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Biodiversity Impacts of
Background Paper Nr. 1
Prepared for IUCN / UNEP / WCD
By Don E. McAllister, John F. Craig, Nick Davidson,
Simon Delany and Mary Seddon
ii Biodiversity Impacts of Large Dams
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imply the expression of an opinion on the part of IUCN, UNEP or UNF concerning the legal status of
any country, territory, or area, or of its authorities, or concerning the delimitation of its frontiers or
The views expressed in this publication do not necessarily reflect the views nor the official positions of
IUCN, UNEP or UNF.
This report has been made possible by funding from the UN – Foundation.
© 2001 International Union for Conservation of Nature and Natural Resources and the United Nations
Reproduction of this publication for educational or other non-commercial purposes is authorised with-
out prior written permission from the copyright holder provided the source is fully acknowledged.
Reproduction of this publication for resale or other commercial purposes is prohibited without prior
written permission of the copyright holder.
iiiBackground Paper Nr. 1
Although occupying a smaller area compared to land and oceans, freshwaters are home to a relatively
high proportion of species, with more per unit area than other environments (10% more than land and
150% more than the oceans). While only about 45,000 species of freshwater animals, plants and
microorganisms have been scientifically described and named, scientists estimate that at least an
additional million more species remain to be named.
Freshwater biodiversity is unevenly distributed. High numbers of species or endemics are found, for
example, in the Amazon, Congo, Nile and Mekong basins. Such species-rich areas are called ‘hotspots’
and dominate other patterns or trends.
Already human uses of freshwater threaten the survival of freshwater, brackish, coastal and terrestrial
biodiversity. The global level of threat for mainly terrestrial vertebrates is 11-25%, while for freshwater
groups it is 13-65%. While this paper focuses on the impacts of large dams on freshwater biodiversity,
the significant effects of large dams on terrestrial biodiversity should not be overlooked.
About 60% of the world’s river flow is regulated. There are more than 40,000 large dams and more than
100 dams with heights >150 m. Reservoirs cover a total area in excess of 500,000 km
Dams and their associated reservoirs impact freshwater biodiversity by:
· Blocking movement of migratory species up and down rivers, causing extirpation or extinction of
genetically distinct stocks or species.
· Changing turbidity/sediment levels to which species/ecosystems are adapted in the rivers af-
fects species adapted to natural levels. Trapping silt in reservoirs deprives downstream deltas
and estuaries of maintenance materials and nutrients that help make them productive ecosys-
· Filtering out of woody debris which provides habitat and sustains a food chain.
· Changing conditions in rivers flooded by reservoirs: running water becomes still, silt is depos-
ited, deepwater zones, temperature and oxygen conditions are created that are unsuitable for
· Providing new habitats for waterfowl in particular for overwintering or in arid regions which may
increase their populations.
· Possibly fostering exotic species. Exotic species tend to displace indigenous biodiversity.
· Reservoirs may be colonised by species which are vectors of human and animal diseases.
· Flood plains provide vital habitat to diverse river biotas during highwater periods in many river
basins. Dam management that diminishes or stops normal river flooding of these plains will
impact diversity and fisheries.
· Changing the normal seasonal estuarine discharge which can reduce the supply of entrained
nutrients, impacting the food chains that sustain fisheries in inland and estuarine deltas.
· Modifying water quality and flow patterns downstream.
· The cumulative effects of a series of dams, especially where the impact footprint of one dam
overlaps with that of the next downstream dam(s).
· Other human activities, including agriculture, forestry, urbanisation and fishing, although these
are primarily land-based.
International agreements and organisations have established standards for minimising the negative
impacts of human activities on biodiversity. Pertinent legal instruments include the World Charter for
Nature, the Convention on Biological Diversity, and Agenda 21, while international organisations such
as the World Bank, the World Business Council on Sustainable Development and the The World Con-
servation Union (IUCN), have contributed to the development of accepted standards. The standards
involve conservation of species and ecosystems, the recovery of degraded ecosystems, the conser-
iv Biodiversity Impacts of Large Dams
vation of ecological functions or processes, securing adequate information for decision making, adher-
ence to the Precautionary Principle, and the adherence to high standards for environmental impact
assessments. A short evaluation shows that many of the standards have been transgressed in the
A number of recommendations are made. They include:
· Avoid the coincidence of environmental impacts of dams with areas rich in biodiversity — ‘hotspots’
· Avoid blocking migratory species
· Maintain natural seasonal and daily river flow cycles
· Maintain discharge volume as much as possible
· Sustain water quality — temperature, oxygen, sediment & other levels
· Avoid cumulative effects of dams — limit their number and proximity
· Take into account the impacts of other human activities when planning dams
· Apply high environmental impact assessment standards
· Involve environment staff early and at high levels in planning and construction
· Enhance delivery and conservation in extant dams
· Decommission ineffective dams & restore river ecosystems and species
· Use landscape management to make dams more effective and to protect biodiversity
· Establish protected areas to enhance the efficiency of dams and conservation of biodiversity
· Improve needed knowledge bases through research
· Explore and reduce the impacts of dams on terrestrial biodiversity
Freshwater species and ecosystems are among the most imperilled. Dams are a principal threat to
freshwater diversity and that threat is largely mediated through loss of habitat frequently involving
modifications to the natural flow regime and to blockage of migrations.
This offers a challenge to the dam construction and management community. Can this community
make courageous and profound changes in their initiatives and, in so doing, find new opportunities for
returns on corporate balance sheets and the Earth’s biodiversity balance sheets?
vBackground Paper Nr. 1
Many people and organisations around the world have generously donated their time, information and
publications to assist us. We gratefully acknowledge their assistance. The following assisted with litera-
ture and other valuable information: Dr Patricia Almada-Villela, Cambridge, UK; Dr Eugene K. Balon,
Guelph University, Guelph, Canada; Dr Juraj Holcík, Slovak Academy of Sciences, Bratislavia; Dr
Rosemary H. Lowe-McConnell, Strethwick, UK; Dr M.I. Stiassny, American Museum of Natural His-
tory; Carmen Revenga, World Resources Institute; Dr Simon Stuart and Dr Ger Bergkamp of IUCN -
World Conservation Union; Dr Tony Whitten and Dr Gonzalo Castro, World Bank. Elisabeth Janssen,
Dianne Murray and Hilary Craig proof-read versions of the manuscript.
In memory of Don McAllister, who recently passed away.
viiBackground Paper Nr. 1
Table of Contents
Executive summary .................................................................................................................... iii
Acknowledgements .................................................................................................................... v
1. Introduction ............................................................................................................................ 9
2. Patterns of freshwater biodiversity .......................................................................................... 17
3. Impacts of dams on biodiversity .............................................................................................. 23
4. Standards for minimising negative impacts on biodiversity....................................................... 47
5. Impacts of dams vis à vis standards........................................................................................ 51
6. Recommendations on dams and biodiversity .......................................................................... 53
7. Beyond dams: other solutions................................................................................................. 57
References ................................................................................................................................ 59
Background Paper Nr. 1 9
This study investigates the interaction between dams and biodiversity particularly the impacts of large
dams on freshwater organisms (see Oud & Muir 1997 for a definition of a large dam). In addition the
following were analysed: biogeography, the application of techniques developed elsewhere in the planning
and construction of dams, the minimising of dam impacts on biodiversity and the application of
ecosystem-based management to enhance the performance of dams. As the analysis was carried out
within a short time period it cannot be considered definitive. There have been many publications on the
impact of dams on animal biodiversity although the data are weak in a number of areas including
plants and small organisms.
Dams, including large dams, are constructed because of the potential benefits that they bring:
· Water for increased food production - 250 million hectares of agricultural land are under irrigation
and use three-quarters of the water supply
· Generation of electric power without releasing atmospheric pollutants or greenhouse gases -
hydropower contributes 20% of electricity production
· Control of floods.
· Drinking water. Of the Earth’s 6 billion people, 1.5 billion are without access to reliable sources
of drinking water
The large expenditures involved with the construction and operation of large dams and the benefits for
agriculture and power generation, are of considerable long-term economic importance.
Biodiversity is considered at four levels: genetic; species; ecosystem and ecological function. Normally
it is the indigenous or native components of biodiversity that are examined; exotic or alien species are
a separate component, although interacting with the native species. Many species are yet to be
discovered, scientifically named and classified, especially in tropical regions and in some taxonomic
groups that are poorly studied, for example the nematodes, algae, bacteria and fungi.
Perturbations to a species status can be measured in simple terms by reduced population sizes,
extirpations (loss of populations from a part of the species range), or extinction (loss of all individuals
of a species). Finer levels of species loss are provided globally by IUCN:
· Extinct (EX)
· Extinct in the Wild (EW)
· Threatened (Critically Endangered (CR) (Endangered (EN) (Vulnerable (VU)
· Under inadequate data: Data Deficient (DD)
· Under evaluated: Not Evaluated (NE)
Conserving habitats and ecosystems is the key to species conservation. Here habitat may be defined
as the place where an organism lives or living space and an ecosystem as the interaction or functioning
between a community of organisms and their nonliving environment (Odum 1963). The earth can be
divided into a series of biogeographical regions, or biomes, ecological communities where certain
species of organisms co-exist within particular climatic conditions. Within a biome there are several
local factors which affect the distribution of species. Degradation of habitat leads to lowered population
size and loss of habitat to extirpation. Humans mainly induce extinctions by causing habitat loss (Wilcove
et al. 1998). The importance of all components of the ecosystem including primary producers, herbivores,
carnivores, detritivores and recyclers and their ecological function (Mosquin 1994) should be considered
in the design of dams. In addition there are certain special linkages between species, e.g. freshwater
mussel larvae (glochidia) are parasitic on the gills of fishes for part of their life cycle. In many cases
specific fish hosts are required.
10 Biodiversity Impacts of Large Dams
Status of the world’s freshwaters
Revenga et al. (1998) describe the watersheds of the world, their ecological value and their vulnerability.
The impacts to the freshwater environment by various activities or sectors are summarised in Table
Table 1.1 Sector threats to freshwater environments
Sector Measure of threat Impacts (Add to each, biodiversity loss)
Agriculture 11% of land in crops, 26%
in pasture. 3/4 of human
water withdrawals, 250
million hectares under
Runoff of toxic pesticides (fish kills); fertilisers
and manure (eutrophication); soil (turbidity and
siltation). Overgrazing (loss plant cover, bank
Deforestation 50% of world's forests lost;
instead of selective
Soil erosion (turbidity and sedimentation. Rapid
runoff. Loss stream food/habitat (leaves, wood,
insects). Changed hydrological cycles.
Dams 60% world's river flow
regulated. 15% world's
precipitation held in
500,000 km2 of reservoirs.
Blocking of movement of
local- and long-distance
migrations in neighbour-
hood of dam.
Fish migrations blocked; stocks lost. Seasonal
flows changed; flows reduced. 25 million km
river habitat modified. Flood plains & deltas lost.
Lowered fish production.
Sediment/turbidity/nutrient changes. Running to
Industry and urban
Release toxic substances,
untreated sewage. 1/4 of
human water withdrawals.
Fish kills and advisories. Impaired reproduction.
Eutrophication. Reduced flows.
Escape of alien species.
Competition with and loss of native biota.
Spread alien pests and diseases. Loss of native
habitats. Genetic pollution. Eutrophication.
Simplification of river
structure. 500,000 km of
river altered for shipping.
Loss of habitats, flood plains and wetlands.
Fishing Over-harvesting. Gear
Reduced populations, loss of stocks, changed
food webs, and habitat loss.
Acid rain Reduction of pH (increase
in acidity) of lakes and
streams down to 4.5 or
lower in thousands of water
bodies in North America
Reduction of populations or extirpation of
species of molluscs, amphibians, fishes, etc. in
water bodies. Development of skeletal
abnormalities. Deposition of aluminium on fish
and per capita
Doubled to 6 billion since
1975. Per capita
consumption doubled since
Population/consumption rate increases magnify
each sector impact above. Humans use 54% of
geographically & temporally accessible water.
Background Paper Nr. 1 11
The table shows that indirect landscape and direct waterscape changes have had profound impacts
on the freshwater environment including:
· River seasonal flow patterns (levelling)
· River flow volume (reduction)
· Accessibility of species to river segments (blockage of migrations)
· Input of organic matter (leaves, wood, insects) (reduced)
· Toxicity (increased)
· Turbidity & sediments (both increases & decreases to natural levels)
· Nutrient levels (increased).
Freshwaters, and especially rivers
and wetlands, are amongst the
world’s most severely impacted
and have received many of the
direct effects of human activities.
Long-term and quality data on
river discharge patterns are poor.
Historical hydrological data are
rare for many rivers. However data
from old air photos and satellite
images, including the new
Radarsat images, which can be
made through clouds, offer a
promising source of information in
particular the extent of flooding in
the river floodplains.
An analysis by Postel (1996)
indicates that humans presently
use 54% of the geographically and
seasonally accessible runoff.
Demands by the year 2025 may
increase to more than 70% of accessible runoff. The existing and growing friction between countries
over shared waterways shows that humans are already facing water shortages that are international in
scope. Problems already exist at the national and local level between and within sectors such as
agriculture, domestic users and industry. The fixed supply of water poses profound problems for how it
will be shared by aquatic life and increasing demands of humans.
Status of the world’s biodiversity
Assessing the status of biodiversity at the global level is difficult because the evaluation is incomplete
and uneven. Birds are quite well assessed, oligochaetes poorly, and many microbiota not at all. The
IUCN Red Lists ease the task by bringing together what is known and applying uniform criteria. However,
summaries do not separate freshwater biota from biota in other environments. Table 1.2 summarises
terrestrial and freshwater vertebrate data at the global level. The level of threat for dominantly terrestrial
vertebrates is 11 to 25%, while the remaining values for groups occurring more frequently or uniquely
in freshwater range from 13 to 65%. This gives a sense that, globally, freshwater species are more at
risk than terrestrial species. Waterfowl are more threatened, 12.7%, than land birds, 10.8%. Similarly,
freshwater mammals are more threatened, 65%, than all mammals, 25%.
