Hydrological Effects of Dams

Wednesday, October 10, 2007
(Excerpted from Silenced Rivers: The Ecology and Politics of Large Dams by Patrick McCully)

Riverine ecosystems and human societies have evolved with, and often become dependent upon, seasonal changes in river flows. All storage dams alter to some extent these seasonal patterns, in most cases ironing out hydrological extremes by storing floods and increasing dry period flows. The exact nature of the impacts, however, will depend on the design, purpose and operating regime of the dam and the size of the reservoir.

A dry river in the US Southwest
A dry river in the US Southwest
Lori Pottinger
Dams and barrages used to divert water, especially for irrigation, reduce, sometimes calamitously, the downstream flow. Undoubtedly the biggest ecological disaster caused by river diversion is the shrinking of the Aral Sea in Central Asia. Evaporation from the sea used to be matched by inflows of water from the Amu Darya and Syr Darya rivers (the Oxus and Jaxartes of classical times). Since the 1960s, however, the building of an extensive network of dams and canals for cotton irrigation has virtually eliminated the flow of water into the sea. In 1995, the area of the Aral Sea was only 30,000 square kilometres, compared to some 64,500 square kilometres in 1960. Its volume has dropped by more than three-quarters. A commercial fishing industry which once supported 60,000 workers ground to a halt in 1982 with what was left of the once freshwater lake now more saline than the oceans. By the early 1990s, 20 of the 24 fish species once caught in the sea had disappeared; the number of bird species found in the Amu Darya Delta had declined from 319 to 168, the delta forests had died, and only 30 out of 70 species of mammals remained.

The salt-encrusted, dried up bed of the lake is now known as the Akum Desert. Wind-blown dust from the new desert, laden with heavy metals and other toxins from fertilizers and pesticides used upstream, has been detected as far away as Alaska, and together with the heavily contaminated water supply has had a catastrophic effect on the health of the 3.5 million people living near the sea. The republic of Karakalpakia which surrounds the southern end of the sea has the highest rates of infant and maternal mortality in the former Soviet Union. The incidence of typhoid fever, hepatitis, kidney disease and chronic gastritis have rocketed as much as 60-fold. In the town of Muynak, nearly 70 per cent of the remaining population of 2,000 had 'pre-cancerous conditions' in 1994, according to the town's medical research centre. Life expectancy in Muynak tumbled from 64 years in 1987 to 57 in 1991. More than 80 per cent of women in the area suffer from anaemia and twelve kinds of pesticides have been found in their breast milk.

The USSR Ministry of Water Management wanted to increase the area of cotton in Central Asia so that they could justify building more canals and thus secure their share of government spending: the inevitable decline in the Aral Sea was not only predicted but justified by the planners. A map issued by the USSR Academy of Sciences in 1981 showed the estimated area of dried up Aral Sea bed in the year 2000 being used to grow rice. In 1987, government water planners proclaimed in a magazine article: 'May the Aral Sea die in a beautiful manner. It is useless.'

Estuarine Impacts

Some 80 per cent of the world's fish catch comes from continental shelves. Many of these fisheries are dependent on the volume and timing of the nutrients and freshwater discharged by rivers as well as on estuary habitats. Almost all the fish and shellfish caught along the US coast of the Gulf of Mexico, for example, live in estuaries for at least part of their lifecycle. The productivity of the Grand Banks of Newfoundland, one of the world's greatest fishing grounds, is directly related to the amount and seasonality of freshwater and nutrients flowing out the mouth of the St. Lawrence. The alteration of estuarine flows by dams and diversions is therefore, together with overfishing, a major cause of the precipitous decline in many sea fisheries.

Dams and diversions have done to the commercial fisheries of the saltwater Black, Azov and Caspian seas almost as much damage as they did to those of the freshwater Aral Sea. The discharge of the Volga into the Caspian Sea has been reduced by almost 70 per cent; that of the Dniester, Dnieper and Don into the Black and Azov seas by around half. The salinity in the estuaries of these rivers has increased by up to four-fold and that in their deltas up to ten-fold. The most valuable commercial fisheries in the seas have now been reduced by 90 to 98 per cent. Sturgeon catches in the Caspian Sea are only one to two per cent of historical levels and have been totally eradicated in the northwestern Black Sea and Sea of Azov (a northeastern appendage of the Black Sea). Michael Rozengurt, a Russian oceanographer now living in the US, estimates the combined economic losses to the fishing industries of the Black, Azov and Caspian in the decade between 1977 and 1987 at $35 billion dollars.

