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82 | Environme ntal Flows

both surface runoff and subsurface ow (including return ows) of precipitation and

snowmelt which are related to the climate. Humans have no direct control over the

climate but their activities have started affecting the climate and the climate change is now

projected to have signicant effects on the river ow regimes all over the Earth (Arnell et

al. 2001, Döll and Schmied 2012, Schneider et al. 2012). The impacts of climate change onriver ow in India have been examined by Rao (1993, 1995), Jha and Sharma (2008) and

Gosain et al. (2011). The amounts of precipitation entering the rivers is affected by various

human activities in their catchment and the effects of deforestation, plantation, agriculture,

urbanisation, and other land use changes have been investigated through many eld studies

as well as modeling techniques (e.g.; Negi 2002, Bruinzeel 2004, Brauman et al. 2007, Wei

et al. 2008, Cui et al. 2012, Ellison et al. 2012, Liu et al. 2012).

However, the single most important human intervention impacting the ow regimes

of the rivers is the construction of a physical barrier across the river bed with the primary

objective of preventing the ow downstream, partly or fully, so that it can be diverted

or stored for various other uses. These physical barriers range from a low weir to gated

barrages and mighty dams. Weirs are constructed to divert ow into a canal, and are usually

low in height so that some water continues to ow over the crest to downstream reaches

except during the very low ow period. Weirs are also constructed on small coastal streams

to prevent tidal ingress (such as along the coast in Gujarat) but the streams are turned from

their estuarine character into shallow freshwater reservoirs. Barrages are similar structures

with adjustable gates are installed over the weir to regulate the water surface at different

levels at different times. All barrages have a considerably large pondage (storage) that may

extend to several hundred meters behind them depending upon the gradient and the height

of the gates. In Maharashtra (India) there are also Kolhapuri Type weirs (KT Bandharas) which are provided with sluice gates right over the leveled river bed and therefore, do not

store any water when the gates are opened. Large dams store water of which a signicant

amount is a dead storage. They are provided with sluice gates at a higher level, usually

nearer the top of the dam, for releasing water when required. Some dams are provided with

a bottom outlet for ushing the sediments and releasing water from deep below the surface.

Generally, the water is abstracted by diversion at higher levels into canals.

Far more numerous dams have been constructed for the hydropower generation than

for other purposes and the discussions on instream ows (environmental ows) have had

their beginning in the impacts of these projects (see chapter 6). In South Asia also, the

hydroelectric power project (HEPs) have been at the centre of the debate on environmentalows. The hydropower plants are of different types: the diversion or run-of-the-river (RoR)

hydropower plants (with low or high heads, storage (or impoundment) hydropower plants

and pumped storage hydropower plants. All along the Himalayan belt, hundreds of the so-

called run-of-river hydropower plants are actually based on large storages that block the

river completely. Chamera I HEP is based on a 140 m high concrete dam on River Ravi.

The river ow from the reservoirs is diverted into head-race tunnels (HRTs) which carry

it to penstocks for delivery to the turbines and the release through tail race tunnels (TRT)

back into the same river (sometimes into another river) many kilometers downstream. The

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Environm ental Flows | 83

reservoirs usually extend upstream to several kilometers, depending upon the height of the

dam and the gradient. The HEPs have often been developed (or are in different stages of

development) in cascades on the same river and its tributaries so close to each other that the

TRT of the upstream project opens just above the tail reach of the reservoir of downstream

project, thereby leaving no space for the river water to ow (Figures 1 to 3).

IMPACTS OF DAMS

The impacts of dams, particularly large dams, have been debated globally for many decades in

the context of the submergence of forest and agricultural lands and settlements, displacement

of people and the issues related to their rehabilitation, and economic development benets

(Goldsmith and Hildyard 1984, McCully 2001, Rangachari 2006). Until recently, the

ecological and environmental aspects, however, have attracted relatively little attention

because these impacts were not felt, experienced or observed immediately or in the short

term. The dams impact areas both upstream and downstream of their siting, and throughall stages of their development – before and during construction as well as through their

operation. The impacts of dams and other diversion structures depend upon the type, size

(storage capacity relative to the river ow; but also area and depth), mode of operation and

geographic location of the reservoir behind them. They invariably alter the ow regime – its

magnitude, duration, frequency, timing and ow velocity. The ow regime in turn affects

the physico-chemical characteristics of the habitat, the dynamics of sediment transport,

the river morphology, the connectivity with the oodplains and the biota. The continuity

and longitudinal connectivity of the river is disrupted. Various ecological/environmental

impacts of the dams have been discussed in several recent publications (McCartney et al.

2001, Robson et al. 2011). Here we examine briey only the impacts related to the alterationof the ow regime. These are divided into upstream and downstream impacts though some

upstream impacts have their inuence on the downstream environment as well. It must also

be appreciated that where a dam is constructed upstream of an existing dam, the reach of

the river below it has already been impacted upon by the existing dam’s upstream impacts.

New dams are also created some distance downstream of the TRT outfalls to utilise the water

released from an existing HEP for further power generation. The river stretch between the

two dams is thus affected by both the upstream impacts and the downstream impacts, and

may sometimes lose its riverine character completely.

Upstream ImpactsThe construction of a dam converts a section of the owing stream into a stagnant water

body. Depending upon the geographic location and height of the dam, the reservoirs vary

in shape (from linear in mountainous areas to dendritic in low gradient areas where several

tributaries are affected), shoreline to surface area ratio, and maximum and mean depths.

Three zones can usually be identied: a riverine zone where the owing conditions still

prevail, a transitional zone in the middle section, and a lacustrine zone closer to the dam

(Figure 4; Rast and Straskraba 2000). The abstraction of water determines the amplitude

and frequency of uctuations in the water level that result in the exposure of the littoral

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F

i g u r e 1 .

E x i s t i n g a n d p r o p o s e d h

y d r o p o w e r p r o j e c t s i n u p p e r r e a c h e s o f R .

G a n g a

( c o u r t e s y S A N D R P ,

N e w D e l h i )

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F i g u r e 3 .

E x i s

t i n g a n d p r o p o s e d h y d r o p o w e r p r o j e c t s i n A r u n a c h a l P r a d e s h ( c o u r t e s y S A N D R P ,

N e w D e l h i )

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and tail reach areas of the reservoir. These changes in the hydrological regimes are of an

extremely high order and magnitude in rivers with high seasonal variation in their ow. The

operation of ‘peaking’ power plants utilises the ow accumulated over a long period of one

or more days for power generation for a couple of hours only. Thus, the upstream habitats

experience a diurnal cycle of deep ooding and exposure (drawdown) instead of the small

ows that occurred in the river channel before the construction of the diversion structures.

Such frequent water level uctuations on a daily basis affect all kinds of organismsas well as their physico-chemical environment. The temperature of water in a reservoir is

usually higher in the winter and lower in the summer than it would be in the river before the

dam. These temperature changes directly affect the aquatic biota and also the microclimate

of the region around the reservoir. The lacustrine section of the reservoir usually becomes

thermally stratied with a deeper layer of water (hypolimnion) with low temperature (see

Uhlman 1982 for south Indian reservoirs) although the currents generated by large water

level uctuations can prevent thermal stratication in smaller and shallow reservoirs.

Another effect of the dam is on the ow velocity and turbulence which come to naught and

Figure 4. Characteristic of three zones in a reservoir(reproduced with permission, from Rast and Strakraba 2000)

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hence, most of the coarser sediments carried by the river are deposited in the reservoirs.

Since the Himalayan rivers are known to carry sediment loads that are highest in the world,

many reservoirs get lled much earlier than expected.

Further, signicant changes occur in the water quality of the reservoirs as a result

of release of nutrients from the sediments, and following death and decomposition of the

vegetation submerged during lling of the reservoir (e.g., Chang and Wen 1998). The increased

availability of nutrients and the still water conditions promote the growth of phytoplankton

(eutrophication). A recent study shows that frequent water level changes caused by the

withdrawal of water for hydropower generation caused replacement of macrophytes in the

littoral zone by green benthic algae which could support high macroinvertebrate biomass

(Thompson and Ryder 2008). The inputs of both allochthonous (brought by the river) and

autochthonous organic matter (by phytoplankton) gradually result in anoxic conditions

in the hypolimnion and consequently further changes in water quality. The altered ow

regimes and increased nutrient levels in the reservoirs are known to be responsible for their

rapid colonisation by exotic invasive plants (such as Salvinia, Pistia and water hyacinth in

many reservoirs of tropical Asia and Africa), sh, molluscs and insects (for references, see

McCartney et al. 2001, McCartnety 2009). The operationally induced water level uctuations

in the reservoirs, have signicant impacts on the littoral and riparian vegetation which may

be completely eliminated or develop with a different species composition (cf. Nilsson and

Jansson 1995).

Creation of the reservoirs has further consequences for the fauna in the affected area.

All terrestrial animals disappear from the submerged areas and their populations decline

with the loss of the habitat. Most impacted are the species dependent upon riverine forests,

and other riparian ecosystems, and those adapted to the fast-owing conditions of the mainriver course (McAllister et al. 1999). The riverine sh are unable to adapt to the lacustrine

conditions and may survive for some time close to the shores of the reservoirs. The overall sh

diversity and populations soon decline and are often replaced by exotic species. Hoeinghaus

et al. (2009) report that the shery of the Parana River (Brazil) changed drastically after

the formation of Itaipu Reservoir as the high-value migratory species were replaced by

reservoir-adapted introduced species. The migratory sh species are the most affected as

they are unable to reach their spawning, nursery and feeding habitats. Dams or barrages in

the downstream reaches prevent migration of the sh from the sea or estuary to freshwater

habitats upstream (Marmulla 2001). The inability of the migratory species to move upstream

does not affect them alone but has some serious consequences for other biota as well througha variety of species interactions. Greathouse et al. 2006) observed that the loss of migratory

freshwater shrimp and sh populations in Puerto Rico (USA) from high-gradient streams

above large dams dramatically increased the populations of epilithic algal biomass, ne

benthic organic matter, ne benthic inorganic matter and non-decapod invertebrate biomass

which constituted their basal food resources and invertebrate competitors and prey. The

effects were however much smaller in case of low gradient streams.

One of the consequences of deposition of sediments behind dams and barrages is

the development of shallow water or waterlogged habitats which are quickly colonised by

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various macrophytes, support macroinvertebrates and turn into suitable habitats for a wide

range of avian fauna besides other wildlife. Some of these human-created biodiversity-

rich habitats are sometimes utilised by important fauna, particularly migratory waterfowl,

and are therefore, recognised for their conservation value. A few are even designated as

nature reserves, wildlife sanctuaries or internationally important wetlands. In India, Pongdam, Harike, Ropar and Kanjli are among the Ramsar sites. Even Loktak lake – another

Ramsar site is a former oxbow transformed into a reservoir for hydropower generation by

constructing a barrage. Asan Conservation Reserve near Dehradun, Jayakwadi and Ujni

wildlife sanctuaries in Maharashtra are also similar wetland habitats that developed with

time behind the dams or barrages. It is not readily realised, however, that these newly

developed habitats harbour also a variety of disease vectors such as bilharzia-carrying

snails, mosquitoes and intermediate hosts for ukes that do not survive in running waters.

