Dams (environmental effects)

Most dams are built to control flood hazards, to store water for irrigation or other uses, or to produce electricity. Along with these benefits come environmental costs including riparian habitat loss, water loss through evaporation and seepage , erosion , and declining water quality . Farther-reaching consequences of dams include changes in groundwater flow and the displacement of human populations.

Riparian, or stream–side, habitats suffer both above and below dams. Valuable ecological zones that support specialized plants, riparian environments, and nearby shallows provide food and breeding grounds for birds, fish, and many other animals. Upstream of a dam, impounded water drowns riparian communities. Because reservoirs can fill hundreds of miles of river channel, and because many rivers have a long sequence of dams and reservoirs, habitat drowning can destroy a great deal of river biodiversity . Downstream, shoreline environments dry up because of water diversions (for irrigation or urban use) or because of evaporation and seepage losses in the reservoir . In addition, dams interrupt the annual floods that occur naturally on nearly all rivers. Seasonal flooding fertilizes and waters flood plains and clears or redistributes debris in river channels. These beneficial effects of flooding cease once a river is dammed.

Dams and reservoirs alter sediment deposition in rivers. Most rivers carry large amounts of suspended silt and sand, and dams trap sediments normally deposited downstream. Below the dam, erosion reshapes river channels once sediment deposition ceases. If erosion becomes extreme, bridges, levees, and even river deltas can be threatened. Meanwhile, sediment piling up in the still waters of the reservoir behind the dam decrease water storage capacity. An increasingly shallow reservoir also becomes gradually warmer. Oxygen content decreases as the water temperature rises; fish populations fall, and proliferating algae and aquatic plants can begin to block the dam's water intakes. In arid regions a higher percentage of river water evaporates as the reservoir becomes shallower. Evaporating water leaves behind salts, which further decrease water quality in the reservoir and river.

Water losses from evaporation can be extreme: Lakes Powell and Mead on the Colorado River lose about 3 billion cu ft (1 billion cu m) of water to evaporation each year; Egypt's Lake Nasser, on the Nile River, loses about 45 billion cu ft (15 billion cu m). Water losses also result from seepage into bedrock. As river water enters groundwater, water tables usually rise around a reservoir. In arid regions increased groundwater can increase local fertility (sometimes endangering delicate dry-land plant species ), but in moister regions excessive groundwater can cause swamping. Evaporation from exposed groundwater can leave higher salt concentrations in the soil . The most catastrophic results of reservoir seepage into groundwater occur when saturated rock loses its strength. In such events valley walls can collapse, causing dam failure and disastrous flooding downstream.

Perhaps the most significant environmental effect of dams results from the displacement of human populations. Because people normally settle along rivers, where water for drinking, irrigation, power, and transport are readily available, reservoir flooding can displace huge populations. The planned Three Gorges Dam on China's Chang Jiang (Yangtze River) will displace 1.4 million people and flood some of western China's best agricultural land. A series of dams on India's Narmada river will inundate the homes of 1.5 million people along with 600,000 acres (150,000 ha) of farm land. In both cases, people will need to find new places to live and clear new land to grow food. Such ripple effects carry a dam's influence far beyond its immediate proximity.

Where dams are needed for power, they can have a positive effect in offsetting environmental costs associated with other power sources. Hydropower is cleaner and safer than nuclear power . Water turbines are also cleaner than coal-fired generators. Furthermore, both nuclear and coal power require extensive mining, with environmental costs far more severe than those of even a large dam.

[Mary Ann Cunningham Ph.D. ]

RESOURCES

BOOKS

Goldsmith, E., and N. Hidyard, eds. The Social and Environmental Effects of Large Dams. (3 Vols.) New York: Wiley, 1986.

PERIODICALS

Esteva, G., and M. S. Prakash. "Grassroots Resistance to Sustainable Development." The Ecologist 22 (1992): 45-51.

OTHER

Driver, E. E., and W. O. Wunderlich, eds. Environmental Effects of Hydraulic Engineering Works. Proceedings of an International Symposium Held at Knoxville, Tennessee, Sept. 12-14, 1978. Knoxville: Tennessee Valley Authority, 1979.