Dams and resources
Dams and resourcesThe Environmental Impacts of Large Dams
Land and water are ecologically linked in a natural system called a watershed. From the smallest
droplet to the mightiest river, water works to shape the land, taking with it sediment and dissolved materials
that drain to watercourses and, in most cases, eventually to the sea. So is the river a product of the land it
inhabits--the type of rock and soil, the shape of the land, and the amount of vegetation are some of the
factors that determine the river's shape, size and flow.
When these ties between the land and the river are broken by a large dam, the consequences are felt
throughout the watershed, as well as by the web of life it supports. Of all the ways to tamper with or harm a
river, a large dam usually has the most immediate and far-reaching effects because of the huge changes it
causes to river circulation system.
Some 40,000 large dams, most of which were built in the past 50 years, now obstruct the world's
rivers. More than 400,000 square kilometers--an area larger than Zimbabwe, and 13 times the size of
Lesotho. Volta Reservoir behind Ghana's Akasombo Dam, flooded 4% of that nation's land area. In the
United States, whose 5,500 large dams make it the second most dammed country in the world, we have
stopped building large dams, and are now spending great amounts of money trying to fix the problems
created by existing dams.
Although the impacts of large dams have been well documented for some time now, in case after
case, new ones are proposed whose environmental impacts are downplayed or even ignored. A suvey
showed that most dams were built without the consideration of downstream effects, even when these
impacts could be predicted to cause massive coastal erosion, pollution, and other problems.
Reducing the flow of water from a river changes the landscape it flows through, which in turn can
affect the ecosystem's flora and fauna. A dam holds back sediments, especially the heavy gravel and cobbles.
The river, deprived of its sediment load, seeks to recapture it by eroding the downstream channel and banks,
and other riverbank structures. Riverbeds are typically eroded by several meters within a decade of first
closing a dam; the damage can extend for tens or even hundreds of kilometers below a dam. Within nine
years of closing Hoover Dam in the US, the riverbed below the dam had lowered by more than 4 meters.
Riverbed deepening will also lower the groundwater table along a river, threatening vegetation and local
wells in the floodplain and requiring crop irrigation in places where there was previously no need. The
reduction of riverbed gravels reduces habitat for many fish that spawn in the gravelly river bottom, and for
invertebrates such as insects, molluscs, and crustaceans.
Before the Aswan High Dam, the Nile River carried about 124 million tons of sediment to the sea
each year, depositing...
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