Downstream Impacts

Downstream of the dams the ow regimes change drastically. Most of the large dams andhydropower projects do not release water other than the small amounts of seepage or leakage

from the diversion structure and gates. Depending upon their storage capacity relative to the

river’s discharge, excess ow is released through the sluices provided at different levels. Even

in case of barrages, the river downstream may receive no ow during the low ow season

because of high demand of water for irrigation and domestic uses during the dry period (also

hot in the monsoonal south Asian region). The ow can be expected to be near normal ow

in the rainy season if there is no storage upstream. Thus, in general, the downstream ow

regime is altered in magnitude (volume), frequency, duration, timing and the amplitude

(difference between low ows and peak ows). The ecosystem responses to the alterations in

ow characteristics and consequent changes in landscape interactions, along with suggestedmitigation measures, are reviewed by Renöfält (2010) and a summary is provided in Table 1.

Each upstream project affects the hydrological regime of the downstream area and thereby

the next project below it (Figure 5). The velocity of ow is also regulated by the process of

release from the diversion structures. The annual river ow downstream decline heavily

(Goes 2002) with large decrease in the peak ows though the dry season ows are increased

steeply (Beilfuss and dos Santos 2001). Flow regulation results in lower ood magnitudes

and but longer stream ow duration (Elliott and Hammack 2000).

The hydropower projects often claim that because the ows diverted through the

tunnels to the powerhouse is released back into the river, there is no change in the ow. A

fairly long stretch of the river between the dam and the TRT outfall either remains dry orexperiences a highly altered ow regime if any signicant tributary joins it. After the TRT

discharges water into the river, very signicant and important hydrological changes occur

in the downstream reaches, especially in case of projects with peaking power generation.

The rate and velocity of discharge from the TRT are governed by operational factors (Figure

6). The ow regime downstream of the TRT outfall is then altered to daily pulses of very

large volumes for a short duration followed by a long period of no additional ow. Under

natural conditions the river would experience continuous small ows with very only small

diurnal changes except when the ow is derived solely from the snow melt. Only smaller

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Table 1. Impacts of hydropower projects on ows, ecosystem responses to these impacts

and management options (modied from Renöfält et al. 2010)

Flow

attribute

Change in ow Ecosystem response Management option

Flow ⁄ Hydrology

Magnitude Increase in variability

Organisms and organic matterushed down

Reduce range of ow variation

Flow stabilised Competitive species becomedominant

Productivity and decompositionrate decrease

Exotics invade and increaseRiparian species fail to establish

Increase seasonal variationin ow

Periodically ush thechannels to remove exotics

Frequency Increase in variability Increased erosion and stress to biota

Reduced habitat availability

Reduce frequency of ow variation

Decrease in variability

Flushing of sediments decreases

Competitive species becomedominant

Increase seasonal variationin ow

Periodically ush thechannels to remove exotics

Timing Change ofseasonal ow

variables

Reduced habitat availability

Life cycles disrupted, growthdeclines and succession patternaltered

Exotics invade

Provide ow for seasonal owpeaks and higher minimum

ows

Remove exotics suitably

Duration Prolonged lowows

Changes in abundance anddiversity

Organisms experiencephysiological stress

Increase seasonal high ows

Prolongedinundation

Changes in riparian communities Reduce high ows

Shortened low

ows

Greater availability of aquatic

habitats

Increase seasonal low ows

Smaller oodpeaks

Terrestrial organisms startcolonising

Increase duration of seasonalood peaks

Rate ofChange

Rapid changein river level

Organisms washed out andstranded

River banks collapse byundercutting

Establishment of riparian biotaaffected

Reduce rates of change

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snow-fed streams at high altitudes experience ash ood like ows for 2-4 hours in the

afternoon and no ow during the night. It should be pointed out that hydro-peaking can

have severe ecological effects on a river. Depending on the rate of discharge acceleration

benthic invertebrates and also juvenile and small sh can get washed away with the ush, which results in decimation of benthic fauna, reduction of sh biomass and also changes to

the structure of sh populations. During the down-surge benthic invertebrates and sh can

get trapped in pools that might dry out later on so the animals either die or become easy prey

for predators (ICPDR 2013).

Unusually high volumes of water may sometimes be released for navigational and

recreational purposes, thereby affecting the river stretch between the dam/barrage and the

point of interest. Similar changes in ow regimes occur also due to managed ow releases

for other desired benets downstream.

Landscape Interactions

Velocity Flowing waterhabitatsconverted into

standing waterhabitats

Declines of lotic species andincrease of lentic species

Remove dams

Longitudinal Longitudinalconnectivityinterrupted bydams, barrages,etc.

Communities fragmented due toreduced migration ⁄ dispersal

Sediment redistribution decreases

Nutrients and organic matterretained in reservoirs

Remove dams

Facilitate migration acrossdams

Restore riparian and aquatichabitats

Flush out the sediment fromreservoirs

Lateral Floodplainsdisconnected

Reduced colonisation andrecruitment of riparian organisms

Decline in number of species

Ecological functions such asprimary production decline

Facilitate connection betweenriparian/oodplain areas andthe river channel throughoverbank ooding

Suitably modify ripariantopography/slope to enhanceriparian vegetation

Vertical Ground waterdisconnectedfrom Surface

water

Decline in number of species

Reproductive failure in sh

Decline in river water quality(changes in dissolved oxygen andtemperature)

Improve exchange betweensurface and hyporheichabitats

Remove ne sediments fromthe gravel beds

Temporal ReducedHeterogeneity New habitats cannot be formed(e.g., by sediment redistribution)

Non-indigenous species increase inplace of native species

Declne in habitat diversity andhence, lower beta diversity

Enhance habitat diversity bymodifying the ow regime

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Figure 5. Effect of an upstream HEP on the ow regime (ow duration curve) of the downstreamproject in Himachal Pradesh. The data are for years 1986, 1994 and 2006

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These changes in the ow regime have a direct impact upon the channel morphology,

water quality, composition of biotic communities and various ecosystem processes (see

Hildebrand 1980a,b, Loar and Sale 1981, Turbak et al. 1981). In general, the aquatic habitats

shrink to small area within the channel, the riparian areas remain and oodplain-channel

interactions are reduced signicantly with a change the extent, depth, duration and timing

of ooding. Various impacts of hydropower projects are shown in Figure 7. Cortes et al.

(2011) provide a comprehensive review of the impacts of hydropower dams on the rivers in

general and sh in particular.

Impacts on Sediments

The dams effectively reduce the sediments transported downstream and the impacts of the

reduced sediment loads extend far beyond those of the river channel morphology, oodplain

dynamics and fertility and river biota to the deltaic zone. The impacts of ow regulation on

the deltas of Rivers Nile, Ganga-Brahmaputra, Mekong and Yangtze are among the well

known examples (Biswas 1992, Bohannon 2010, Yang et al. 2005, Xu and Milliman 2009,

Xue et al. 2011, Gupta et al. 2012).

Figure 6. Daily discharge (m3 s-1) for an average year before (a) and after (b) regulation(reproduced with permission, from Cortes et al. 2002)

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The nature and amount of sediments received downstream changes drastically as

the coarser sediments are retained behind the diversion structures. However, if and when

the coarser sediments are ushed out periodically during maintenance or during the

ood season, their dispersion over the river bed and transport downstream creates manyproblems for the channel morphology and the biota. In the absence of volume or velocity of

ow, ner sediments settle down in the river bed, and smother the vegetation, periphyton,

benthicalgae and macroinvertebrates, and affect also the feeding and growth of other

organisms. Flushing by periodic high ows results in greater erosion. Downstream of a dam,

reduction in sediment load in rivers can also result in increased erosion of riverbanks and

beds, loss of oodplains (through erosion and lower accretion) and degradation of coastal

deltas. Aggradation of the river bed may however occur if the sediment are entrained into

the river from the oodplain and tributaries but are unable to move downstream. Further

changes in the stream bed occur due to gravel and sand mining facilitated by easy access to

the river channel. Increased agricultural activity on the oodplain, often extending right onto the dry river bed, durther disturbs the sediments, lls up small water bodies and pools,

and promotes meandering and shifting of the river channel in successive years (see Raman

and Gopal 2007, 2009).

Impacts on Water Temperature

The temperature regime of the river water downstream is altered in two ways. First, in the

absence of ow from upstream, the temperature of the remaining water in the channel rises

during the summer and drops down during the winter. The extent of these changes depends

Figure 7. Impact of hydropower dams on important habitat and biotic components of riversFew other impacts on upstream systems and oodplains are not shown.

(adapted from Vovk Korz 2008)

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also upon the geographical location and the stream bed characteristics (rocky, gravelly,

sandy, etc.). Average water temperature below the Danjiangkou dam on the Hanjiang River

(China) are reported to have increased during winter and decreased during the summer

season by 4-6 C (Liu and Yu 1992). This altered temperature regime has an inuence also

on the areas adjacent to the rivers in the absence of its moderating inuence. Second, therelease of water from the reservoirs alters the temperature regime of the downstream areas

signicantly over long stretches, depending upon whether the water is released from the

surface or the deeper layers (for references, see McCartney et al. 2009).

Impacts on Water Quality

All human activities which modiy the river ow affect the water quality (Table 2). The lack of

ow has a direct signicant inuence on the water quality because of the absence of dilution,

reduced oxygen availability and consequently, large reduction in the assimilation capacity of

the organisms. The downstream areas may experience an increase in salinity caused by the

reduced freshwater ows or a decrease in salinity dure to ushing by freshwater releases.Such salinity changes occur especially in the estuarine reaches and coastal regions where

the regulation of freshwater ows alters the interaction with the tidal uxes. Under low ow

conditions water quality degradation is caused by many variables (Nilsson and Renöfält

2008; Table 3). The discharge of wastewater or organic wastes entering the river with the

storm runoff invariably creates complex situations of degraded water quality. The issue

of water quality is among the most intensely debated issues concerning rivers as people

abstract increasingly more water. This is best exemplied by River Yamuna in Delhi where

it has no natural ow but receives only the sewage which is partially treated. Many smaller

rivers throughout India have turned for their entire course into sewage drains. However, it

Table 2. Ecological impacts of direct or indirect modications of the river ow (from Chapman 1996)

Activity/modication Effects

Construction of locks Enhancement of eutrophicationPartial storage of ne sediments may result in anoxic interstitial waters

Damming Enhancement of eutrophication (bottom anoxia, high organic matterin surface waters, etc.)Complete storage of sediments resulting in potential sh kills duringsluice gate operation (high ammonia, BOD and TSS)

Dredging Continuous high levels of TSS, and resultant silting of gravelspawning areas in downstream reachesRegressive erosion upstream of dredging areas may prevent sh migration

Felling of riparian woodland Continuous high levels of TSS, and resultant silting of gravelspawning areas in downstream reachesIncreased nitrate input from contaminated groundwaters

Flood plain reclamation andriver bed channelisation

Loss of ecological diversity, including specic spawning areas Loss of biological habitats, especially for sh

BOD Biochemical oxygen demand; TSS Total suspended solids

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Table 3. Examples of situations when combinations of water quality variables and

extreme discharge conditions can produce environmental problems

(reproduced with permission, from Nilsson and Renofalt 2008).