Nutrients carried to sea during the flood season once caused a huge bloom of plankton at the mouth of the Nile. This plankton was grazed by great shoals of sardines which accounted for 30-40 per cent of the annual Egyptian sea catch. After the closure of the Aswan High Dam and the elimination of the annual flood, however, the sardine catch fell from 18,000 tonnes to less than a thousand tonnes in the late 1960s. The catch has since risen to a few thousand tonnes but this is attributed to improvements in fishing technology and greater numbers of boats. Shrimp catches at the mouth of the Nile decreased by two-thirds after the nutrient supplies were cut off. Landings of other fish in 1970 were 77 per cent below pre-dam levels.

Estuarine mangrove forests are valuable nurseries for fish and shrimps as they provide cover and also food when they shed leaves, flowers, fruit and twigs. Nearshore fish catches in several tropical areas are proportional to the mangrove cover of the adjacent coast. Mangroves are also directly used by people for fuel, animal fodder and fibre. The 80 per cent reduction in the discharge through the Indus Delta because of dams and barrages in Pakistan and India has killed off almost all the delta's mangrove forests which once covered a quarter of a million hectares (although mangroves can tolerate salinity much better than other plant species they still require freshwater to thrive).

River Plumbing

"The unregulated Colorado was a son of a bitch. It wasn't any good. It was either in flood or in trickle."

Floyd Dominy
US Bureau of Reclamation Commissioner, 1969

The major hydrological impact of hydro dams is to impose on the river an unnatural pattern of flow variations. As Wallace Stegner puts it, 'a dammed river is not only stoppered like a bathtub, but it is turned on and off like a tap.' In Quebec, peak electricity consumption occurs during winter when river flows are naturally at their lowest because water is locked up in snow and ice. To meet the demand for electricity during cold weather, dams and diversions have increased the winter flow on the La Grande River by eight times (from 500 to 4000 cubic metres per second) and in order to store water for the following winter have eradicated the spring flood (flow reduced from 5000 to 1500 cubic metres per second). Interbasin diversions compound the effects of dam operation on rivers: redirecting water from the Eastmain River into the La Grande to increase generation has doubled the La Grande's total average annual discharge into James Bay, while reducing by 90 per cent flows to the Eastmain estuary.

Superimposed upon the seasonal post-dam pattern of downstream flows are short-term daily or even hourly fluctuations in river levels, sometimes of as much as several metres, due to releases to meet peak demands for power. The link between water releases and power demand means that river levels downstream of Glen Canyon now change not according to rainfall in the Colorado Basin but because of factors like the drop in electricity use on Sundays and public holidays. Releases from Glen Canyon Dam cause daily river level fluctuations of one-and-a-half metres compared to natural daily changes of a few tens of centimetres. Increases in demand for power from Kariba Dam on the Zambezi River can cause the downstream water level to rise by five metres in just half an hour.

Flow alterations on this scale have numerous ecological consequences. Rapid water level fluctuations speed up erosion downstream and can wash away the trees, shrubs and grasses along its banks. Without the riparian vegetation to hold it in place, the bank then erodes even faster. Riparian vegetation provides food and shelter for riverside creatures and branches on which birds such as kingfishers can wait for their prey to swim by. It also prevents the river becoming dangerously hot during the summer by providing shade. Furthermore, leaves and twigs falling into the river are an important source of food for insects and other aquatic fauna.

Varying releases through dams also effect reservoir levels. Rapid reservoir fluctuations can prevent fish spawning by alternately exposing and submerging the favoured nesting areas in shallow waters. Nests of waterfowl may be similarly affected. The fluctuations also prevent riparian and marsh vegetation from growing along the reservoir shore and so renders lifeless the nearshore shallows — usually the most biologically prolific areas of natural lakes and ponds. The six hydro-reservoirs on the La Grande river have submerged some 83,000 kilometres of natural shorelines with their fringing woods and shrubs; the shores of the reservoirs, meanwhile are broad, lifeless banks of mud, rock and dead trees.

Cutting off the Floodplain

"In my view, nature is awful and what we do is cure it."
Camille Dagenais, former head of Canadian
dam engineering firm SNC, 1985

Even if flood control is not an intended consequence of a project, a storage dam will almost always delay floods downstream and reduce the size of average flood peaks, commonly by more than a quarter (even a flood control dam, however, may have little effect upon extremely large and infrequent floods — making the 'flood control' offered by dams often dangerously deceptive for people who move onto the downstream floodplain). The Warragamba Dam in Australia, for example, reduced the 'mean annual flood' (a flood likely to recur on average every 2.3 years) by more than half, while the size of the flood likely to recur every 50 years barely changed.