Data for North American animals is more complete and the proportion of selected groups of animals at
risk is given in Table 1.3. These data indicate that freshwater animals are much more at risk, 39 to 68%,
than predominantly terrestrial ones, 15 to 17%.
Molluscs. Extinctions are a significant problem in terrestrial, freshwater and marine molluscs. Of all
the species that became extinct since 1600 AD, 37% were mollusc species, which is more than any
other group evaluated (birds 17%, mammals 14%, fish 14%, reptiles 3%, and all others 15%). These
percentages refer to the total known globally. It should be made clear that the percentages are affected
by the degree to which the group has been studied and the number of species in the group, e.g. the
status of birds is better studied than that of molluscs, but there are more mollusc than bird species.
Table 1.2 Proportion of terrestrial and freshwater vertebrates
Group Proportion Threatened (%)
Mammals - all 25
Land birds 11
Waterfowl (freshwater) 13
Turtles, tortoises and terrapins 38
Amphibians 25 (estimated)
Freshwater fishes 33 (estimated)
Freshwater mammals (freshwater
dolphins and otters)
12 Biodiversity Impacts of Large Dams
The 1996 IUCN Red List of Threatened
Animals lists 12 bivalves and 216
gastropods as extinct, and 114 bivalves
and 806 gastropods as threatened, for a
total of 228 extinct and 920 threatened
terrestrial, freshwater and marine
molluscs. About 18% or 145 of the
threatened molluscs are spring molluscs.
Data on threatened freshwater molluscs
are given in Table 1.4).
Spring snails in the critically endangered
category in Austria, Australia and United
States are threatened by over-abstraction
of water from their habitat or by pollution.
African freshwater molluscs are
threatened by decline in quality of water,
pollution, damming, and increased
Table 1.5 and 1.6 provide information on
the status of North American and
Australian freshwater molluscs (since the
latter table was made, a further 66 freshwater species have been described, many from the family
Wilcove et al. (1998) reviewed the threats to imperilled species in the United States. They found that
the chief threats to freshwater molluscs were habitat degradation and loss, 97%, pollution, 90%, and
alien species, 17%. The chief causes of habitat loss were water development, 99%, pollution, 97%,
dams and other barriers to flow, 96%, and agriculture 64% (note that more than one threat can act on
a given species so the threats do not add up to 100% although they indicate their relative importance).
More recently in North America, alien zebra mussel invasions have become the major factor in loss of
native mussel diversity (Ricciardi & Rasmussen 1999).
Fishes. The 1996 IUCN Red List of Threatened Animals lists 617 freshwater fishes (including euryhaline
species); about 6% of the known number of freshwater species. The Red List has evaluated only a
fraction of freshwater fishes, therefore a conservative estimate gives 20% as extinct, endangered or
vulnerable, or more realistically 30-35% (Stiassny 1996).
Table 1.3 The proportion of selected North American animals
at risk (from Stein & Flack 1997)
Group Proportion at risk (%)
Freshwater fishes 39
Freshwater crayfishes 51
Freshwater mussels 68
Table 1.4 Freshwater molluscs in the IUCN 1996 Red List of threatened animals
Risk category Bivalves Gastropods Total molluscs
Extinct 12 14 26
Critically endangered 85 60 145
Endangered 24 86 110
Vulnerable 8 194 202
Near threatened 66 35 101
Data deficient 4* 104* 108*
Total listed 199 493 692
Background Paper Nr. 1 13
In the United States, freshwater fishes have been threatened by water development (91%), dams,
impoundments and other water barriers (64%), pollutants (55%) and agriculture (45%) (Wilcove et al.
Birds. According to the 1996 Red List, 1,107 species or 11% of all bird species are threatened and 104
are extinct. Amongst the more threatened of bird groups are the aquatic Gruiformes (rails and cranes)
with 54 species, and the partially aquatic Coraciiformes (kingfishers and bee-eaters) with 11.5%
threatened, while 18% of the Podicipediformes (grebes) are threatened. Extinct aquatic birds include
the Colombian grebe (Podiceps andinus) and the Atitlan grebe (Podilymbus gigas). Thirteen per cent
of globally threatened birds are (freshwater) water-birds. There are 90 species of critically endangered,
endangered and vulnerable water-birds. This suggests that water-birds are slightly more threatened
than land birds. Habitat loss and degradation are the key factors affecting threatened species. This is
generally due to the loss and change through drainage and land reclaiming, converting natural wetlands
into urban and industrial lands in Europe and into agricultural land in North America.
Plants. There are about 270,000 scientifically described species of vascular plants, but the true number
may be in the order of 300,000-350,000 species (WWF & IUCN 1994). Two important documents on
global plant biodiversity have been published in recent years, Centres of plant diversity, in three volumes
(WWF & IUCN 1994, 1995, 1997), and the 1997 IUCN Red list of threatened plants (Walter & Gillett
1998). About 78% of the world’s plants are tropical (the zone between the Tropic of Cancer and the
Tropic of Capricorn). More than 40,000 plant species, about a quarter of the world’s tropical plant
diversity, occurs in Colombia, Ecuador and Peru, while Brazil has between 40,000 and 80,000 species.
Tropical and subtropical (areas north and south of the tropics but outside of the temperate zone) Asia
has at least 50,000 species and Southern Africa has 21,000 species of plants, of which 80% are
The Red list of threatened plants demonstrated that 32,112 species or 11.9% of the world’s 270,000
vascular plant species, are threatened, and 374 or 14% are extinct. The counts are for terrestrial and
Table 1.5 USA Federal register of threatened status of North American freshwater
mollusc fauna (after Bogan 1998)
Taxa 300 601 901
Candidate taxa 61 20.3 173 28.0 244
Threatened taxa 5 1.7 0 0 5
Endangered taxa 57 19.0 9 1.5 66
Extinct taxa 35 11.7 42 7.0 77
Table 1.6 Threatened status of the freshwater mollusc fauna in Australia (after Beesley et al. 1998)
Status NSW QLD VIC SA WA NT TAS Total
Taxa 42 56 37 34 31 27 42 178
Endemic to State 11 24 16 11 20 8 86 176
Threatened 1 13 5 9 1 0 60 88
NSW = New South Wales, QLD = Queensland, VIC = Victoria, SA = South Australia, WA = Western
Australia, NT = Northern Territory and TAS = Tasmania
14 Biodiversity Impacts of Large Dams
aquatic species combined. Of these, 6,522 species are classed as endangered. Thirty-two countries
have at least 5% of their native species threatened. The main countries (excluding small islands)
having high proportions of threatened species, 11-29%, include the USA, Jamaica, Turkey, Spain,
Australia, Sri Lanka, Cuba, Panama, Japan and Greece.
In the United States about half of the potentially extirpated species are either obligate or facultative
native wetland species (LaRoe 1995). (The Ramsar Convention in 1971 adopted a wide definition for
wetlands: areas of marsh, fen, peatland or water, whether natural or artificial, permanent or temporary,
with water that is static or flowing, fresh, brackish or salt, including areas of marine water the depth of
which at low tide does not exceed six
metres.) These wetland plant species
may be affected by changes in the
aquatic environment mediated by dams.
Terrestrial species may be affected by
reservoir flooding, de-watering
downstream of dams; construction of
transmission lines, access roads or
canals or through lowered water tables.
Walter and Gillett (1998) list the numbers
of species, the number globally
threatened by category and the
threatened percentage for each family of
vascular plants. The status of selected
freshwater plants based on their Red List
is presented in Table 1.7.
This small sample illustrates the level of threats, 7.0-31.6% in ‘water-loving’ plants is about as high as
in all vascular plants (terrestrial and aquatic) 11.7%. The mean, 14.3, and the midpoint, 19.1, of this
sample are both higher than the mean of the family values for terrestrial and aquatic plants combined.
This suggests that freshwater plants are more threatened than land plants.
About half of the world’s wetlands have been lost over the past century (Myers 1997). In Asia more
than 5,000 km
of wetland is lost every year due to agriculture, irrigation, dam construction, etc.
Large dams are usually >15 m from foundation
to crest. Dams of 10-15 m can also be defined
as large dams if they meet the following criteria:
crest length 500 m or more; reservoir capacity
of at least one million cubic metres; maximum
flood discharge of at least 2,000 m
difficult’ foundation problems, or ‘unusual design.’
Major dams meet one or more of the following
criteria: at least 150 m high; having a volume of
at least 15 million m
; reservoir capacity of at
least 25 km
; or generation capacity of at least
There are 306 major dams in the world and 57
are planned in the near future (Revenga et al.
1998). Table 1.8 lists watersheds with more than
five major dams.
Construction of large dams includes the creation
of access roads, preparation of the reservoir,
Table 1.7 Status of selected families of freshwater plants
Family % of threatened species
Water lilies, Ceratophyllaceae 16.7
Water lilies, Nymphaeaceae 8.0
Water poppies, Limnocharitaceae 31.6
Water plantains, Alismataceae 13.3
Reeds and sedges, Cyperaceae 7.0
Frog-bits, Hydrocharitaceae 14.0
Duckweeds, Lemnaceae 9.7
Table 1.8 Watersheds with more than five major dams
Watershed Number of dams
Tigris and Euphrates 7
Background Paper Nr. 1 15
excavation, construction of buildings and dams within and between river diversions, digging of canals
and erection of power lines. Forested reservoir basins provide a particular challenge. Leaving some
trees may provide fish habitat although leaving trees in any quantity may pose problems for future
fishing, water quality and turbine safety. While most reservoirs tend to trap sediments, e.g. a new delta
is being formed within Lake Nasser (Saad 2000), in some cases e.g. South Indian Lake Manitoba,
(Bodaly et al. 1984), the exposure of clay soils to shifting reservoir water levels increases erosion and
downstream sediment discharge.
The quantity of water discharged, when it is discharged during diel and seasonal cycles relative to the
river’s natural flow pattern and abiotic characteristics of the discharge such as temperature, oxygen,
turbidity, and water quality significantly affect downstream biodiversity.
The value of biodiversity
Globally terrestrial and aquatic ecological functions have been calculated to be minimally worth US$33
trillion per year, almost twice the value of the global gross national product, some $18 trillion (Costanza
et al. 1997), although the figure contains the value of some biological resources as well as functions.
Costanza et al. (1997) indicated that the annual per hectare total global flow value of inland water
systems, US$6,579 x 10
exceeded that from all other non-marine ecosystems combined - US$5,740
. Ecological functions, although not ordinarily included in gross global or national/domestic products
nevertheless make significant contributions to economies. Freshwater ecosystems are economically
more valuable than terrestrial ones. In many developing countries, fishes, including those from freshwater
make a notable contribution in animal proteins to an otherwise carbohydrate-based diet. In the Amazon,
the per capita consumption rate is 67 kgyr
higher than in many areas (Chao et al. 1999). In Tonie Sap,
Cambodia, 100,000 tonnes of freshwater fish are caught annually, which source alone would provide
a per capita 10 kgyr
Biodiversity has many kinds of values and potential benefits for humans and the world as a whole.
Before it is diminished, those responsible may well wish to consider the Precautionary Principle and
take action to conserve it before components of it are permanently lost, even when the evidence for
loss is not as strong as might be desired. That approach is advocated by the Convention on Biological
Background Paper Nr. 1 17
2 Patterns of freshwater biodiversity
Although freshwaters comprise only 0.8% of the surface area of the world and they have fewer species
than other systems, Table 2.1 shows that freshwaters contain more species per unit area than terrestrial
and marine environments. This is particularly notable for fishes.
Global biodiversity patterns in the three study groups, molluscs, fishes and vascular plants is presented
in Table 2.2. Transitional ecosystems such as estuaries and land-water interfaces are not presented.
The total number of species (species richness) is only one measure of biodiversity. Other measures
include species abundance. For example molluscs can form significant proportions of the benthos;
80% of the biomass of the River Thames at Reading is composed of freshwater unionid mussels
(Berrie & Boize 1992). Holcík (1999) states that long term investigations in the Czech and Slovak
Republics show a large biomass of fishes per unit area. In the mountains, the mean fish biomass
varies from 27-80 kg ha
, in foothill streams from 90-500 kg ha
and in the lowland streams from 300-
600 kg ha
, while in human-made lakes it varies from 65-200 kg ha
. Diversity indices, which measure
the relative richness, evenness, rarity and abundance, require quantitative sampling and at present
there are little data for aquatic ecosystems.
Species richness can change from the headwaters to the river mouth. This may be related to changes
in stream order, water temperature, oxygen, current, turbidity and available nutrients. The small
headwater streams may have low numbers of fishes that increase downstream as the number of
available habitats increases. Further and larger downstream segments of the river may have moderate
numbers of species because lower habitat variety, and river estuaries, where salinity varies, may also
have moderate species numbers. Higher numbers of fish species are found in the Tennessee-
Cumberland plateau drainage of the Mississippi, USA, than in the adjacent mainstream Mississippi;
this may reflect more numerous habitats in the varied topography of the former, compared to the more
constant gradient of the latter. Different species may characterise different river segments. Trout (Salmo)
and sculpins (Cottus) may occupy headwaters, minnows and catfishes (Cyprinidae and Ictaluridae)
mid-river sections, and euryhaline (salinity-level tolerant) species are found in the estuaries.
Freshwater molluscs generally increase from the headwaters to the river mouth. This again is related
to an increase in habitats in the floodplain areas in the middle or lower reaches.
Latitude-longitude grids have the disadvantage that the size of their grid cells shrinks towards the
poles, a disadvantage in making comparisons of numbers of species if the study area spans several
Table 2.1 Species richness of the world's major environments
Environment Area of world
No. living species % Richness:
Freshwater 0.8 2.4 3.0
Terrestrial 28.4 77.5 2.7
Marine 70.8 14.7 0.2
Symbiotic N.A. 5.3 N.A.
18 Biodiversity Impacts of Large Dams
degrees of latitude. It is generally accepted that the number of species tends to increase from the poles
to the tropics. Arctic waters can therefore be expected to contain fewer species than ones in the
tropics. In many freshwater lakes in the Arctic there is only one fish species, the Arctic charr, Salvelinus
Table 2.2 Number of species per environment in study groups (world counts)
Group Total species Land species Freshwater
Fishes 24,618 0
Birds 10,100 9,043
1 The few species of fishes, like mudskippers and climbing perch edwell out of water intermittently, or
for short periods, are ignored.
2 The 499 or 2.0% of fish species that move between the sea and freshwater are omitted.
3 To keep the table compact the midpoint values for marine, terrestrial and freshwater were used.
4 Of the freshwater molluscs 4,000 or 80% are found only in rivers.
5 Number of aquatic plants calculated on the basis of the proportion in USA and Canada (Reaka-Kudla
et al. 1997).
Figure 2.1 Plant biodiversity hotspots in South America
Background Paper Nr. 1 19
alpinus. The greatest freshwater fish diversity is found in Southeast Asia, tropical South America and
central Africa although many fishes have not been scientifically described. Brown (1994) demonstrated
the latitudinal trends in African freshwater molluscs which decreased in diversity from the tropics
towards the Mediterranean and southern Africa.
McAllister et al. (1986) found that species diversity of North American fishes was related to a measure
of ‘aridity’. Data indicated that more species were found in moist compared to arid areas. The study
showed at what critical level of moisture fish diversity began to increase rapidly.
Although arid areas may be poor in species, desert springs or other water bodies may be rich in
endemic organisms, for example, there are vast numbers of endemic spring-snails and fairy shrimps in
the arid west regions of the USA.
‘Hotspots’: areas rich in species and endemics
‘Hotspots’ are geographic areas rich in species. ‘Hotspots’ often dominate over latitudinal and other
gradients as shown for plants (Fig. 2.1) and animals (Fig. 2.2). McAllister et al. (1997) listed countries
that are rich in absolute numbers of aquatic species and in the number of species per unit area. There
Table 2.3 River basins and sub-basins with the highest number of native species per
unit area (in descending order) and the number of associated large and major dams
(Revenga et al. 1998)
River or sub-basin Number of large or major dams
Kapuas, Indonesia, Borneo 0
Rio Negro-Amazon, northern South America 0
Chao Phrysa, Thailand 3
Hong, China 3
Xing Jiang (Hsi Chiang), China 7
Lake Victoria-Nile, Africa 1
Susquehanna, United States 124
Ohio-Mississippi, United States 711
Mekong, Cambodia, Laos, etc. 4
Alabama-Tombigbee, United States 103
Orinoco, northern South America 10
Lake Ontario, United States and Canada 2
Madeira-Amazon, South America 0
Magdalena, Colombia 5
Uruguay, South America 2
Hudson, United States 53
Fly, New Guinea 0
Yalu-Jiang, North Korea and China 3
Yangtze, China 17
Parana-Paraguay, South America 0
20 Biodiversity Impacts of Large Dams
is some evidence that ‘hotspots’ for different groups of freshwater organisms are correlated, for example
McAllister et al. (1997) found that the numbers of amphibian and fish species in 12 ‘mega-diversity’
countries were highly correlated (r=0.937). Revenga et al. (1998) catalogued the number of fish species,
endemic bird areas and percentage area of wetlands for most of the world’s primary watersheds. From
this was determined the twenty richest basins (Table 2.3). The number of species varies with size of
basin; generally larger basins have more species.
Global ‘hotspots’ of freshwater mollusc species include the Mobile Bay and Tennessee River basin
faunas in the United States (North America has the richest freshwater mollusc faunas in the world); the
lower Mekong River of southeast Asia (160 species of which 72% are endemic; the upper Mekong has
been studied by the Chinese although data are not available); the northern Western Ghats, India (71
species, 18% endemic); the Lower Uruguay River and Rio de la Plata (93 species, 37% endemic);
lower Zaire (96 species, 25% endemic); Lake Tanganyika (83 species, 64% endemic); Balkans region
(190 species, 95% endemic); Lake Baikal (nearly 180 species, about 67% endemic). Note that the
freshwater molluscs display very high levels of endemism. Alfonso and McAllister (1994) used an
equal-area grid to show geographic patterns of freshwater molluscs, marine fishes and terrestrial
mammals in the region surrounding the Great Whale River Hydroelectric Project (Fig. 2.3). Their approach
helped to identify gradients in species numbers and determine any ‘hotspots’ in species including
Global, regional and national ‘hotspots’ are often the dominating feature in geographic patterns of
biodiversity. Hence it is vital that they should be taken into account in evaluating prospective dam sites.
However more accurate identification of freshwater ‘hotspots’ is needed. If a standardised method was
used for different groups it would assist in identifying common underlying factors and assist in application
of the data to environmental impact analyses.
Figure 2.2 Animal biodiversity hotspots in Southeast Asia
Background Paper Nr. 1 21
From data provided by Revenga et
al. (1998), the 11 most species-rich
watersheds were determined (Table
2.4). Kottelat and Whitten (1996)
gave 298 species (equal to 37.0/
) for the Mekong
watershed instead of 244 by
Revenga et al. (1998). The number
of fish species per 100,000 km
given in the table although a better
method is to plot the species and
area data on log-log axes and see
which basins lie above a line of best
fit (Fig. 2.3) (Groombridge 1992;
McAllister et al. 1997).
In Table 2.4 the Nile River and Lake
Victoria watersheds were combined
(Revenga et al. 1998 listed them
separately). This results in a value
inflated by a few percent for those
species shared between the river
and the lake. The Lake Victoria sub-
basin has 343 species and 121
, while the rest of the Nile watershed has only 129 species and 4 species/100,000
. The difference in diversity is due to the rich flock of endemic cichlids in Lake Victoria (now largely
extinct due to the introduction of the predatory Nile perch, Lates niloticus).
The Amazon, Congo, Nile, Paraná and Yangtze watersheds are the most species rich, with the Amazon
far ahead of the others. When area is taken into account, the Indonesian Kapuas and Thailand Chao
Phryas watersheds are significantly richer. However area correction will not cover ‘hotspots’ lying partly
within a basin any more than they will within a country. Using Revenga et al.’s (1998) data for sub-
watersheds shows this quite clearly. The Rio Negro, sub-watershed of the Amazon has 600 species
with 83 per 100,000 km
, the Ohio sub-watershed of the Mississippi has 281 species and 57 per
Figure 2.3 Plot of fish species on log-log axes
Table 2.4 Number of fish species in the world's 11 most species-rich primary watersheds
Watershed/continent Number of fish species Number of species/100,000
Amazon, South America 3,000 49
Congo, Africa 700 13
Nile-Lake Victoria, Africa 432 12
Mississippi, N. America 375 12
Paraná, South America 355 14
Yangtze, Asia 322 19
Kapuas, Indonesia, Asia 320 360
Orinoco, South America 318 33
Xi Jiang (Pearl), Asia 290 71
Mekong, Asia 244 30
Chao Phrya, Thailand 222 124
22 Biodiversity Impacts of Large Dams
, while Lake Victoria, a sub-watershed of the Nile, has 343 species and 121 species per
Using as closely-spaced geographic grid as the data will permit, would provide the most precise
localisation of species or endemic species ‘hotspots’. ‘Hotspot’ analysis can be a useful tool in evaluating
potential impacts of different dam sites.
Phylogenetic and ecological diversity
Species are the most common units used in evaluating biodiversity. Yet species are only one unit in a
hierarchy: Since families are more distinct genetically, ecologically and behaviourally, and generally
have a more ancient origin than species, many taxonomists and conservationists would give a higher
priority to conservation of the sole species representing a family than to one of a family with numerous
Alien (exotic) species
Disturbed environments created by dams can foster populations of alien species and that diversions
associated with dam projects may enable the invasion of such species. In the environmental assessments
of dams, alien species should be separated from indigenous ones, and not tabulated in measurements
of local indigenous biodiversity. In general, dam projects should not foster the introduction of alien
Background Paper Nr. 1 23
3 Impacts of dams on biodiversity
Species movements up and down stream
There are a number of different migratory patterns of river-dwelling species. These include the well-
known anadromous fishes e.g. salmon and hilsa (Fig. 3.1) and the catadromous fishes such as eels.
Adults of anadromous species migrate up rivers to spawn and the young descend, while the reverse
occurs with catadromous species. But many other freshwater fishes move up rivers or their tributaries
to spawn, while the glochidia larvae of freshwater mussels ‘hitch rides’ on host fishes. To help counteract
the drift downstream of their larvae, some aquatic insect adults such as mayflies and stoneflies fly
upstream to lay their eggs (Hynes 1970). Dams block these migrations to varying degrees. However
most waterfowl are able to fly over dams. Reservoirs provide waterfowl habitat and may aid longer
migrations by providing ‘stopover’ sites. Waterfowl is used in the present study to cover all wetland bird
families (e.g. in the Ramsar Convention), including divers, grebes, cormorants, Anatidae (swans, geese,
ducks), coots and rails; “shorebirds” (synonymous with waders); and some other wetland bird families
notably gulls, terns, herons and egrets.
The blockage of fish movements upstream can have a very significant and negative impact on fish
biodiversity. Many stocks of Salmonidae and Clupeidae have been lost as a consequence. In the
Columbia River, USA, more than 200 stocks of anadromous, Pacific salmonids became extinct. Sturgeon
populations in the Caspian Sea rely on hatcheries, mainly in Iran, since Russian dams block natural
spawning migrations. Hydroelectric dams in the Amazon basin have halted the long distance upstream
migration of several species of catfishes and interrupted the downstream migration of their larvae. On
the Araguaia-Tocantins River basin, Brazil, several species of migrating catfish have been drastically
reduced in abundance as a result of dams; catches in the downstream fisheries have been reduced by
McDowall (1992, unpublished information) noted that diadromous fishes (those that migrate at regular
phases of their life history between freshwater and the sea, comprise about 250 species, <1.2% of all
fishes species but form 3% of those classed as endangered. He observes that amongst them are
species of great importance to fisheries, out of proportion to their number. Due to their occupation of
connected habitats through which passes are needed at two or more life history phases, they pose
special problems for conservation. In particular the diversity of habitats used, the extensive areas
occupied, the spatial separation of the habitats and the need for fish passage between them. Fishways
must be designed to assist not only upstream and downstream migrations of large, fast-swimming
migrants such as salmon that can pass substantial barriers, but also those of the lesser-known climbers
like eels and gobies that require continuous dampened surfaces on which to move.
Holcík (1999) stated that while dramatic declines in migratory species such as lampreys, sturgeons,
salmons and clupeids were well known in European rivers, other fishes, the so-called resident or non-
migratory fishes which perform in-stream movements require attention. These include the European
minnow, Phoxinus phoxinus, the Japanese sculpin, Cottus pollux, the grayling, Thymallus thymallus
and Balon’s ruffe, Gymnocephalus baloni. Even small-sized species such as the white bream, Abramis
bjoerkna, were found to migrate up to 60 km from the place they were tagged. Some of these small-
sized species are among the most endangered. Damming can contribute towards their decline as in
the unique and critically endangered percid, the asprete, Romanichthys valsanicola, endemic to the
Arges River in Romania (Craig 2000). Artificial barriers also lead to the dramatic decline of the
endangered cyprinid fish, Anaecypris hispanica in Iberia.
24 Biodiversity Impacts of Large Dams
In general, a river is a one-way system for molluscs, as many molluscs can only move downstream by
drifting or being dislodged by flood events and moved downstream. But some species with a larval
form can move significant distances upstream with the aid of a third party, e.g. host fish during the
Movement of matter up- and down-stream
Nutrients. Conventionally, rivers have been regarded as the one-way transfer of matter downstream.
Experiments by the Fisheries Research Board of Canada several decades ago showed that removal
of spawned out sockeye salmon carcasses from streams reduced the growth of fry in the following
year. Recently there has been greater appreciation that migrating species carry nutrients upstream.
Reimchen (1995) proposed that intensive coastal catches of Atlantic salmon in the Queen Charlottes,
Canada reduce the nutrients in adjacent stream riparian zones and estuaries. The contribution of
nutrients from both Atlantic and Pacific salmon carcasses has been linked to riparian tree growth
Figure 3.1 Some migratory freshwater species
Background Paper Nr. 1 25
(Kavanagh 1999). Data from 45 watersheds in British Columbia suggests that up to half the nitrogen
stored in giant old-growth trees originates from sockeye salmon, using the nitrogen isotope N
tag. The largest source of N
is in the ocean. Cederholm et al. (1999) reviews the contribution of
Pacific salmon carcasses to the flow of nutrients and energy for aquatic and terrestrial ecosystems.
Anadromous fishes carry nutrients in their bodies as well as gametes up river. The fishes and their
eggs are used for food by a variety of aquatic and terrestrial predators, scavengers and detritivores.
Decaying bodies, eggs and faeces of the consumers provide nutrients for the algae and other plants.
Cederholm et al. (1999) calculated that in the Columbia River, USA prior to dam construction, spawning
salmon contributed to 45,150 metric tonnes of fish bodies to the aquatic and terrestrial ecosystems. By
1997, following construction of multiple dams and impacts of other human activities, only 3,400 metric
tonnes were contributed, 8% of the pre-dam level. The decreased production could be self-perpetuating,
since small stocks produce lower amounts of in-stream nutrients for themselves, as well as other
species. To a degree fertilisation from the lower riparian vegetation will also affect fish productivity. The
fall of insects, leaves and twigs into streams, which serves as direct or indirect food can be expected to
decline, as riparian vegetation growth diminishes.
There is no reason to suppose that the upstream transport of nutrients is restricted to anadromous
salmonids. It can be expected that andromous hilsa in southeast Asia, Arctic whitefish of the genera
Coregonus and Stenodus, lampreys such as Petromyzon marinus, and New Zealand retropinnids
such as Stokellia anisodon would also transport nutrients upstream during their spawning migrations.
Turbidity. Reservoirs trap suspended particles, reducing turbidity downstream. Many species are
adapted to natural turbidity. For example turbid water catfishes have small eyes, refined senses of
smell and touch in their sensitive barbels. The turbid water helps conceal the fish and other biota from
visual predators like birds. When normally turbid water becomes clear below dams, the indigenous
species may find themselves at a disadvantage. Other animal species may move in, filter feeders and
aquatic vegetation may flourish. Sediment burrowing species may find their habitat has diminished.
Flood plain ecosystems and deltas may no longer be replenished by the annual transport of sediment.
Silt and increased turbity, above natural levels, can interfere with primary production (Arthington &
Welcomme 1995). In the Mekong River system, silt levels increased following deforestation. This resulted
in siltation of the river, lakes and swamps threatening the river fisheries.
Large organic debris (LOD). This consists of branches and tree trunks that fall into the river because
of age, storms, beaver activity and eroded banks (Maser & Sedell 1994; Bryant & Sedell 1995; Stevens
1997). Numerous organisms feed on LOD which is often the first link in the food chain. Trees can also
play a complex role in creating habitats e.g. they divert, slow and speed up current flow, they shelter a
variety of biota from currents and predators and create feeding stations. On land, LOD helps stabilise
slopes and reduces erosion, and is converted into humus which helps hold water and moderates the
runoff. In estuaries, along shores of lakes and coastal areas LOD functions as a source of food, energy
In the Santilla River, Georgia wood represented 4% of the total habitat, yet supplied 60% of the
invertebrate biomass and 78% of the drifting invertebrates (Bryant & Sedell 1995).
Dams tend to ‘sieve out’ LOD. The logs and branches may become waterlogged and sink, drift onto the
shore or are removed by booms or other systems designed to protect turbines. If the integrity of
downstream ecosystems is to be maintained, then LOD input must be sustained.
Species and materials may move laterally away from the river, extending the effect of river changes to
a band of varying width, parallel to the river.
Watering wildlife. As long as there is sufficient river flow below the dam, wildlife such as deer, antelope
and elephants will come to the water, especially in the dry and hot season for drinking. Hippopotamuses
will use water of sufficient depth as a day-time refuge, emerging to forage at night. Many birds may fly
in to drink. These lateral movements can extend to several kilometres from the river. The reservoir
itself, however, may serve as a source of water during the dry season or droughts, to wildlife living
Watering terrestrial vegetation. Water release protocols can lower water tables lateral to the rivers
which may affect vegetation there. According to LaRoe (1995) riparian ecosystems along most major
26 Biodiversity Impacts of Large Dams
western rivers of the U. S. have changed as a result of water development and flood control. Losses of
riparian forest downstream of dams have been reported throughout western North America. Cottonwood-
willow stands are being replaced by non-native woody species such as Russian olive and tamarisk.
This may result in diminished LOD input.
Loss of riverbank forests. The extinction of species may be related to the loss of gallery forests
adjacent to rivers which became submerged following dam construction. The land-snail, Anthinus
albolabiatus, was formerly endemic to gallery forest adjacent to the Uruguay River but became extinct
after the formation of the Salto Grande Dam, Uruguay (Mansur unpublished information). This factor is
of concern with regard to proposed dam projects in South Africa, where much of the remaining forest
is preserved on the steep sides of valleys, which are also suitable sites for dam construction.
Maintaining populations and gene flow. The main stem of a river performs two related ecological
functions to biota in tributaries. Periodically, populations of a tributary stream species, particularly fish,
may go extinct and may be restocked from nearby rivers. Secondly individuals, as described above,
may ascend a non-home tributary and contribute to the resident population’s genetic diversity. For
example salmonids are known to home to their natal streams for spawning, using celestial and olfactory
cues. A small percentage of migrants are known, through tagging studies, to stray into adjacent tributaries
or rivers. Straying can serve to repopulate streams and also contribute to the genetic diversity of
populations through gene flow.
A single dam and more significantly multiple dams along a given river interfere with the genetic bridging
function of the mainstem. Thus dams on the main stem can influence species diversity in lateral
tributaries, even though there may be no changes in water flow or quality characteristics. In the Murray
River, Australia, river regulation has lead to the separation of billabongs (oxbow lakes) from the main
channel habitat and thus for many molluscs in particular freshwater gastropods. Since river regulation
was introduced some mollusc species have become extinct.
Oxbows, wetlands and springs. Oxbows, ponds, lakes and wetlands are often isolated in the floodpain
from the river’s main stem. These may be replenished with water, biota, sediment and nutrients during
natural, seasonal floods. Levelling out of river discharge has been known to prevent these periodic
linkages with the mainstem. Most of the 50 species of fishes (many endangered) in the Austrian Danube,
depend on the connection between the river and its backwaters (Schiemer & Spindler 1989; Balon &
Transmission lines. Transmission lines affect biodiversity on land. The use of herbicides to control
plant growth under power lines probably reduces native plant diversity in favour of weed species,
which are often exotic. Baker (1999) reported that power lines corridors can serve as refuges for rare
species. Tree trimming may provide a haven for native ‘sun-loving plants’. Shrub communities may
flourish under power lines and provide habitat for nesting and migratory birds. Only nine snoutbean
plants were thought to exist in all of Kentucky, USA. However an East Kentucky Power Plant Corporation
(EKPC) study showed that about two thousand specimens survived under their power line. EKPC
adjusted their mowing schedule and removal of woody plants and educated other utilities about protecting
native plants. Utility rights of way may harbour rare birds, amphibians, reptiles, tree snails, mammals
and other species.
Above the reservoir. Water quality, flow and seasonality of flow are not normally disrupted in the
upstream area above the reservoir so impacts are generally less than for the reservoir and downstream
areas. Nevertheless, the dam and the reservoir affect migratory movements of species into and out of
this upstream area. The genetic exchange with downstream segments is reduced or prevented. A
study was made of molluscs upstream in a braided river that enters a reservoir on the River Inn in
Austria (Foeckler et al. 1991). Data shows that there was a decline of 10 species upstream of the
reservoir. This was due to channelisation of the braided area, an increase in the overall river gradient
and a consequent reduction in the active floodplain area. The extirpation of freshwater mussel populations
upstream of the dam construction at Lake Pepin on the Mississippi River, USA, was due to the lost
migratory fish host species, skipjack herring, Alosa chrysochloris (Eddy & Underhill 1974).
Reservoirs. In the construction of reservoirs, the clearing of vegetation, movement of earth and rock,
the presence of humans and machinery, bringing in construction materials, use of explosives, noise,
Background Paper Nr. 1 27
and reducing or cutting off river flow and increasing turbidity, will affect biodiversity. Removal of forests
or other vegetation over a wide area, excavation, earth and rock movement and reduction in river flow
are the most significant. Some of the on-site activities are mirrored in off-site disturbances such as the
mass displacement of earth and rocks and road building.
During reservoir filling the river and any associated wetland areas become inundated. Riffles, runs and
pools of the river are lost beneath the rising waters, leading to the extirpation (or extinction) of habitat
sensitive riverine species with tightly defined niche requirements. Fishes in rivers are generally well
adapted to flowing water. Similarly molluscs are often restricted to specific habitats within the river
system, e.g. some species are bottom-dwelling filter feeders, others live in weeds at the edge of the
The transformation of a river to a reservoir therefore poses a problem for the resident, mainly riverine
species that are not adapted to the new conditions. Lacustrine fishes have been introduced into reservoirs
in a number of cases e.g. Lake Kariba although this may pose new problems. In eastern Canada, lakes
and streams, which have emerged fairly recently from glaciation, contain a number of fish species able
to dwell in both habitats. However some species like the longnose dace, Rhinichthys cataractae, and
rainbow darter, Etheostoma caeruleum, are adapted to running water only.
Table 3.1 Factors affecting the life cycle stages of unionid molluscs
(amended after Chesney & Oliver 1998). Items highlighted in bold are
directly impacted (after impoundment) and those in italics indirectly
Factor Life cycle stages affected
Exploitation (pearl fishing, shells
for buttons or seeding)
Fish host stock size Glochidium
Mussel stock size Fertilisation, glochidium numbers
River bed Adults, juveniles
Flow regime Fertilisation, glochidium
infection,settlement, juvenile and adult
Suspended solids Adults, breeding and brooding
Eutrophication Adults, juveniles
Nitrogen Adults, juveniles
Phosphate Adults, juveniles
Dissolved oxygen All stages
Conductivity Adults, juveniles
Calcium Adults, juveniles
pH (acidity) Adults, juveniles
Interstitial particulates Juveniles
Interstitial water chemistry Juveniles
Industrial pollutants All stages
Pesticides All stages
28 Biodiversity Impacts of Large Dams
Molluscs generally show a drop in species richness from pre- to post-impoundment. Unionid mussels
are exposed to a number of changes in the reservoir impoundment (Table 3.1). The most important
ones include: changes in the fish host population size (needed for the glochidia stage of the mussel
larvae), reduction in the mussel population size (fertilisation success), eutrophication (affecting adult
and larvae) and dissolved oxygen levels (low levels of oxygen in the profundal zone would eliminate
mussels from that zone). Eutrophication can limit the interstitial habitat of post-glochidial juveniles.
Raised nitrate and phosphate levels are especially deleterious to juveniles. Organic debris can clog
benthic interstitial spaces. Some of these effects can extend downstream of the dam. Table 3.1 shows
the environmental and biotic factors which influence the various stages in the life cycle of unionid
Extinction of 38 out of 42 taxa of molluscs in the Mobile Bay, Alabama, USA, basin occurred when the
big river shoal fauna were covered by deep standing water in the impoundments and subsequently
buried under increased siltation (Bogan 1998). However molluscs may comprise an important part of
the benthic fauna of some reservoirs. In the man-made Lake Kariba, molluscs made up nearly the
entire biomass of the benthic animals (prosobranchs 4.1% and bivalves 95.8%) (Machena & Kautsky
In the Tennessee River Basin, U.S., several molluscs are under threat of global extinction following the
construction of dams and the subsequent regulation of flow. A number of gastropods of the family
Pleurocercidae are under threat as they persist on clean-swept shoal areas below dams on the river
(Bogan 1998) and three other species have been extirpated from the river (Haage & Thorp 1991). Over
85 mussel species were known in the Cumberland River of the upper Ohio-Tennessee River basin
prior to the construction of impoundments and locks between 1916 and 1923 (Blaock & Sieckel 1996).
In Kentucky portion of the lower Cumberland, for example, there were 25 species in 1911, 15 in 1981
and only 4 in 1994, for a total of 21 extirpations.
Extensive mussel beds contribute to the health of rivers by their filtration power. Reductions in those
beds reduces this ecological function. However replacement mollusc faunas have developed in some
River sections with steep gradients or escarpments sometimes offer optimal conditions for locating
hydroelectric and other dams. However, those locations may provide special fast-water habitats for
species with only scattered distribution, or local endemics. In some cases species may multiply following
construction of a dam. The status of a freshwater pulmonate, Bulinus truneatus, changed from rare to
common in Lake Volta, Ghana. This led to an increase in the level of urinary schistomiasis infections in
the region (Brown 1994).
Williams et al. (1989) listed 12 darter species (family Percidae) as endangered or threatened, and 9 of
special concern in Tennessee, USA, a state which has many dams due to the activities of the Tennessee
Valley Authority. The species of darter found in streams, now flooded by reservoirs in the Tennessee
River system (Neves & Angermeier 1990), had well defined river run and pool niche requirements. In
Texas, USA, the filling of a reservoir was involved in the extinction of the spring-dwelling Amistad
gambusia, Gambusia amistatensis (Miller et al. 1989).
In some tropical reservoirs the overall number of fish species has increased, although several riverine
species have disappeared, e.g. Lakes Kariba, Zambia and Zimbabwe, and Ayame, Côte d’ Ivoire,
(Kolding & Karenge unpublished information; Gourène et al. 1999). Tilapias of the family Cichlidae are
usually the most successful in these lakes.
The extent of entrainment of larval and juvenile fishes in hydroelectric turbines varies according to
flushing duration, depth of extraction and species present (Walburg 1971; Travnichek et al. 1993).
Passage through turbines of young anadromous salmonids, en route downstream, is a well-known
source of mortality. In the catadromous eels, family Anguillidae, it is the downstream migrating adults
which are killed by the turbines.
Reservoirs promote waterfowl and many dams have substantial populations. The type of shoreline,
shallow, with fringing vegetation, supports greater species diversity and larger numbers compared to
steeply shelving mostly deep water sites. A substantial number of dammed sites support nationally or
Background Paper Nr. 1 29
internationally important waterfowl assemblages. In more arid areas, creating dams increases numbers of
birds able to remain all year round in otherwise largely seasonally dry places. The presence of a dam has
substantively altered the migration phenology and distribution of some species.
Of 957 Ramsar sites (Frazier 1999) 25% had natural lake types and 10% had artificial wetland types; 78%
of the latter were dammed sites. Some of the Ramsar sites with dams supported internationally important
waterfowl populations, though these sites were small in number compared to natural wetlands. The
significance of such artificial wetlands for waterfowl is difficult to interpret. Nevertheless some dammed
sites did prove suitable for large waterfowl assemblages.
A study of natural lakes and
dammed reservoirs in
Switzerland (Table 3.2)
showed that waterfowl species
diversity was considerably
higher on natural lakes than on
dammed lakes. Nevertheless
there is considerable overlap in
the number of species on the
two types of water bodies. The
most common five species
were the same on natural lakes and dammed lakes, common coot, tufted duck, mallard, pochard and
great crested grebe (Fig. 3.2). Damming of rivers has increased the number of open water sites available
to wintering waterfowl.
The United Kingdom is of major importance for wintering waterfowl which use the Eurasian-African flyways.
Because of its relatively mild winters, its wetlands seldom freeze over, and, in severe winter weather in
continental Europe, it additionally serves as a cold weather refuge for waterfowl species. Table 3.3 lists the
number of natural and artificial wetlands in the UK which support internationally important numbers of
Figure 3.2 Great crested grebe (Podiceps Cristatus)
Table 3.2 Waterfowl species diversity in Switzerland
Natural lakes Dams
Number of sites 8 6
Number of species per site 14-30 11-20
Total number of species 33 23
30 Biodiversity Impacts of Large Dams
Table 3.3 shows that estuaries and natural inland waters are of great international importance for
wintering waterfowl. Estuaries and coasts tend to have higher numbers of species because they attract
waders as well as waterfowl, while inland systems generally support only wildfowl in internationally
important numbers. A much smaller number of artificial wetlands are of international importance for
wintering waterfowl. Only 11 support one or more internationally important populations and only three
have large overall wintering populations >20,000 waterfowl.
For several species a large proportion of nationally important lakes are artificial ones: 40% or more of
important sites are artificial for eight waterfowl: little grebe, great crested grebe (Fig. 3.2), great cormorant,
gadwall, northern shoveller, pochard, tufted duck and common coot. A further analysis showed 27
species on five natural lakes in the UK, as compared with 33 species on artificial lakes, in contrast to
the situation in Switzerland. On the negative side was the fact that two exotic species, the Canada
goose and the ruddy duck were present on dammed lakes. Those species have expanding populations
and are of conservation concern.
Much of South Africa is arid and has few natural permanent water bodies. Almost all permanent water
bodies are dammed sites constructed for water storage. There are at least 517 major reservoirs and
numerous small, farm dams. The presence of these new wetlands has had several consequences. At
least 12 impoundments support major and important concentrations of waterfowl. Suitable conditions
have been provided for the Pelecaniformes (pelicans, darters and cormorants), 70% of the global
population of the South African shelduck during moulting, and refuges for species of national
conservation concern such as the pink-backed pelican, Pelecanus rufescens, and the Caspian tern,
Hydroprogne caspia. Negative impacts in overall waterfowl assemblages in southern Africa are due to
the loss of many of the former natural marshes and riverine habitats through reduction in river flow,
removal of seasonal flow variability and consequent changes in sediment movement and channel
stability. In addition poor dam management may cause sudden major releases of water, causing major
downstream floods in areas that have had little or no flood activity for years. That can affect species
that use unvegetated river banks and sand banks between river channels.
Running compared to still water impacts. The construction of reservoirs converts lotic (running)
into lentic (still water) habitats. Species dependent on running water will diminish or disappear. In
almost all cases, the diversity of fish species will drop (McCully 1996). Reservoir fisheries are one of
the frequently claimed benefits of impoundments. The changes in catches following impoundment are
variable. However the catches in new reservoirs frequently go through a “boom and bust” cycle
(Welcomme 1995), with catches initially increasing following filling of the reservoir and then declining.
Therefore impact assessments of dams should be based on the long term catches.
Table 3.3 Natural and artificial wetlands in the UK with internationally important numbers of wintering
Wetland type Estuaries and
Inland wetlands Dams and
Sites with > 20,000 waterfowl: Ramsar
Number of sites 42 12 3
Average number internationally important
4.7 2.8 2.0
Other sites with 1 or more
internationally important species:
Ramsar criterion 6
Number of sites 15 49 8
Total number of sites supporting
internationally important species
52 60 11
Background Paper Nr. 1 31
The shore-edge (marginal) ecosystem also changes. A study of the Upper Mississippi River, USA
(LaRoe 1995), an area with many dams, showed that open water and marsh habitats generally increased
between 1891 and 1989, although at the expense of grass-herb, woody terrestrial, and agricultural
habitats. For example, in ‘Pool 8’, open water and marsh increased from 3,600 hectares in 1891 to
9,500 hectares in 1989.
The edges of new reservoirs are often exposed to erosion, while deeper areas are sedimented. In the
Upper Mississippi River sedimentation rates of one to three cm per year have been measured (LaRoe
1995). Erosion was more prevalent in shallow areas and sedimentation at deeper depths, the processes
converging between 0.9-1.5 m.
Creation of new sublittoral and profundal zones. Water oscillations in the littoral zone reduce its
suitability for species requiring stable conditions. Some mobile species, such as shorebirds, may find
this habitat suitable for feeding.
The nature of the deeper or profundal zone will depend on the climate, preparation of the reservoir
prior to filling and other factors. In boreal and arctic areas the deeper waters are normally cooler than
the surface waters and this provide habitats for both warm- and cool-loving species. If the trees and
other vegetation are not cleared from the reservoir, then decomposition commonly leads to low oxygen
levels in the profundal zone usually only suitable for anoxic microorganisms.
Weeds, exotics and diseases. The changed or fluctuating conditions in the reservoir may lead to
opportunities for weed or exotic species e.g. the water hyacinth, Eichhornia crassipe.
Increases in the number of mollusc-borne diseases following dam construction in various countries.
For example at least four genera of mollusc-borne human diseases have increased as a result of
impoundments in Thailand (Woodruff & Upatham, 1992).
Following the construction of Lake Volta Ghana, the gastropod, Bulinus truneatus colonised the lake,
replacing Bulinus globosus and other species which were not able to withstand the lake-level fluctuations.
This lead to an increase in the level of urinary schistomiasis in villages around the lake (Brown 1994).
The stocking of fishes for anglers in Texas, USA, reservoirs has resulted in the introduction of exotic
“floater” freshwater mussel groups, transported as glochidia on the fish hosts (Howells et al. 1996).
Mercury. Mercury is in a harmless inorganic form in many soils (McCully 1996). Bacteria which feed
on decomposing matter in a new reservoir transform the mercury into methyl mercury which passes
up the food chain from plankton to fishes and those species that feed on fishes, including humans.
Biomagnification results in higher mercury levels as mercury ascends the food chain (Rosenberg et al.
1997). In the La Grande Phase of the James Bay, Canada, hydro project it was found that reservoir fish
became contaminated with mercury at levels exceeding World Health Organisation (WHO) standards
(Dorcey et al. 1997). Sixty-four percent of the Cree living in the La Grande estuary had blood mercury
levels far exceeding WHO standards (McCully 1996). Mercury can also negatively affect wildlife that
prey on mercury-contaminated fishes. Loons are severely affected by mercury pollution. Sport fishing
also plays a role in the local economy, and mercury contamination creates unfavourable publicity.
Sedimentation. Reservoirs tend to serve as sediment traps since river velocities and carrying capacity
for particles decrease in reservoirs (McCartney et al. 1999). However sometimes fluctuating water
levels in reservoirs erode the shores and add to the turbidity of the reservoir discharge. Sedimentation
can degrade habitat both in the reservoir and below the dam, as well as reduce storage capacity. Many
of the molluscan extinctions in the Mobile Bay, USA drainage, following multiple impoundments, were
due to siltation (Bogan 1998). The degree of tolerance to silt cover depends on the species of mollusc.
Suspended silt may reduce the feeding efficiency of filter-feeding bivalves and other species.
About 50 km
of sediment, nearly 1% of global reservoir capacity, was estimated in 1997 to be trapped
behind dams. Keeping sediments flowing through reservoirs will benefit both reservoir life and the life
of downstream ecosystems such as flood plains and deltas. Many of the existing potential solutions,
such as dredging and sluicing, have economic or environmental limitations.
32 Biodiversity Impacts of Large Dams
In the downstream segment, most of the impacts of a dam are negative. In a preliminary assessment
of 66 case studies of the impact of dam construction on fishes, based on qualitative information, 73%
of the impacts were negative and only 27% were positive. About 55% of the impacts were below the
dam and linked to fish migrations and to floodplain access.
In the distribution of molluscs along a 240 km stretch of the Little River in Oklahoma, USA there were
mussel extinction gradients downstream from large impoundments (Vaughan & Taylor 1999). With
increasing distance from the dam there was a relative increase in mussel species richness and species
abundance. Only those stretches furthest from the dam contained the relatively rare species. Richness
declined below each successive dam, with a multiplier effect.
Overall volume of flow. Some reservoirs have been filled by cutting off all or almost all flow downstream
of the dam, e.g. Cabora Bassa, Mozambique (Jackson 1989) with the consequent loss of organisms.
Large aquatic species such as sturgeons, crocodiles and dolphins require minimal flows in which to
navigate and feed. Such species may be affected by reduced flows including a reduction in the area of
habitat utilised. This may lead to smaller populations, reduced growth rates and, where populations are
already at risk, extirpation or extinction.
A certain level of downstream flow is needed to maintain a minimum volume and area of habitat,
oxygen concentration and other ‘desirable’ in-stream conditions and avoid lethal temperatures. Normal
seasonal flow patterns are a key to maintaining river biodiverstiy. Balancing reservoirs may help avoid
pulse discharges, delay peak discharges and reduce them to an ecologically acceptable level and
guarantee a certain minimum discharge (Moog 1993).
Constructing dams just above large tributaries can moderate changes to downstream flow patterns. If
the tributary has a similar seasonal flow cycle to the mainstem, then the downstream flow pattern and
seasonal cues will be less impacted than if the mainstem dam had been sited below the tributary.
Seasonal variability of flow and flood plains. The pattern of flow of a river undergoes a regular
series of changes with the seasons. The patterns can differ profoundly from region to region e.g. in an
Indian river the peak flow may be during the monsoon, in an Arctic river during snow-melt or ice break-
up. The expansion and contraction of the river controls living space and access to particular habitats.
It is to these profound seasonal patterns that the species in a drainage basin adapt. It is from river flow
events that species take cues to migrate, spawn, etc. The rhythm of the river is thus tied intimately to
the life of river species. Dams alter the natural flow of rivers as is shown in the Colorado River, USA,
following construction of the Lake Powell River dam (Fig. 3.3).
Figure 3.3 Discharge before (dotted line) and after (continuous line) dam
construction in the Colorado River, USA
Background Paper Nr. 1 33
Some species are adapted to strongly seasonal flow regimes with flash flooding, as in certain river
systems in Australia. Drastic declines in the molluscs of the Murray-Darling River system have been
attributed to: predation and sediment disturbance by introduced fish such as the common carp, Cyprinus
carpio (Fletcher et al. 1985), changes in flow patterns through intensive flow regulation after
impoundments and possible changes in algae, bacteria and fungi, which form potential mollusc food
A study by Almada-Villela (unpublished information) in the Naga Hammadi barrage area of the lower
Nile, Egypt, showed the certain presence of 34 species and the questionable presence of an additional
3 species for a possible total of 37 species. Boulenger (1907) identified about 72 species in the Lower
Nile. It was deduced that approximately 35 species have disappeared from the Lower Nile or have
become very rare, since the construction of the Aswan High Dam and other dams and barrages in the
Water table changes. Water diversion for irrigation may lower the downstream water table adjacent to
the river. Further, the ‘levelling out’ of floods, may reduce seasonal recharging of the water table. The
supply to springs, artesian flows and cave streams may also be affected.
Cave streams and some aquifers sometime have species characterised by reduced or absent eyes,
loss of pigmentation and enhanced non-visual sensory systems. According to David Culver (personal
communication) there are 1,300 described obligatory cave species in the United States alone, while
the undescribed species probably number several times that value. Many cave organisms are restricted
to a single cave or cave complex. Cutting off the cave water supply, either temporarily or permanently,
may lead to extinctions. When selecting potential dam sites, planners should be alert to known cave
dwelling speciesor check for the presence of unknown species.
Of the world’s 110 described species of cave fishes, a high proportion are threatened. Proudlove
(1997) reviewed the conservation status of hypogean (underground) fishes. The percentage of
threatened hypogean fish species has risen from 18% in 1977 to 87% in 1996. Most of the species
have very limited distribution. The primary threats are habitat degradation (e.g. quarrying and siltation
due to dams and logging), hydrological manipulations (e.g. water removal for human consumption or
irrigation and damming), environmental pollution (e.g. eutrophication and poisoning from agricultural
and industrial runoff), overexploitation (capture for the aquarium trade) and introduced exotic animals.
Abiotic changes. In summer, in temperate lakes, solar radiation heats the eplimnion but not the
hypolimnion. The hypolimnion is often anoxic. In the autumn the lake undergoes mixing and some heat
is transferred to the bottom layers. Water discharge from the dam is usually below the epilimnion.
Therefore in summer the water discharged into the river below the dam is colder and has less oxygen
than normal and in winter it is warmer (Neves & Angermeier 1990). These physical changes can effect
the biota for long distances down the river. Discharges from the reservoir are variable usually resulting
from the requirements for hydroelectric power and not related to natural cycles. Flow below the dam
can rapidly alter from almost standing water to torrential flows and water depth, water velocity, oxygen
concentration, temperature, suspended solids, pollutants and chemical composition can change in a
very short period of time.
Many of the effects experienced downstream of a dam are in reverse of those produced in the reservoir
above them. Heat, silt, inorganic and organic nutrients retained in the lake are lost to the stream below.
Large annual variations in water level in the reservoir results in a decrease in the annual variation in
water level in the efferent river.
Inland deltas. As noted previously, dams trap sediments, diminishing the downstream supply. An
inland delta and flood plain, including a network of oxbow lakes, in the middle Danube, was supplied by
sediment during natural seasonal floods (Balon & Holcík 1999). The area produced an impressive
harvest of trees, fishes and cereals. Biota included 65 species of fishes, 11 of amphibians, nine reptiles,
41 mammals and 242 birds. Construction of a series of dams, dredging the river channel and construction
of a canal deprived the delta and flood plain of the annual supply of silt and resulted in severe alterations
in the benthos and zooplankton communities as well as the change and decrease of species diversity
and biomass. The inland delta was lost eliminating spawning, feeding and overwintering grounds for
fishes. Amongst fish species lost from the affected area were the percids, Zingel streber and Zingel
34 Biodiversity Impacts of Large Dams
zingel, both classed in the IUCN Red List as vulnerable, and the salmonids, Hucho hucho, classed by
the IUCN Red List as endangered, and Salmo labrax m. fario (= Salmo trutta m. fario) (Holcík personal
communication). The European beaver, Castor fiber, has left the territories influenced by the dam. The
mean annual fish catch has dropped by 87%.
Anti-gradient, thermal transport. North-flowing rivers such as the McKenzie River, Canada, transport
warmer water into the Arctic than that from local tributaries. This enables some species to range
further north. The same phenomenon would occur in reverse in the Southern Hemisphere. Rivers
flowing towards the tropics or down from higher cooler altitudes would permit cool water fishes to
extend their ranges.
Dams or series of dams may affect anti-gradient thermal transport and the ranges of aquatic species
which depend on them. However the thermal transport affects more than just the species in the river. In
the Arctic the northern limits of the tree-line, appears to be extend adjacent to north flowing rivers, as
well as next to large lakes which act as reservoirs of summer heat. It is not clear whether reducing the
flow of anti-gradient thermally transporting rivers, or the effects of reservoirs and discharge from either
epilimnion or hypolimnion will affect the downstream and lateral distribution of species. Analysis of
data from Russian rivers with older dams might be useful.
Water basin connections
When waters of one basin are diverted into another, impacts can be expected from changes in volume
and seasonality of flow. New biota from the source basin may invade the recipient basin and compete
with the native species. If all the water is diverted from the source basin, this will obviously have
serious impacts on any unique species or genetically different stocks.
Individuals washed down irrigation canals, especially if there is a drop from the impoundment, may no
longer be able to return and may not be able to maintain viable populations in the new habitat. In the
Nicola River, western Canada, considerable numbers of Pacific salmon fry pass down into irrigation
ditches, depleting the river populations. Screen systems have been installed in Canada and the USA.
Estuarine and marine impacts
Many of the effects in estuaries are similar to those upstream, e.g. loss of habitat and changes in
seasonal flow, turbidity and productivity. Water withdrawal on the North Caspian had the following
effects (Rozengurt & Hedgepeth 1989): 1) the mean salinity increased from 8-11ppt; 2) the estuarine
mixing zone was compressed and moved up to the delta; 3) the nutrient yield, especially phosphorus,
and sediment load were reduced by as much as x2.5 and x3, respectively; 4) biomass of phytoplankton,
zooplankton, and benthic organisms were decreased by as much as x2.5; and 5) a substantial part of
the Volga flood plains that served as a nursery ground for many valuable fishes was transformed into
drying swamps or desserts. This led to a progressive deterioration and significant decline in natural
recruitment. Commercial catches fell by as much as three to five times for three sturgeon species, x10
for bream (Abramis brama), pikeperch (Stizostedion lucioperca), Caspian roach (Rutilus rutilus), and
carp (Cyprinus carpio), and nearly x100 for the commercial fishery of Caspian herrings (Alosa kessleri
volgensis). The sevruga, Acipenser stellatus, has been saved from extinction by release of fry reared
in hatcheries over the last two decades.
Estuarine deltas. Deltas below impoundments tend to shrink, reducing habitats, because of the capture
of sediments by impoundments. The Nile Delta, Egypt, has shrunk at a rate of 125 to 175 m yr
(Rozengurt and Haydock 1993), and more saline water has invaded inland. The Danube Delta, central
Europe, shoreline is receding at a rate of up to 17 m yr
, threatening benefits from tourism to birdlife
(Pringle et al. 1993). The delta support large populations of bird species that are generally widespread
over Europe; some 170 species of birds breed in the delta, including pelicans, herons, ibises and terns.
Impoundments upstream, including 7 major dams, on the 2,860 km long river, channelisation and the
loss of the nutrient absorption capacity of upstream floodlands has meant nutrients and other pollutants
are affecting delta water quality. Bird populations are at a fraction of their historical numbers. So although
reservoirs may provide new habitat upstream their impacts on birds may be negative in the long-term.
Salinity, nutrients and reproduction. Decreased discharge rates can result in an increase in salinity
in estuaries and change the composition of species in this zone. The effects of increased salinity on
fishes of the Nile Delta, Egypt, has been documented by several authors. Abramovitch (1996), for
Background Paper Nr. 1 35
example notes that out of 47 commercial fish species in the Nile prior to the construction of the Aswan
High Dam, only 17 were still harvested a decade after its completion. The annual sardine harvest in the
eastern Mediterranean has dropped by 83%, probably the effect of a reduction in nutrient-rich silt
entering that part of the sea. The effect of lowered nutrient input is generally the greatest in the first
year of life of the fishes.
Rivers can increase nutrient levels at river mouths by two processes: entrainment and transport. Satellite
imagery can be helpful in evaluating the nutrient contribution of rivers. Reference to the October 1999
colour satellite photographs of nutrient levels show high levels at the mouths of the Yangtze, Mekong,
Ganges, Indus and Volga in Asia, Amazon, Plate and Orinoco in South America, and moderate levels
near the mouths of the Fraser and Columbia rivers in North America. The Colorado, Nile and Congo
rivers have weak nutrient levels, the first two probably because of high abstraction rates from large
reservoirs, leaving little discharge into the sea. A series of maps throughout the year should be consulted
because of different seasonal discharge patterns.
Entrainment. The surface outflow of freshwaters in estuaries, results in a return current of deeper,
nutrient-rich waters. These nutrients contribute to the high productivity of estuaries. Reduction of flow
may therefore reduce import of nutrients. There are numerous impoundments in the North American
Great Lakes and St. Lawrence River basin. It is estimated that the spring and summer runoff at the
entrance to the Gulf of St. Lawrence has been reduced by between one third and one half (Neu 1975).
Kerr and Ryder (1997) proposed that many decades of anthropogenic activity have altered the Laurentian
Great Lakes ecosystem and the devastating changes that took place in the northwestern Atlantic
ground fisheries can be related to this.
Coastal fish catches adjacent to deltas with large upstream volumes of impoundments have declined
seriously from 1950 to 1990, e.g. the Egyptian Mediterranean to 18% and the western Black Sea, Sea
of Azov and Caspian Sea to <3% of the original catches (Rozengurt & Haydock 1994). Based on
world-wide experience, no more than 25-30% of the historical river flow to the estuary can be diverted
without disastrous ecological consequences to the receiving estuary (Rozengurt & Haydock 1981).
Economic losses in the Black, Azov and Caspian seas total about $3 billion per year (Rozengurt
Interaction with non-dam impacts
Dams often interact with the effects of other human activities. McAllister (1995) classes the potential
interaction of human impacts as either neutralising, additive or synergistic He postulated that synergistic
interactions, those where harmful affects were amplified, were more likely, considering that genomes
by selection are adapted to a certain set of undisturbed natural environmental conditions. Species
classed as endangered are more likely to become extinct. Factors to be considered include climate
change (Watson et al. 1996) agriculture, forestry, industry and municipal effects (McAllister et al. 1997).
The impacts of dams on the biodiversity of molluscs, fishes and waterfowl are summarised in Tables
3.4, 3.5 and 3.6. Table 3.7 provides a summary.
Tables 3.4 and 3.5 show that upstream impacts are generally less than those in the reservoir or
downstream. The exception to this generalisation is the migratory species that move up and downstream
and use such movements to maintain genetic diversity (in a stock or the survival of a stock) or the
survival of a species in that part of the basin, or in their entire global range.
For molluscs and fishes the change from running to still waters (and other related conditions) in a
newly established reservoir is profound. Most species highly adapted to currents will be extirpated and
the reservoir diversity will drop. The reservoir becomes stocked with ecologically flexible native species
or with exotics. One exception is in African reservoirs where fish numbers may increase. Reservoir
fishery harvests usually increase after impoundment, but then drop.
For waterfowl the situation is different. Reservoirs provide new habitat for over-wintering in cool regions
and for residence in warm arid regions which have few natural water bodies. Thus new reservoirs can
increase waterfowl populations, though their diversity will not be as high as that in natural lakes. The
effects of impoundment on birds that existed in the reservoir basin and downstream of the dam have
not been well studied.
36 Biodiversity Impacts of Large Dams
Table 3.4. Dam impacts on freshwater molluscs
Case studies Biodiversity increases Biodiversity decreases Source
N. American mussels
- downstream and
None noted. Extinction curve, taking into account functionally extinct
species is 4.2% per decade, result of all impacts
including dams, but excluding zebra mussels.
USA mussels. None noted, but alien zebra
mussel and Asian freshwater
clam populations are expanding.
Lead threats to 102 imperilled species are: habitat
degradation - 97%, pollution - 90% and alien species -
17%. Habitat loss from: water development - 99%;
pollutants - 97%, dams - 96% and agriculture - 64%,
but these impacts are being overtaken by those of
alien mollusc invasions.
Wilcove et al.
Current decline of freshwater mussels in the
Mississippi Basin will have a detrimental impact on the
entire ecosystem as they play a vital role in sediment
mixing and nutrient recycling. Given their dominance in
terms of biomass, removal could have long-term
effects, as yet unknown.
Stein & Flack
Demise of freshwater mussels due to host species
(skipjack herring) movements being blocked by dam.
Eddy & Underhill
River Inn, Austria. One new freshwater mollusc
recorded in river.
10 species lost due to changes in flow regime,
increase in gradient. Loss of temporary habitats
(Segmetina nitida, Viviparus connectus).
Lake Kariba, Zambezi
River, Zambia and
Reservoir supports 7 species of the 25 gastropods
known from the river system. Two species, Gabiella
balovalensis and G. zambica; endemic to area. Under
Lake Volta, Ghana. Bulinus truncatus invades. Vector
of urinary schistosomiasis.
Bulinus globosus populations decline, unable to use
new habitat. Bivalves and prosobranchs dominate
animal benthic biomass.
Australia: overview. Decline in species due to impoundments. Walker (1985)
One introduced non-native
species of gastropod.
Decline from 18 gastropods to 1 native species due to
3 factors: changes in biofilms, predation by carp and
Sheldon & Walker
Bivalve Velesunio ambiguus has
increased in abundance.
Bivalve Alathyria jacksoni has declined in abundance.
Lower Murray River
None noted. Some molluscs become locally distinct because
billabongs isolated when normal flooding stops.
Boulton & Lloyd
Background Paper Nr. 1 37
Summary USA rivers. None noted due to dams.
Increase in range of two species,
zebra mussel, Dreissena
polymorpha and Asian freshwater
mussel, Corbicula fluminals which
are abundant where they occur
and adversely impact native
294 species disappeared, decreasing 42-84% of pre-
impoundment levels with an average loss of 70% of
50% loss amongst 100 species. Bogan (1998).
Mobile Bay basin,
Species richness: 38 of 42 extinctions, 90% when big
river shoal fauna impounded, covered by deep water
Species richness: Loss of very diverse fauna with over
70 species in 35 genera. Estimated decline: 50%.
Van der Schalie
et al. (1993).
Species richness: 6 species
(1981) and 5 species (2 new)
Species richness: 10 species were extirpated. 1994
survey showed some new arrivals did not take.
Species abundance: Most species now recorded at
low % of total fauna (1.28-3.12%). Decline in overall
diversity and predominance of a few species.
Blaock & Sieckel
USA. Dam created
Species richness: loss of 43 species in dammed
stretch. Decline from 59 to 16 species.
Norris, Clinch River,
USA. Dam created
Species richness: loss of 28 species in dammed
stretch. Decline from 40 to 12 species.
River, Alabama, USA.
Dam created 1954.
Species richness: loss of 21 species - decline from 50
to 29 species. 6 species present in river listed as
endangered and 5 as candidates for Federal Register
of protected species; all globally threatened. No
decline in unimpounded sections in 1970s. Causes for
decline: increased water depth, decreased current
and loss of gravel substrate following siltation.
Williams et al.
Species richness: Loss of 42 species in dammed
stretch - decline from 60+ to 18 species.
None noted. Reduced mussel abundance with cumulative impact of
multiple dams giving an overall mussel extinction
gradient downstream from large impoundments. Only
stretches furthest from the dam contain relatively rare
Vaughan & Taylor
(1600 km2. Parana
None noted. Species richness: 3 of 7 taxa from this group of
prosobranchs in the river (genus Aylacostoma) are
extinct in the Wild (EW). Only extant in captive
holdings of Argentine Museum of Natural Science.
Increase in water depth changed the well-lit, clear bed
with oxygenated water to dense growth of algae with
(updates as pers.
Salto Grande Dam,
Species richness: Terrestrial gastropod (Athinus
albolabiatus), formerly endemic to Gallery Forest next
to Uruguay River has been proposed as Extinct for
IUCN Year 2000 list following dam building
38 Biodiversity Impacts of Large Dams
Ubolratana (or Nam
Pong) Dam, Thailand.
Species abundance increase of 3
species of prosobranch Bythnia
funiculatus, B. siamensis and B.
goniomphalos in subgenus
Digniostoma which transmit
Opisthorchis viverrini causing
outbreaks of Opisthorchiasis.
Can live at high densities, usually
found in rice fields, canals, ponds
None noted. Extinction curve, taking into account functionally extinct
species, is 4.2% per decade - result of all impacts
including dams, but excluding zebra mussels.
Current decline of mussels in the Basin will have a
detrimental effect on the entire ecosystem as they
play a vital role in sediment mixing and nutrient
recycling, and given biomass dominance, their removal
could have long-term effects, as yet unknown
Stein & Flack
River, Kentucky, USA.
None noted. 21 of 25 mussel species disappeared between 1911
Center Hill, Cany
Fork, USA. Dam
Loss of 32 species with impact 12 km downstream of
dam - decline from 39 to 7 species.
Alabama, USA. Dam
No decline in impounded sections in 1970s. Williams et al.
Loss of very diverse fauna with over 70 species in 35
genera. Estimated decline of 70%.
Van der Schalie
et al. (1993).
USA. Dam created
Loss of 35 species with impact 18 km downstream of
dam - decline from 39 to 4 species.
River Inn, Austria. None noted. 38 species lost due to changes in flow regime,
increase in gradient: loss of temporary habitats,
increasing pollution, glochidial fish host loss and
Foeckler et al.
Rivers of Franche
Declined from 48 to 39 species due to anthropogenic
activity including hydropower developments.
Background Paper Nr. 1 39
Table 3.5. Overview of dam impacts on freshwater fishes
Case studies Biodiversity increases Biodiversity decreases Source
Alien species noted to be the third
threat to freshwater biodiveristy.
Prime threats to freshwater biodiversity were: habitat loss -
94%, pollution (including siltation) - 66%, and alien species -
53%. Prime causes of habitat loss were: water
development incl. dams - 91%, dams and other river
barriers - 64%, and pollutants - 55%.
Wilcove et al.
None noted. Acipenser transmontanus endangered, numbers reduced.
Dams prevent movement between riverine sections.
Beamesderfer et al.
Prochilodus lineatus increased by
taking advantage of the
connection between the reservoir
and the floodplain above the
ins Basin, Brazil.
Increase in Prochilodus nigricans,
Semaprochilodus brama, Anodus
elongatus and Pimelodina
Ribeiro et al. 1995.
Asian (Oriental). None noted. None noted.
In the long-term the three native species, Barbus sclateri,
Chondrostoma polylepis willkommiiand Leuciscus
pyrenaicus have been almost entirely replaced by the
exotic, Micropterus salmoides.
Bia Basin - Lake
Species number has recently
increased from 44 to 65 including
Distichodus rostratus and Citharinus eburneenis no longer
Gourène et al.
1999; Koné &
Teugels in press.
Pellonula afzeliusi and
Oreochromis niloticus increased.
Alestes baremose, Mormyridae and others declined after
Petr 1967, 1968,
1971; Petr &
Cichlidae, Cyprinidae and
Bagridae increased in number in
the lake compared to the River
Mormyridae, Cyprinidae, Citharinidae and Bagridae
decreased. A loss of about 23 species formerly in the river
that were not subsequently found in lake
Balogen & Ibeun
In ten years from closure, the
number of species in the lake
increased from 28 to 40.
Mormyrids, cichlids and silurids
increased. The characid,
Hydrocynus vittatus also
increased as did benthic species.
Synodontis zambezensis is now
the most abundant fish.
Several species of the Zambezi River such as Opsaridium
zambezense and Distichodus mossambicus disappeared.
Other early abundant species, Clarias gariepinus, Labeo
spp., Barbus spp. and Distichodus spp. declined rapidly in
the early 1960s. Protopterus annectens has now become
extinct in the lake basin.
1995; Kolding &
40 Biodiversity Impacts of Large Dams
Australian None noted. None noted.
In the Kenney Reservoir, White River, exotic fish became
dominant (90%) over native species. This has been a
feature of many of the reservoirs on the Colorado River
Stanford & Ward
1986; Martinez et
In the Norris Reservoir 35 species disappeared including
four families, Petromyzontidae, Anguillidae, Cottidae and
Cyprinodontidae, and several genera and species of
Cyprinidae and Percidae.
ins River Basin,
Richness (217 species) not
Barthem et al.
1991; Ribeiro et al.
The number of species decreased from 107 to 66-83
because the Xianjiang Dam blocked migrations.
Zhong & Power
The original, 8 native, riverine
species colonised the lake.
Stocking increased this number to
Three of the four most common species in the river,
Chondrostoma toxostoma, Leuciscus cephalus and Barbus
fluviatilis declined, in particular in the downstream area of
Brun et al. 1990.
Six of the 21 species in the lake disappeared or became
very low in number. These included
Silurus glanis, Barbus
plebejus cyclolepsis, Anguilla anguilla
brama. Perca fluviatilis and Esox lucius lost spawning
habitat (destruction of reed beds and shallow water areas).
lucioperca now present.
Volga, Russia. The number of species in four
major reservoirs increased from
44 to 48. Thirty nine species are
resident. Nine species immigrated
or were introduced but none of
these reproduce naturally. They
will probably disappear as
stocking has discontinued.
Seven species, mainly anadromous rheophils, disappeared. Poddubny & Galat
1995; Gertsev &
Total catch of
declined by 90%.Forty seven
species of fishes have disappeared in the lower Nile.
Central Delta of
the Niger River,
Cichlidae, Clariidae and
Decline in Gymnarchus niloticus, Polypterus senegalus and
Gnathonemus niger (reproduction linked to the floodplain)
and Citharinus citharus and Clarotes laticeps (feed in the
Background Paper Nr. 1 41
South Africa. Dams have prevented or disrupted the migrations of several
vulnerable and rare species. They have also negatively
affected the conditions required by rheophilic species.
Barbus serra, B. capensis, Labeo seeberi
(Olifants River system), B. tenius (Gourits and Keurbooms
system - Braam Raubenheimer Dam on the Crocodile
River), Austroglanis sclateri (Vaal-Orange system),
Hippocampus capensis (dam on the Keurbooms River),
Syngnathus watermayeri (dams on the Bushmans and
(along the east coast of
southern Africa) Chiloglanis emarginatus (Fig Tree and
Morgensen Dams, Pangold River system) and Opsaridium
zambezense (eastern Transvaal, Swaziland and Natal).
Those impacted by dams include: Maccullochella
macquariensis and Maccullochella sp. both endangered,
and Prototroctes maraena, vulnerable.
Wager & Jackson
The diversity of fish has not
In the river system 15-16 species of fish are threatened and
5 are vulnerable. Flow regulation is implicated because
floods are essential for reproduction.
Walker & Thoms
Fish diversity decreased with increase in water regulation. Gehrke et al. 1995.
Ptychocheilus lucius reduced in the Green River
catchment and the White River (Taylor Draw Dam).
Endangered species are: P. lucius, Gila elegans, G. cypha,
and Xyrauchen texanus.
Holden & Stalnaker
1975; Carlson &
Martinez et al.
1994; Stanford &
More than 200 stocks of anadromous salmonids have
become extinct. The state of the 214 native, naturally
spawning stocks of Pacific salmon, steelhead and sea-run
cutthroat trout (Oncorhynchus spp.) from the Pacific north-
west are: endangered = one, facing high = 101 or
moderate =58 risk of extinction or are of special concern
=54. Eighteen of the high risk stocks may already be
Williams et al. 1989;
Nehlsen et al. 1991;
Wissmar et al.
1994; Devine 1995;
Losos et al. 1995;
Ryman et al. 1995;
special concern, has had much of its
habitat eliminated. Low species diversity below the
Garrison Dam due to the discharge of cold, hypolimnion
water from Lake Sakakawea.
Hesse et al. 1989;
Williams et al. 1989;
Wolf et al. 1996.
In the South Fork Holston River 43 species were found
before impoundment compared to 17 collected in the
tailwater of the operating dam. Thirty two species were
sampled before construction of the Watauga and Wilbur
dams compared to 13 in the tailwaters after impoundment.
Dams on the river have obstructed the migration of some
commercially valuable fish species including
Pseudoplatystoma corruscans and Salminus maxillosus.
Two other commercially important species, Piaractus
mesopotamicus and Brycon orbignyanus were eliminated
after the dam was closed.
1987; Agostinho et
al. 1994; Agostinho
& Zalewski 1995.
42 Biodiversity Impacts of Large Dams
ins River Basin,
Richness (190 species) was not
significantly affected. Migration of
Hypophthalmus spp. not directly
interrupted by the Tucurui Dam.
Ten previously abundant species drastically reduced. Long
distance migrations of
Prochilodus nigricans, Anodus
elongatus, Brachyplatystoma flavicans, B. filamentosum,
interrupted by the dam.
Ribeiro et al. 1995.
Amazon River. Hydroelectric dams in the Amazon Basin interrupt the
migrations, both upstream by adults and downstream by
larvae, of the catfish Brachyplatystoma filamentosum, B.
flavicans, B. vaillanti, Goslinia platynema and Lithodoras
Barthem et al.
- Petit Saut,
Decrease in the number of taxa of juveniles from 51 to 48.
Species richness (of all age groups caught by gill netting) in
general declined from pre-dam through filling to the
stabilisation period (54 species pre- to 47 post-
impoundment). There were 34 species common to both
pre- and post-impoundment, 20 species present before the
dam but not after and 13 not found before but captured
Triportheus rotundatus (Characidae) practically
disappeared after dam closure.
Ponton & Copp
1997; Ponton &
Merona & Albert
Dams, e.g. the Farakka Barrage, have nearly eliminated the
anadromous Hilsa ilisha (Clupeidae) in the riverine
stretches. Other major carp species reduced (50% of 1964
levels) in the lower Ganges.
Jhingran & Ghosh
1992, 1995. Temple
& Payne 1995.
tributary of the
Macrura reevesii and Clupanodon thrissa migrations
blocked by dams and fish virtually disappeared by 1970.
Cirrhinus molitorella, also affected.
Liao et al. 1989.
Macrura reevesii eliminated from the river. Zhong & Power
Acipenser sinensis migrations affected. Zhong & Power
Decline in Probarbus jullieni (Cyprinidae). Dudgeon 1992.
Dams on the river have reduced access to spawning
grounds of Alosa alosa, Acipenser sturio and Petromyzon
marinus. Biodiversity has been reduced because of loss in
habitat variation resulting from river regulation.
ruget 1992; Crivelli
Upper Rhône -
Chondrostoma nasus, usually
considered sensitive to river
engineering, increased in
abundance. Diversity increased in
the lotic habitats.
Penaz et al. 1995.
Smolt production of the Hunder strain of brown trout, Salmo
trutta, has been permanently reduced.
The anadromous Vimba vimba and the rheophilous
Chondrostoma nasus disappeared.
Penczak et al.
Volga, Russia. Alosa kessleri volgensis virtually disappeared from the
Volga - North Caspian
Rio Tera, Spain.
Salmo trutta persisted. Cobitis calderoni, Leuciscus carolitertii, Rutilus arcasii,
Gobio gobio and Barbus bocagei disappeared after
Garcia de Jalon &
Background Paper Nr. 1 43
Two thirds of large-sized stocks of Salmo trutta have
become extinct in <100 years due to migratory
Environmental changes brought
about by water regulation
improved conditions for Cottus
gobio and Salmo trutta below the
Crisp et al. 1983.
Table 3.6. Overview of dam impacts on waterfowl
Case studies Biodiversity increases Biodiversity decreases Source
800 Ramsar sites. Five regularly support > 20,000
waterfowl; 6 support > 1% of
Not evaluated, but number of Ramsar sites involving
dams with internationally important waterfowl
populations is small compared to natural wetlands.
232-4,272 birds on reservoirs
with overall diversity of 23
species and 753-88313 on
natural lakes with overall diversity
of 33 species.
Not evaluated. Davidson &
Three reservoirs support >20,000
waterfowl and 8 support 1 or
more internationally important
species (natural wetlands: 12 and
Not evaluated. Davidson &
At least 12 reservoirs support
major and import. concentrations.
Adds habitat to dry landscape.
Only one area studied- 2 of 13 waterfowl present
before inundation disappeared, including 1 Red Data
species, 2 decreased in abundance, 7 little change,
and 2 common species increased in abundance.
Danube Delta in
None noted. Populations of many of the 160 species of waterfowl
down to a fraction of their historical numbers, due to
habitat decline, in part from upstream impoundments
Pringle et al.
44 Biodiversity Impacts of Large Dams
The impounding of rivers has terrestrial impacts on biodiversity. The biodiversity of land flooded by
reservoirs, and floodplains, wetlands, oxbows and other river valley aquatic ecosystems deprived of
normal flooding may be diminished or lost.
River ecology is tied to that of estuaries in the transport of silt, nutrients and seasonally different
volumes of river discharge. This is important in the physical maintenance of delta and coastal habitats
and the nutrient-based estuarine food chains. The nutrient plume of rivers can extend far out to sea. So
regulation of rivers can influence even ocean species and ecosystems.
The addition of each new dam in a river contributes to the fragmentation of habitat and separation of
populations. Gene flow, hitherto bidirectional, becomes unidirectional, downstream, reducing genetic
diversity. Each new dam also prevents natural restoration of upstream populations lost through natural
or anthropogenic causes. One of the biggest cumulative impacts may be that a greater proportion of
running water is converted to still reservoirs habitat. Table 1.8 shows that there are 10 basins with 6 to
14 major dams.
The Itaipu Reservoir, Brazil, is sited below a floodplain and hence enhances migratory fishes. The
species inhabit the floodplain, then, when mature, migrate down into the reservoir (Agostinho et al.
1994; Agostinho & Zalewski 1995). However, the floodplain will disappear when a new dam being built
will cause it to go underwater.
Table 3.7. Overall summary of dam impacts
Biodiversity increase Biodiversity decrease
Mollusc. None noted. Decrease in freshwater molluscs.
Fishes. None noted Decrease in migratory species.
Waterfowl. Not evaluated. Not evaluated.
Molluscs. Increases not noted except
species abundance, especially
where molluscs dominant in
In 66 cases around the world, an average of 70% of species were lost.
Extinction or extirpation rates of up to 50% or even 90% are reached in
Fishes. Increase in overall diversity noted
only in some African reservoirs.
On most continents overall fish diversity declines despite the frequent
invasion of a few exotic species.
At least 12 reservoirs support
major and import. concentrations.
Adds habitat to dry landscape.
Only one area studied- 2 of 13 waterfowl present before inundation
disappeared, including 1 Red Data species, 2 decreased in abundance, 7
little change, and 2 common species increased in abundance.
Waterfowl. Reservoirs increase populations,
providing new habitats,
sometimes in significant numbers
of important species.
Diversity of waterfowl tends to be higher on natural water bodies than
reservoirs. Delta populations of birds may suffer impacts from lowered
water quality, delivery of sediments and other results of upstream
impoundments and channelisation.