Water quality variables Low ows High ows

Pollutants Concentrations can reach toxiclevels

Can be washed out from adjacent,otherwise unooded uplands;dilution reduces but does noteliminate risk for toxicity

Drugs PPCP:s (Pharmaceuticals andPersonal Care Products) can

become toxic; natural estrogenscan feminize sh

Nutrients Can lead to eutrophicationand acidication; N levels can become toxic

Removed from watercourse bydownstream transport, uptake by riparian vegetation anddenitrication

Salts Can lead to acidication,mobilization of toxic metals andinvasion of salttolerantspecies

Organic Matter &sediments

Considerable addition thatincreases turbidity, which reducesprimary productivity and may

increase acidity and threatensh production Organic mattercan reduce pH Sedimentationof transported inorganic matterrestructures channel

High temperature Lowers oxygen content, makescontaminants more toxic, lowersproductivity

Low temperature Surface ice cover leads toreduced oxygen Open water and

low air temperatures can fosterexcessive formation of frazilice and anchor ice that damageaquatic biota Open water andtemperatures rising from belowto above 0°C lead to meltinganchor ice that can jam up andproduce local oods and uplandice that damage riparian andupland biota

If high ows occur during periods with low temperatures and surface

ice, water can be forced on top ofthe ice, often leading to oods, orthe ice cover may break up and runthe risk of jamming

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must be emphasised that the assimilation capacity of the rivers without any ow alteration

is also not unlimited, and ow alone cannot take care of the problem of pollution.

Impacts on Ground Water

Another major impact of the altered ow regimes is on the ground water aquifers along therivers and under their oodplains as the inltration is drastically reduced. Peak ows for short

periods and of a lower magnitude such as those experienced by the regulated rivers during

the rainy season restrict the time and land area under ooding for groundwater recharge.

Similar effects occur also in the hilly regions depending upon the geology, topography (river

bed slope) and substrate characteristics. A lowering of the groundwater table close the river

channels is experienced throughout the India and in neighbouring countries because of the

excessive abstraction and absence of groundwater recharge (CGWB 2011, Soni and Singh

2013; Figures 8a,b).

Impacts on Aquatic Biodiversity The biota of the river, its riparian zone and the oodplain are impacted by the combined

and interacting inuences of altered ow regimes, sediment characteristics and the physico-

chemical quality of water. The overall consequence is a decline of biological diversity, and

often the invasion by undesirable species. The changes in the macroinvertebrate communities

downstream of the reservoirs have been investigated in numerous cases. Physical variables

such as the water depth and current velocity, and sediment characteristics have been

observed to inuence their species composition, density and biomass (e.g., Xiaocheng et al.

2008, Rehn et al. 2008, Wu et al. 2010, Mantel et al. 2010, Maroneze et al. 2011, Carlisle et al.

2012). While a signicant reduction in the species richness of molluscs has been observed in

Figure 8a. Annual hydrograph of groundwater of the Yamuna oodplain near Palla Temple, Delhi(data from CGWB5). Monitoring well located 700 m from the river (from Soni and Singh 2013)

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many studies (Layzer et al. 1993, Seddon 2000), there are also reports of spread of pulmonate

snails which are vectors of schistosomiasis and other diseases (e.g., Brown 1994).

Assani et al. (2006) however observed that a signicant reduction in the frequency

and magnitude of the minimum discharges during the growing season (April–September)

and the formation of numerous new geomorphological features (islets, pebble bars and rock

outcrops) in the low ow channel downstream of a dam in Belgium, was accompanied witha large increase in the number of vascular plant species downstream from the dam. Changes

occur in the species composition and diversity of riparian and oodplain vegetation even

with relatively small changes in water levels (Chauhan and Gopal 2005). Cogels et al. (1997)

reported rapid expansion of aquatic macrophytes along River Senegal after stabilisation of

water levels whereas in the alluvial oodplain of Zambezi, Beilfuss et al. (2001) observed

an increase in density of woody species and drought tolerant grasses which displaced ood-

tolerant species. The riparian forests, grasses or other plants may even be eliminated by

larger changes in the ow regimes (e.g., Nilsson et al. 1997). Another recent study on the

impact of dams showed the importance of ow variability to the development of riparian

vegetation. Bejarano et al. (2011) observed that the annual ow uctuations had beenreduced below the dam and that this effect decreased further downstream with the entry of

tributaries. Reduced ow variability triggered establishment of trees and shrubs towards the

mid-channel along the entire study reach. Shrubs were most impacted by ow regulation in

terms of lateral movement, but the effect on trees extended furthest downstream. Species

richness after regulation increased for trees but decreased for shrubs though the species

composition was affected by other factors.

The impacts of altered ow regimes on sh diversity and sh populations have

received considerable attention throughout the world (Table 4). In the river channel,

Figure 8b. Annual hydrograph of groundwater of the Yamuna oodplain at Burari,Delhi (data from CGWB5). Monitoring well located 1700 m from the river

(reproduced with permission, from Soni and Singh 2013)

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degradation of water quality favours algal blooms and/or lamentous algae whereas the loss

of food and feeding and breeding sites results in the loss of sheries. In contrast to a shift

in sh species composition from riverine to lacustrine one in the reservoirs, downstream

of dams, sh populations are affected by their inability to migrate to upstream habitats

and disconnection of the river and oodplain, in addition to the reduction in ow itself. Water temperature inuences many important ecological processes. It affects growth in

freshwater sh, both directly and indirectly, through feeding behaviour, food assimilation,

and the production of food organisms. Temperature has a direct relationship with the

availability of oxygen as its solubility changes more rapidly at lower temperatures (0.405

mg/L reduction from 0°C to 1°C) than at higher temperature (0.261 mg/L with temperature

Table 4. Signicance of alteration of different components of ood regimes to sh

Characteristics Biological signicance

Smoothness of oodcurve

• Loss of eggs and nests through stranding or exposure due to rapidchanges in level.

• Stranding of eggs and fry of some species

Increased rapidity ofchanges to level

• Submergence of nests to great depths• Rapid currents may sweep fry and larvae past suitable sites• Increases stranding mortalities of sh mortalities of sh in temporary

oodplain pools.

Acceleration of ows atinappropriate times

• Pelagic eggs and larvae in drift swept past suitable nursery sites

Slowing of ow atinappropriate time

•Pelagic eggs and larvae settle out of drift and fail to reach sites orkilled by anoxic conditions

Failure of ood peak toarrive at correct time

• Failure of migratory species to mature, migrate and spawn.

Decoupling of ood peakfrom temperature peak

• Failure of thermally sensitive sh to spawn• Reduced growth rate of fry and adult.

Extensive abstraction of water in dry season

• Fish killed by excessive de-oxygenation.• Fish gets more exposed to shing pressure and predation.• Failure in maintain dry season refuges.

Reduction in depth of

ooding

• Reduction in spawning success.

• Reduced food and shelter areas for fry and adult.• Overall drop in production. Failure in ooding may result in yearclass spawning of species.

Flooding period reduction • Reduction in growing season small sh falls to predators

Interruption to continuityof ooding.

• Loss of eggs and fry unable to sustain in drift to colonize.

Flow • E-ow requirement, timing of ow, speed of discharge, water level.

Habitat connectivity • Removal / mitigation obstruction to sh movement, maintain accessto inowing tributaries in lakes / impoundments

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100 | Envi ronment al Flows

increase from 10 to 11 °C and 0.128 mg/L with rise in temperature from 30 to 31°C). Thus,

even a small increase in water temperature in the hilly regions has greater implications for

the coldwater shes such as trout. Lessard and Hayes (2003) examined the effects of release

of water with higher summer temperature from an impoundment on downstream sh and

macroinvertebrate communities in a cold-water stream. They observed that increasingtemperatures downstream coincided with lower densities of cold-water sh species such as

brown trout ( Salmo trutta), brook trout ( Salvelinus fontinalis) and slimy sculpin (Cottus

cognatus) while overall sh species richness increased downstream (Figure 9 a,b). Density

of Cottus bairdi, another cold-water species, was not affected by temperature changes below

the dams. Macroinvertebrates showed shifts in community composition below dams that

increased temperature. Changes in the physiochemical conditions, primary production and

channel morphology may however, benet some species but not without having an adverse

effect on most of the native species.

Figure 9 a. Relation between sh populationdensity (loge transformed) and mean summer

temperature ( °C) at upstream (open diamonds)and downstream (closed squares) samples.

(reproduced with permission, from Lessard and Hayes 2003)

Figure 9 b. Regression of differences in shspecies richness on differences in mean summertemperature between above dam and below dam

reaches (R2 = 0.79).

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In India, the impacts of ow alteration on riverine sheries have been reported in

many publications (Vass et al. 2006, 2011, Das et al. 2007, Pathak and Tyagi 2010) though

the Ganga-Brahmaputra river systems have received most attention. However, it needs

to be pointed out that these studies do not provide any quantitative relationship between

changes in sheries and river ows because the ow data are classied (not accessible), andthe high levels of pollution from the domestic and industrial wastewaters mask the effects

of changes in ow regimes.

During the past fty years, the sh catch from River Ganga has been declining

regularly (Table 5; Figure 10). Though the extent of decline in the total catch and its species

composition varies in different sections of the river, the major carps and large sized catshes

( A. aor, A. seenghala, W. attu) declined sharply while the smaller species exhibited only

slight changes. During 2001-08, the sheries improved in general, mainly due to invasion

of exotic species, C. carpio and O. niloticus. These changes can be attributed to several

Table 5. Decline in sh catch in river Ganga (from Pathak et al. 2010)

Period MajorCarps

Largecatshes

Hilsa Exotics Others Total

1961-68 424.91 201.35 97.17 null 211.96 935.39

1972-80 135.17 98.55 9.66 null 197.86 441.25

1981-90 155.73 99.40 4.31 null 247.59 507.03

1991-00 28.91 62.74 4.51 null 178.20 274.36

2001-06 38.58 40.56 1.20 64.27 223.41 368.01

Figure 10. Changes in sh catch in River Ganga during 1955 to 2003 (from Pathak et al. 2010)

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other factors besides the reduction in river ows caused by hundreds of diversion structures

on various tributaries (for details see Vass et al. 2011). It is noteworthy that in Murray

Darling basin also, exotic species like the introduced carp, Cyprinus carpio, expanded with

simultaneous decline of native sh (Gehrke et al. 1995). The oodplain habitats have also

decreased signicantly with the construction of embankments and the degradation and/orloss of oodplain water bodies.

Most of the Indian migratory shes are potamodromous. The big catsh, Pangasius

pangasius travels the longest distance upstream for reproduction. Fishes in the hill

streams migrate both up and downstream periodically in response to rising water levels or

temperatures to access food or to new habitats or spawning areas or for the post-spawning

movement (Table 6). Numerous dams and hydropower projects in the hills, particularly

the Himalayan belt, are now affecting the trout and mahaseer. Indian mottled eel,

Anguilla bengalensis is the only noteworthy catadromous migrant which descends the R.

Brahamaputra from its hill stream tributaries around November-December every year for

spawning in the Bay of Bengal.

Hilsa (Tenualosa ilisha) is the most popular anadromous species which used to

travel longest distance upstream for breeding in the rivers of all major rivers (Ganga,

Brahmaputra, Mahanadi, Godavari, Krishna, Cauvery and Narmada. For Hilsa, the post-

Table 6. Migration pattern of migratory sh species in India

Species Spawning time

J F M A M J J A S O N D

Tor putitora(Golden mahaseer)

U U U U U D DD

August-Sept.

Tor tor(Deep bodied mahseer)

U U U U D DD

May-August

Acrossocheilus hexagonolepis(Copper mahseer)

U U U U U D DD

May-June; August – Sept.

Schizothorax richardsonii(Snow trout)

U U U March-May

Schizothorax progastrus U U U March-May

Hilsa Tenualosa ilisha U U July-Augustanadromous

Pangasius pangasius U U U June-August

Catla catla U U U U May-August

Labeo rohita U U U U May-August

Cirrhinus mrigala U U U U May-august

Indian mottled eel, Anguillabengalensis

D D June-July (Catadromous)

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monsoon and late winter run is the major run returning to the estuary for growth and

survival in brackish or low salinity waters. During the early monsoon or late summer hilsa

migrated upstream up to Allahabad in the Ganga, and Mettur in Cauvery (Ghosh 1965,

Sreenivasan 1976). This migration has been severely affected by the dams, barrages and

anicuts on Hanga, Cauvery and other rivers but the dramatic changes in hilsa have attractedgreater attention in the lower reaches of River Ganga because of the Farakka barrage which

was constructed for augmenting river ow to the Hooghly estuary of River Ganga to meet

the needs of the Kolkata Port. The barrage became operational in 1975. Soon thereafter the

catch composition of the sheries immediately above Farrakka changed considerably as

the hilsa declined sharply (Table 7). Hilsa now occurs only downstream of the lock gates of

the feeder canal at Farakka.

The diversion structures and altered ow regimes have impacted the mammal and

bird populations also (Nilsson and Dynesius 1994). Throughout south Asia dolphins have

disappeared from many rivers such as River Ganga upstream of Middle Ganga Barrage

at Bijnor, and most of its tributaries (Smith et al. 2000, Sinha and Sharma 2003). The

Farakka Barrage on River Ganga has also caused a decline in the dolphin population (Sinha

2000). In China, river dolphin, ( Lipotes vexillifer, has disappeared after construction of

the Xinjiang dam (Liu et al. 1999). On the other hand, beaver populations have increased

following reduced ow in some rivers (Hayeur 2001).

In the long term, reduced ooding alters vegetation communities that may be

important for a wide range of mammal and bird species (Table 8). In arid regions, riparian vegetation may be the only signicant vegetation, and many animals will have adapted

behavioural patterns to t with seasonal ooding. If the ooding regime is altered, changes

in vegetation may place the birds and animals that depend on it at risk. An example is the

decline of the wattled cranes ( Bugeranus carunculatus) on the Kafue Flats after the ow

was altered by the Itezhi-tezhi dam (Kamweneshe and Beilfuss 2002).

Impacts on Estuaries and Backwaters

The effect of changes in freshwater ows on salinity in estuarine and backwater areas was

Table 7. Catch composition (%) at Lalgola during pre- and post-Farakka periods

Group 1963-76 1980-90 1991-00

Major carps 0.33 4.47 9.76

Large catshes 0.12 9.34 13.58

Hilsa 92.02 29.68 16.80

Others 7.53 56.51 59.86

Total (Mg) 121.43 57.31 106.35

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mentioned above in the context of water quality. The changes in the salinity regimes have

many major consequences for the estuarine biota and ecological processes as well as for

coastal sheries. Bunn et al. (1998) have discussed the inuence of river ows on coastal

sheries in some detail. In India and Bangladesh, the impacts have been investigated in

some detail for many years (since the construction of Farakka barrage) in respect of hilsa

and shrimp sheries and the changes in the vegetation of Sundarban mangroves. The major

changes have been summarised by Chauhan and Gopal (2013). The debate has however

been shadowed by importance of ow to the Kolkata Port, and the Indo-Bangladesh disputeon sharing of Ganga waters.

The Farakka barrage has resulted in steep decline in the hilsa catch both upstream

and downstream of the barrage. However, freshwater diversion through the feeder canal

has altered the hydrology and salinity gradient of the Hooghly estuary, and consequently

the extension of the freshwater tidal zone has occurred till further downstream in the Indian

Sundarban. With the habitats gradually turning favourable for Hilsa spawners, breeding

grounds have shifted downstream of the Farakka Barrage, showing species adaptation to

changing situations. These changes are also accompanied by signicant changes in the

Table 8. Reported effects of ow regulation on river biota (adapted from Bragg et al. 2005)

Biotic groups Effects of ow regulation

Macrophytes Large changes in species composition; may be greatly reduced or proliferate indifferent habitats depending upon depth, velocity, and other ow characteristics. Very high ows usually scour the vegetation, substrate and organic matter.

Invertebrates Generally resilient but taxonomic composition and abundance change greatly withow regime.Changes in the substrate characteristics (e.g., sediment texture) affect theinvertebrate assemblages.Flushing and catastrophic drift of Plecopteran larvae reported due to intermittent‘hydropeaking’ ows.

Fish Fish feeding habits change due to changes in invertebrate communities.Scouring, erosion and textural change of spawning habitats due to sudden high

discharge and largw uctuations; also river bank erosion results in greater turbidity(ne sediment load)Swimming affected by high ow velocity and often juveniles concentrate in shallow

water areas or get stranded when ow decreases abruptly.Changes in reproductive development and spawning behaviour of sh.Changes in sh assemblages – species composition and abundance

General Downstream plant and animal communities are affected adversely and drasticallySediment composition below dams is severely altered and affects sh and benthic

biota.Large variations in ow velocity destroy pool/rife relationships; render the stream

banks unstable.

Riparian vegetation altered due to changes in sediment dynamics

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species composition and yields of estuarine sheries which are now being slowly but steadily

replaced by freshwater species.

