlr Gewaumlsserkunde2018)

Supplement The supplement related to this article is availableonline at httpsdoiorg105194bg-16-1657-2019-supplement

Author contributions RSW BW and EC developed the paper con-cept RSW and EC extracted the data for analysis BW providedmentoring and oversight RSW produced the figures RSW wrotethe original draft All authors provided critical review and revisions

Competing interests The authors declare that they have no conflictof interest

Acknowledgements This work is supported by the Decision An-alytic Framework to explore the water-energy-food Nexus in com-plex transboundary water resource systems of fast developing coun-tries (DAFNE) project which has received funding from the Eu-ropean Unionrsquos Horizon 2020 research and innovation programmeunder grant agreement no 690268 The authors thank Luzia Fuchsfor providing graphical support for the creation of Fig 3 Marie-Sophie Maier provided helpful feedback on figure aestheticsComments from two anonymous reviewers greatly improved themanuscript

Review statement This paper was edited by Tom J Battin and re-viewed by two anonymous referees

References

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Bednariacutek A Blaser M Matoušu A Hekera P andRuliacutek M Effect of weir impoundments on methane dy-namics in a river Sci Total Environ 584ndash585 164ndash174httpsdoiorg101016jscitotenv201701163 2017

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Beutel M W and Horne A J A review of the ef-fects of hypolimnetic oxygenation on lake and reser-voir water quality Lake Reserv Manage 15 285ndash297httpsdoiorg10108007438149909354124 1999

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Carpenter S R Caraco N F Correll D L HowarthR W Sharpley A N and Smith V H Nonpointpollution of surface waters with phosphorus and nitro-gen Ecol Appl 8 559ndash568 httpsdoiorg1018901051-0761(1998)008[0559NPOSWW]20CO2 1998

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Rivers-Moore N A Dallas H F and Morris C To-wards setting environmental water temperature guidelines ASouth African example J Environ Manage 128 380ndash392httpsdoiorg101016jjenvman201304059 2013

Rolls R J Growns I O Khan T A Wilson G G Elli-son T L Prior A and Waring C C Fish recruitment inrivers with modified discharge depends on the interacting ef-fects of flow and thermal regimes Freshw Biol 58 1804ndash1819httpsdoiorg101111fwb12169 2013

Rubin J A Gordon C and Amatekpor J K Causes andconsequences of mangrove deforestation in the Volta estuaryGhana Some recommendations for ecosystem rehabilitationMar Pollut Bull 37 441ndash449 httpsdoiorg101016S0025-326X(99)00073-9 1999

Sato Y Bazzoli N Rizzo E Boschi M B and MirandaM O T Influence of the Abaeteacute River on the reproductivesuccess of the neotropical migratory teleost Prochilodus argen-teus in the Satildeo Francisco River downstream from the TrecircsMarias Dam southeastern Brazil River Res Appl 21 939ndash950httpsdoiorg101002rra859 2005

Selge F and Gunkel G Water reservoirs worldwide distribu-tion morphometric characteristics and thermal stratification pro-cesses in Sustainable Management of Water and Land in Semi-arid Areas edited by Gunkel G Aleixo da Silva J A andSobral M d C 15ndash27 2013

Smith L L Oseid D M Kimball G L and El-KandelgyS M Toxicity of Hydrogen Sulfide to Various Life His-tory Stages of Bluegill (Lepomis macrochirus) T Am

Fish Soc 105 442ndash449 httpsdoiorg1015771548-8659(1976)105lt442TOHSTVgt20CO2 1976

Smith V H Eutrophication of freshwater and coastal marineecosystems a global problem Environ Sci Pollut Res 10 126ndash139 httpsdoiorg101065espr200212142 2003

Spoor W A Distribution of fingerling brook trout Salvelinusfontinalis (Mitchill) in dissolved oxygen concentration gradi-ents J Fish Biol 36 363ndash373 1990

Straskraba M Tundisi J G and Duncan A Comparativereservoir limnology and water quality management Springer-Science+Business Media 1993

Sutherland A B Meyer J L and Gardiner E P Effects of landcover on sediment regime and fish assemblage structure in foursouthern Appalachian streams Freshw Biol 47 1791ndash1805httpsdoiorg101046j1365-2427200200927x 2002

Tharme R E A global perspective on environmental flow assess-ment Emerging trends in the development and application of en-vironmental flow methodologies for rivers River Res Appl 19397ndash441 httpsdoiorg101002rra736 2003

Thurston R V Russo R C and Phillips G R AcuteToxicity of Ammonia to Fathead Minnows T AmFish Soc 112 705ndash711 httpsdoiorg1015771548-8659(1983)112lt705ATOATFgt20CO2 1983

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Abstract

Introduction

Impacts of dams on river water quality

Stratification-related effects

Changing thermal regimes

Hypoxia

Phosphorus remobilization and eutrophication

Reduced compounds

Sediment trapping

Altered habitat

Oligotrophication

Elemental ratios

Reversibility and propagation of impacts

How prevalent is stratification of low-latitude reservoirs

Stratification in the tropics

The largest low-latitude reservoirs

Managing water quality impacts of dams

Environmental flows

Aeration

Thermal buffering

Sediment manipulation

Further research needs

More data from low latitudes

Studies of small reservoirs

Better predictions of reservoir stratification behavior

Conclusions

Data availability

Supplement

Author contributions

Competing interests

Acknowledgements

Review statement

References


water quality In cases where investigators have synthesizedknowledge of water quality impacts (Friedl and Wuumlest 2002Nilsson and Renofalt 2008 Petts 1986) the conclusions areinevitably biased towards midhigh latitudes where the bulkof case studies and mitigation efforts have occurred

While there is much to be learned from more thor-oughly studied high-latitude rivers given the fundamentalrole played by climate in river and lake functioning it is im-portant to consider how the low-latitude reservoirs may be-have differently For example the process of reservoir strat-ification which plays a crucial role in driving downstreamwater quality impacts is governed to a large degree by lo-cal climate Additionally tropical aquatic systems are moreprone to suffer from oxygen depletion because of lower oxy-gen solubility and faster organic matter decomposition athigher temperatures (Lewis 2000) Latitude also plays animportant role in considering ecological or physiological re-sponses to altered water quality Studies focused on cold-water fish species may have little applicability to warmerrivers in the subtropics and tropics

Low-latitude river systems are experiencing a high rateof new impacts from very large dam projects A review ofthe construction history of very large reservoirs of at least10 km3 below plusmn35 latitude reveals that few projects werelaunched between 1987 and 2000 but in the recent decade(2001ndash2011) low-latitude mega-reservoirs have appeared ata rate of one new project per year (Fig 1) Given that ongo-ing and proposed major dam projects are concentrated at lowlatitudes (Zarfl et al 2014) a specific review of water qualityimpacts of dams and the extent to which they are understoodand manageable in tropical biomes is needed

Large dams exert impacts across many dimension butin this review we largely ignore the important but well-covered impacts of altered hydrologic regimes Instead wefocus on water quality while acknowledging that flow andwater quality issues are often inextricable We also disregardthe important issue of habitat fragmentation and the manyacute impacts on ecosystems and local human populationsarising from dam construction activities (ie displacementand habitat loss due to inundation) These important topicshave been recently reviewed elsewhere (eg Winemiller etal 2016 Anderson et al 2018)

In order to understand the severity and ubiquity of wa-ter quality impacts associated with dams it is necessary tounderstand the process of lake stratification which occursbecause density gradients within lake water formed by so-lar heating of the water surface prevent efficient mixing Byisolating deep reservoir water from surface oxygen stratifi-cation facilitates the development of low-oxygen conditionsand a suite of chemical changes that can be passed down-stream To address the outstanding question of whether low-latitude reservoirs are likely to stratify and experience asso-ciated chemical water quality changes we devote a sectionof this study to predicting reservoir stratification This analy-sis includes the largest low-latitude reservoirs and is focused