Molluscs. None noted. Moderate to drastic declines with extirpations (up to 84%) and extinctions.
Fishes. 27% of 66 cases reservoir/down-
stream were positive.
In 77% of the 66 reservoirs/downstream impacts were negative, but with
most (53%) being downstream.
Waterfowl. None noted. One Red Data waterfowl disappeared below a South African dam; Many
of the populations of 160 species of water-birds down to a fraction of their
historical numbers, in part from upstream impoundments and
Background Paper Nr. 1 45
In James and Hudson Bay, Canada, river basin impoundments discharge 50% more water in winter
than in the pre-dam era. This has a number of ecological effects in the estuaries and seawards (McAllister
If the dams reservoirs are used for irrigation water supply, then the volume of flow will become
progressively attenuated, as in the Colorado River, USA, where the mouth is virtually waterless.
Dam effects relative to other sectors
There is no doubt that many human activities, other than dams, are degrading freshwater ecosystems
and have, at times, contributed to the extirpation or extinction of individual freshwater species. However
the examples shown in Tables 3.4 and 3.5 indicate that dams have caused the extirpation or extinction
of numerous stocks and species of molluscs and fishes.
In a study of extinction rates of North American freshwater fauna, Ricciardi and Rasmussan (1999)
showed that the mean rate of extinction in freshwater fauna (fishes, crayfish, mussels, gastropods and
amphibians) was 0.5% per decade, while in terrestrial and marine groups (birds, reptiles, land and
marine mammals), the rate was 0.1% per decade. That is extinction is proceeding five times faster in
freshwater than on land in North America. The authors point out that in 1990 only about 40 rivers >200
km remained free-flowing.
Status of species at the global level are not as completely evaluated. The level of threat for predominantly
terrestrial vertebrates is 11 to 25%, while for more aquatic vertebrates it is 13 to 65% (Table 1.2).
Prospects for freshwater biodiversity
The present analysis has focused on what is known on biodiversity and the impacts of dams on three
groups, molluscs, fish and birds. Three questions remain outstanding: how complete are the data,
what does the data imply for other groups of animals, plants and microorganisms, and what are the
prospects for freshwater biodiversity and its relationship to dams?
Completeness of study data. There is a lack of data from many of the developing countries where
much of the world’s biodiversity is located. In most countries, developed and developing, monitoring of
the environment, following baseline studies, is geographically uneven and infrequent. Species may be
declining or have even become extirpated or extinct without human awareness. The problem may be
worse for the smaller, non-commercial or low profile species. Environmental impact assessment reports
are often difficult to secure, they do not enter into the published literature, and their data may not be
Better estimates of biodiversity status come from areas such as North America, Europe and Australia,
involve vertebrates, vascular plants, or a few other high profile groups, and have been tracked by
international environmental organisations such as IUCN’s Species Survival Commission or the Nature
Conservancy. Tracking status is more effective when the taxonomy of a group is fully resolved. Tracking
extinctions is important as a measure of the loss of biodiversity although most resources should be
invested in monitoring the status of populations. Detecting a decline early provides greater options for
Implications of data for biodiversity as a whole. The lack data for many of the world’s taxonomic
groups and geographic areas means that it is necessary to make an estimate the world’s freshwater
biodiversity loss by applying assumed rates of loss to the number of freshwater species.
The mean of globally freshwater threatened species (Table 1.2) is 36%. In the USA Stein and Flack
(1997) estimated it to be 40%. If a midpoint value of 38% is applied to the 44,000 scientifically described
freshwater species, then 16,720 species of animals, plants and microorganisms are threatened. Similarly
applied to the 1.5 million species, scientifically described and yet to be described, then the number of
threatened species is 570,000.
Prospects for freshwater biodiversity in relationship to dams. Data from Revenga et al. (1998)
suggests that the preponderance of the large dams already constructed are in watersheds outside the
tropics (Cancer and Capricorn). It is predicted that tropical locations for dams will be given preference
in the next century. The tropics are ‘home’ to much of the richest freshwater biodiversity (Fig. 2.4). One
46 Biodiversity Impacts of Large Dams
of the main considerations of dam impacts on biodiversity is placement in regard to species-rich areas.
Priority must be given to ensure that the environmental impact of dams does not overlap with biodiversity
In areas rich in biodiversity and productive biological resources, it is also important to take into account
the cumulative impact of dams. Two or more dams may have either serious cumulative or synergistic
Background Paper Nr. 1 47
4 Standards for minimising negative impacts
As a result of the continuing erosion of biological diversity, the inter-governmental community has,
through a variety of mechanisms, adopted a set of standards for minimising harmful impacts on
biodiversity. These standards, which have been adopted in legally binding documents by almost all
governments, are far more exacting and demanding than is generally recognised. In this section of the
report, we review these internationally agreed standards, because they are the most appropriate
biodiversity benchmark against which to assess the impacts of the dam construction industry.
World Charter for Nature.
The World Charter for Nature was adopted by consensus by the UN General Assembly in 1982. It
provides the high-level guiding principles that should govern human responsibility for biodiversity. Its
states that activities which might have an impact on nature shall be controlled, and the best available
technologies that minimise significant risks to nature or other adverse effects shall be used; in particular:
· Activities which are likely to cause irreversible damage to nature should be avoided;
· Activities which are likely to pose a significant risk to nature shall be preceded by an exhaustive
examination; their proponents shall demonstrate that expected benefits outweigh potential damage
to nature, and where potential adverse effects are not fully understood, the activities should not
· Activities which may disturb nature shall be preceded by assessment of their consequences,
and environmental impact studies of development projects shall be constructed in advance, and
if they are to be undertaken, such activities shall be planned and carried out so as to minimise
potential adverse impacts.
These guiding principles have been reaffirmed in a succession of formal intergovernmental agreements.
Convention on Biological Diversity
The Convention on Biological Diversity (CBD) was signed by 156 States in June 1992, and by September
1999 175 countries had ratified the Convention. The Preamble to the CBD starts with similar sentiments
to those in the World Charter for Nature. The objectives of the CBD are the conservation of biological
diversity, the sustainable use of its components, and the fair and equitable sharing of the benefits
arising out of the utilisation of genetic resources.
Article 8 of the CBD identifies the in-situ conservation measures that Parties should seek to carry out.
Article 14 addresses impact assessment and minimising adverse impacts, inter alia in Section 1 (a):
‘Introduce appropriate procedures requiring environmental impact assessment of its proposed projects
that are likely to have significant adverse effects on biological diversity with a view to avoiding or
minimising such effects and, where appropriate, allow for public participation in such procedures’.
A very similar set of standards relating to impacts on biodiversity is included in Agenda 21, which was
adopted by consensus at the UN Conference on Environment and Development in June 1992. Chapter
15 on Conservation of Biological Diversity states that: processes and activities with significant impacts
on biological diversity should be identified; action should be taken for the conservation of biological
diversity through the in-situ conservation of ecosystems and natural habitats; and that the rehabilitation
and restoration of damaged ecosystems and the recovery of threatened and endangered species
should be promoted.
The Convention on the Conservation of Migratory Species of Wild Animals (CMS), adopted in 1979,
states, among its fundamental principles, that: ‘the Parties acknowledge the need to take action to
avoid any migratory species becoming endangered’. This principle is of particular importance for the
48 Biodiversity Impacts of Large Dams
current report, since migratory freshwater species have suffered particularly serious adverse affects
from large dams.
At the national level, numerous laws, action plans, and conservation programmes have enshrined the
principles contained in the documents quoted above.
The World Bank’s Operational Policy 4.04 on Natural Habitats requires that comprehensive analysis
should demonstrate that overall benefits from a project outweigh the environmental costs before
significant conversion of natural habitats is allowed, unless there are no feasible alternatives for the
project and its siting. The World Bank Environment Department study, the Impact of Environmental
Assessment (1997) reported some positive trends but also a wide range of quality in the biodiversity
sections of Environmental Impact Assessment (EIA) reports
The World Business Council for Sustainable Development and the IUCN have prepared: Business and
biodiversity. A guide for the private sector (WBCSD and IUCN 1997). This explains the Biodiversity
Convention, issues and opportunities for the private sector, and describes how companies can engage
with biodiversity issues.
Hydro-Québec, Canada, a major hydro-power company, is moving into wind-generated power. The
company has co-published a report on biodiversity and hydroelectricity (HQ and GDG 1999).
Some governments have high environmental protection standards. The United States Environmental
Protection Act was a pioneer in establishing standards not in outlawing killing endangered species but
in protecting their habitat.
Work on a California dam in 1996 included substantive pro-active environmental activities. Endangered
frogs were relocated. Flooded habitat had to be replaced and artificial ponds which would be self
sustaining were created with large reed beds for amphibians and birds. There were stiff penalties for
any construction worker moving outside the terms and conditions of site work, including loss of job and
financial penalties. Training videos were produced which even visitors to the site had to watch.
A few key points can be distilled from the documents quoted in this section. These are as follows:
· Species and ecosystems have intrinsic value. They should be conserved in their own right, as
well as to provide benefits to humans.
· Every effort should be made to minimise the risk of the extinction of species.
· Urgent steps should be taken to bring about the recovery of threatened species.
· High priority should be given to securing the recovery of degraded habitats and ecosystems.
· Essential ecological functions or processes should be conserved.
· Natural resources should be used sustainably.
· There is concern that biodiversity continues to be lost at a rapid rate.
· Attempts to conserve biodiversity are hampered by inadequate information.
· Lack of information should not be an excuse for lack of conservation action — the Precautionary
· Environmental impact assessments should be thorough, be given sufficient time, and be carried
out in an open and transparent fashion
· Activities that have potential impacts on biodiversity should be the subject of prior environmental
· Activities that are likely to have particularly serious negative impacts on biodiversity should not
· Staff training about biodiversity conservation in project environmental standards and objectives
· In the absence of good information on the likely impacts of particular actions, the precautionary
principle should apply.
Background Paper Nr. 1 49
It is important to realise that the above list summarises the points that have been agreed by the global
community through formal inter-governmental mechanisms.
Environmental impact assessment standards
UNEP and Wetlands International have been promoting economic and environmental impact
assessment. Dorcey et al. (1997) provide a checklist for key potential environmental and social impacts
caused by large dam projects. The public expects high standards for environmental impact surveys
(EIA), an expectation not always met. Standard most frequently breached are:
Lead time. EIAs should begin long before any construction, especially in geographic regions where
biota are poorly known. Lead time should permit sampling through at least one annual cycle, preferably
more to allow for natural variability. Biodiversity monitoring should be continued through the construction
Expertise. Taxonomic knowledge is required. Similar expertise is required in ecological, life history,
fishery and other aspects. It is desirable to complement academic expertise with local indigenous and
Open process. Draft and final EIA studies should be freely available locally, nationally and internationally
with sufficient lead-time before public hearings and final approval processes.
Voucher specimens. Representative voucher specimens of flora and fauna collected should be
deposited in national or regional museums.
‘Arm’s’ length’ analysis. EIA’s should not be conducted by the firms engaged in building the dam, nor
Follow-up. One-, five- and ten-year follow-up biodiversity studies, by an organisation other than the
one which carried out the EIA, should be performed. These test EIA predictions and provide data for
planning future dams. EIAs should consider what are alternative uses of the site, in addition to current
usage and the planned dam. What are the costs and benefits of the various options, including items
that do not enter the normal market such as subsistence fishing? What is the lifetime of the various
options? In some cases dam capacity has been significantly reduced in less than 10 years, in other
cases dams are functioning at good capacity several decades after their construction. Can sedimentation
be mitigated by engineering solutions in situ (dredging and draining sediments) or reduced ex situ by
biological management of the drainage basin? What are the biodiversity effects of these various options?
If heavy sediment deposition is impossible to avoid, is this a factor in possible elimination of this dam
Background Paper Nr. 1 51
5 Impacts of dams vis à vis standards
Goldsmith and Hildyard (1984), Rosenberg et al. (1995, 1997), McCully (1996) and Dorcey et al.
(1997) have reviewed the success of dam building in meeting international standards. Only a brief
coverage is presented here.
Minimise species extinctions. Fifty species of unionid mussels were lost in the Tennesse River
basin, USA, due to multiple impoundments. In nine USA rivers an average of 70% of freshwater mussel
species were lost following impoundments. In the Aral Sea 83% of fish species were lost following
diversions of its tributaries.
Give high priority to recovery of degraded ecosystems. Only about six dams in the USA are at the
discussion or action stage for decommissioning, out of hundreds of potential candidates. Discussions
about mitigating the impacts of Aral Sea tributary dams are about a decade old, with no concrete
Conserve essential ecological processes. Means of avoiding natural sediment trapping by dams
have not been developed despite their impacts on dam efficiency and reduction in biodiversity. Deltas
at the mouths of the Danube (largest wetland in Europe), Nile and Mississippi are shrinking.
Impoundments which deprive floodplains of natural flooding are a primary problem in many of the
world’s river basins.
Use natural resources sustainably. Dams have reduced populations of migratory fishes or caused
extirpation of genetically distinct populations, as well as diminishing estuarine fisheries in most continents.
Surviving mussel fisheries produce a fraction of their former button production; other fisheries are
extinct. Reed production in the Danube is greatly diminished.
Arrest continued loss of biodiversity. There is valid concern that biodiversity loss continues at a
rapid rate. Extinction of freshwater species continue at a high rate in North America; in that continent
freshwater extinctions are five times as high as those on land.
Resolve information inadequacies that hamper conservation. Taxonomic and ecological knowledge
is incomplete and individual expertise and institutional capacity in museums, universities and
governments is declining. Geographic information is sparse and uneven due to a lack of regularly
spaced biological survey sampling. Knowledge of restoration approaches for freshwater ecosystems
Apply the precautionary principle. The lack of full scientific certainty should not postpone action for
preventative measures when there is a threat of significant reduction or loss of biological diversity.
Carry out high standard EIAs. EIAs should be carried out to avoid adverse impacts on biodive