Similar impacts hve occurred in other estuarine systems as well. Several dams and

barrages in the Damodar River basin have resulted in a decreased silt load and lower

deposition of detritus in the estuaries downstream, thereby affecting the sheries. Estuaries

of Krishna and Godavari rivers have become hypersaline, and sh species with low salinity

tolerance have been wiped out (Das et al. 2007). Among other estuarine systems impacted

by changes in river ows, the Vembanad lake (a backwater in Kerala) and the Lake Chilika (a

coastal lagoon on the east coast of India) – both Ramsar sites – have attracted many studies

(e.g., Das and Jena 2008, Dujovny 2009, .Mishra 2012, Abe and James 2013).

Socio-Cultural and Livelihood Impacts

As described in the previous chapter, rivers provide many social and cultural services which

are linked directly to the ow regimes besides their indirect relationship with the river’s

abiotic and biotic components. Alteration of the ow regimes, therefore, has also signicant

social, cultural, economic and health impacts on the human populations which live along

the rivers and depend upon them for livelihoods. As mentioned earlier also, while the socio-

economic impacts in areas upstream of the dam sites have received global attention, little

attention is paid to these impacts along the downstream reaches where usually a much

larger area and greater population are affected. Richter et al. (2010) point out that the

large dams can severely disrupt natural riverine production systems – especially sheries,

ood-recession agriculture and dry-season grazing (Table 9). Few examples given in the

foregoing account show that the capture sheries usually decline and the oodplain and

riparian vegetation changes greatly. Together with a reduction in the area of oodplains, theresource availability and quality fall sharply. It is common knowledge in South Asian region

and many other countries that the introduced exotic species of sh, such as tilapia, are not

preferred by the human consumers and hence have a lower market value. Hoeinghaus et

al. (2009) report that in the Parana River (Brazil), the replacement of migratory species by

introduced species resulted in higher energetic costs of sheries production whereas market

value decreased as a result of consumer preferences. Similar problems of availability, quality

and costs occur everywhere with plant resources, sediments and of course water itself. Large

number of people depends on the oodplains for reeds, cattails, grasses and also fuelwood,

for cultivating various crops, particularly vegetables, and aquatic plants like Trapa and lotus

in shallow water areas, and for extracting sand and gravel. Reduced sediment transport onlyleads to illegal and excessive exploitation. Some communities/social groups are engaged

in water transport, preparing materials to support shing, supporting cultural/religious

activities and various recreational activities for their livelihood. We are not aware of

published quantitative estimates of these activities and affected people. Richter et al. (2010)

estimate that globally 472 million river-dependent people live downstream of large dams

along impacted river reaches and call for “comprehensive assessments of dam costs and

benets, as well as to the social inequities between dam beneciaries and those potentially

disadvantaged by dam projects”.

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At the end, we would like to draw attention to two important studies. Barbier and

Thompson (1991) compared the agricultural, shing and fuelwood benets lost through

reduced ooding downstream against the gains from increased irrigation production

upstream in the Hadejia-Jama’are River Basin in northern Nigeria. They observed that

the irrigation benets can only partially replace the benets lost due to reduced oodplain

inundation. In a recent study, Wang et al. (2009) determined the effects of hydropower

development on watershed level ecosystem services, using 21 indicators and various

methods of economic valuation such as the market value method, opportunity cost method,

Table 9. Fisheries status before and after dam construction. (modied from Richter et al. 2010)

River/Country Fish species Catch or # sh speciescaught

Kafue Gorge damand Itezhitezhi dam,Zambia

Oreochromis andersonii,a key ood-plain-dependent tilapia species

In 1968, 50% of thecatch consisted ofthe commerciallyimportant O.andersonii

By 1983, only 3% ofcatch was made of O.andersonii

Kafue Gorge damand Itezhitezhi dam,Zambia

Increase in predatorspecies: Clariusgariepinus,

Macuseniusmacrolepitus, Labeo maolybdinus, Hepsetus odeo

Families supported by shing: 2600(1977)

Families supported byshing: 1157 (1985)

Urra dam Colombia 6000 Ng/year 1700 Ng/year (early 1990s)

Salandi dam, India 350 Ng/year (1960-1965) 25 Ng/year (1995-2000)

Pak Mun dam,Thailand

265 sh species 50 species have disappeared and otherspecies show marked population declinesFish catch decreased by 60-80%

Kainji dam, Nigeria 30% decrease in catch, and reduction of Mormyridae species from 20% ofsh community to 5% (commercially important)

Aswan High dam,Egypt

Number of harvested shspecies: 47

Number declined to 25 at Assuit and to 14at Cairo; in Kafr el Zayer reach, number and

size increasedXinanjiang dam,China

Macrura reevesii Freshwatersh: 96 species

85 species

Manantali and Diamadams , Mali andSenegal

23,500 Ng/year Reduced by 50% (net loss of 11,250 Ng/ year)

Three Gorges dam,China

Four speciesof carp (silver,

bighead, grass, black)

Commercial harvest:3,360,000 Ng (2002)

1,010,000 Ng (2004),1,680,000 Ng (2005)

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project restoration method, travel cost method, and contingent valuation method. Three

representative hydropower projects in the Jiulong River Watershed in southeast China were

considered. They concluded that biodiversity loss and water quality degradation accounted

for 80-94% of the major negative impacts, the value of negative impacts ranged from 64%

to 91% of the value of positive benets, and that the average environmental cost per unit ofelectricity was about 75% of its on-grid power tariff.

It can be safely concluded that the short-term gains from a reservoir or hydropower

project (over their average life span) may not compensate for the accumulated long-term

loss of river’s ecosystem services. There is a clear urgent need for proper valuation of all

quantitative and qualitative aspects of the water resource development projects.

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