Figure 1 Construction history of the worldrsquos 54 largest reservoirslocated below plusmn35 latitude Project year of completion data arefrom the International Commission on Large Dams (httpswwwicold-cigbnet last access 1 November 2018) Project start data areapproximate (plusmn1 year) and based on either gray literature sourceor for some more recent dams ie visual inspection of GoogleEarth satellite imagery Grand Ethiopian Renaissance abbreviatedas GER Volume in map legend is in cubic kilometers

on physical and chemical processes within the reservoirs thatmay affect downstream water quality

Finally we review off-the-shelf efforts to manage or mit-igate undesired chemical and ecological effects of dams re-lated to water quality The management of dam operationsto minimize downstream ecological impacts follows the con-cept of environmental flows (eflows) The primary goal ofeflows is to mimic natural hydrologic cycles for downstreamecosystems which are otherwise impaired by conventionaldam-altered hydrology of diminished flood peaks and higherminimum flows Although restoring hydrology is vitally im-portant to ecological functioning it does not necessarilysolve water quality impacts which often require differenttypes of management actions Rather than duplicate the re-cent eflow reconceptualizations (eg Tharme 2003 Richter2009 Olden and Naiman 2010 OrsquoKeeffe 2018) we focusour review on dam management efforts that specifically tar-


















212 Hypoxia


















221 Altered habitat







































42 Aeration




















6 Conclusions















References


















































































































Abstract

Introduction




Hypoxia


Reduced compounds

Sediment trapping

Altered habitat

Oligotrophication

Elemental ratios






Environmental flows

Aeration

Thermal buffering






Conclusions

Data availability

Supplement


Competing interests

Acknowledgements

Review statement

References

















212 Hypoxia


















221 Altered habitat







































42 Aeration




















6 Conclusions















References


















































































































Abstract

Introduction




Hypoxia


Reduced compounds

Sediment trapping

Altered habitat

Oligotrophication

Elemental ratios






Environmental flows

Aeration

Thermal buffering






Conclusions

Data availability

Supplement


Competing interests

Acknowledgements

Review statement

References










221 Altered habitat







































42 Aeration




















6 Conclusions















References


















































































































Abstract

Introduction




Hypoxia


Reduced compounds

Sediment trapping

Altered habitat

Oligotrophication

Elemental ratios






Environmental flows

Aeration

Thermal buffering






Conclusions

Data availability

Supplement


Competing interests

Acknowledgements

Review statement

References


































42 Aeration




















6 Conclusions















References


















































































































Abstract

Introduction




Hypoxia


Reduced compounds

Sediment trapping

Altered habitat

Oligotrophication

Elemental ratios






Environmental flows

Aeration

Thermal buffering






Conclusions

Data availability

Supplement


Competing interests

Acknowledgements

Review statement

References












6 Conclusions















References


















































































































Abstract

Introduction




Hypoxia


Reduced compounds

Sediment trapping

Altered habitat

Oligotrophication

Elemental ratios






Environmental flows

Aeration

Thermal buffering






Conclusions

Data availability

Supplement


Competing interests

Acknowledgements

Review statement

References













References


















































































































Abstract

Introduction




Hypoxia


Reduced compounds

Sediment trapping

Altered habitat

Oligotrophication

Elemental ratios






Environmental flows

Aeration

Thermal buffering






Conclusions

Data availability

Supplement


Competing interests

Acknowledgements

Review statement

References













































































































Abstract

Introduction




Hypoxia


Reduced compounds

Sediment trapping

Altered habitat

Oligotrophication

Elemental ratios






Environmental flows

Aeration

Thermal buffering






Conclusions

Data availability

Supplement


Competing interests

Acknowledgements

Review statement

References

Reviews and syntheses: Dams, water quality and tropical ......Many aquatic insects are highly...

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Transcript of Reviews and syntheses: Dams, water quality and tropical ......Many aquatic insects are highly...