River and floodplain ecosystems are closely adapted to the annual cycle of flooding and drying. Many species depend on seasonal droughts or pulses of nutrients or water to give the signals to start reproduction, hatching, migration or other important lifecycle stages. Annual floods replenish wetlands not only with water but also with nutrients, while flooded manure from both domestic and wild animals on the floodplain enriches the river. Floods sweep fish eggs and fry into floodplain backwaters and lakes where they hatch and grow before joining the river again after the next annual floods. Adult fish and other aquatic animals such as turtles also follow the flood to take advantage of the new food sources offered in the submerged shrubs and woods.

For large floodplain rivers the floodplain is just as much a part of the river as the main channel itself. Most fish in the Amazon basin, for example, spend much of their lifecycle in the várzea, the tens of thousands of square kilometres of seasonally flooded forests and grasslands along Amazonian rivers. Some of the várzea forests are flooded for ten months or more each year and some fish and other aquatic species may never make direct use of the main channel. Many Amazonian fish eat the fruits of the flooded plants and play an important role in dispersing plant seeds. The renowned biodiversity of the Amazon rainforest is mainly in the várzea — the much greater area of dry forest is relatively unproductive and poor in species.

What ecologist Peter Bayley terms the 'flood pulse advantage' is the main reason for the astonishing diversity and productivity of rivers and floodplains — on a per unit area basis the diversity of fauna in rivers is 65 times greater than in the seas. Annual floods on tropical rivers are estimated to produce fish yields one hundred times higher than in rivers without floodplains, and, on a per hectare basis, around four times more than in tropical lakes or reservoirs. Most freshwater fish species are found in rivers and floodplains: few are adapted only to life in lakes.

It is generally recognised by biologists that dams and other flood control schemes are the most destructive of the many abuses which are causing the rapid disappearance of riverine and riparian species. Around 20 per cent of the world's 8,000 recognized freshwater fish species are threatened with extinction. Out of the 170 fish species endemic to the heavily dammed Western US, 105 are officially listed as threatened or endangered or are being considered for such a listing. A further 17 Western fish species have been exterminated during this century. The situation of some non-fish freshwater species is even worse: around two-thirds of the several hundred crayfish and freshwater mussel species in North America are on the danger list. In the little studied rivers of the tropics many species which have yet to be discovered by science have almost certainly been extinguished, or are about to be extinguished, by the building of dams (there are three times more known species in the Mekong than in the Mississippi, yet there have been 10,000 times more scientific articles published on Mississippi fauna).

The plants and animals of the river bank and floodplain also suffer when the plain no longer floods — or when the river is in spate at the wrong time. Unseasonably high discharges from dams on the Savannah River in Georgia, for example, killed almost all the bald cypress seedlings along the river banks. Studies on the floodplains of the Missouri and the Pongolo River in South Africa have both shown a reduction in the diversity of forest species after dam construction upstream. The forest on the floodplain of Kenya's Tana River appears to be slowly dying out as it loses its ability to regenerate because of the reduction in high floods due to a series of dams upstream.

The 6,000 square kilometre floodplain of the Kafue River in Zambia, known as the Kafue Flats, was once one of the richest wildlife habitats in the world. The Kafue, a main tributary of the Zambezi, was impounded in the 1970s by the Gorge Dam which permanently flooded much of the Flats and then by the Itezhitezhi Dam upstream, which eradicated the seasonal floods over the remaining part of the plain. Biologist Walter A. Sheppe visited the flats before and after the dams. On his first visit in May 1967, 'the extensive annual floods were largely hidden by a dense growth of emergent grasses reaching to the horizon.' Large herds of antelope grazed the edges of the flooded area and zebra and wildebeest fed on the higher ground. The water and shore were dense with birds. Sixteen years later Sheppe returned to the same spot. This time the lowest part of the plain was covered by Gorge Reservoir and the rest was dry. The productive grasses that had depended on the seasonal floods had been replaced by aquatic plants on the open water, and on the dry former floodplain by a sparse cover of low grasses and scrub. There were few birds, relatively few antelope, and no zebra or wildebeest.

Read the next section in this chapter, The Mitigation Game


Mailing lists

We offer many campaign-specific email lists to help you stay informed.

Sign up for urgent campaign actions and NewsStream: