Tue 27 Dec 2011
Damming large rivers to generate electricity seemed like a good thing when there were only 1 billion or 2 people on the planet. But now with a planet nearing a human die back at 7 billion people and having stressed the planet to nearly its bursting point they are causing more problems then they are worth. I only include a couple of paragraphs here but you have to love, “making America the most damming country” part. You go girl..
Environmental Impacts of Large Dams: African examples
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––have been inundated by reservoirs worldwide. The world’s largest impoundment, the 8,500 sq.km. 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.
The Environmental Consequences of Big 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 1990 internal survey of World Bank hydroelectric dam projects showed that 58% were planned and built without any consideration of downstream impacts, even when these impacts could be predicted to cause massive coastal erosion, pollution and other problems.
The following are some of the more serious environmental impacts of dams on rivers and the life they support. I have concentrated on the kinds of impacts that might affect the Orange River watershed, leaving out other major dam–caused problems that have affected rivers under different ecological circumstances.
Effects on River Systems
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, undermining bridges 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 depletion of riverbed gravels reduces habitat for many fish that spawn in the gravelly river bottom, and for invertebrates such as insects, molluscs and crustaceans. Changes in the physical habitat and hydrology of rivers are implicated in 93% of freshwater fauna declines in North America.
Before the Aswan High Dam, the Nile River carried about 124 million tons of sediment to the sea each year, depositing nearly 10 million tons on the floodplain and delta. Today, 98% of that sediment remains behind the dam. The result has been a drop in soil productivity and depth, among other serious changes to Egypt’s floodplain agriculture. The Aswan Dam has also led to serious coastal erosion, another problem stemming from the loss of sediments in a dammed river. Another example of this problem is along the mouth of the Volta River in Ghana. Akosombo Dam has cut off the supply of sediment to the Volta Estuary, affecting also neighboring Togo and Benin, whose coasts are now being eaten away at a rate of 10–15 meters per year. A project to strengthen the Togo coast has cost US$3.5 million for each kilometer protected. The story is the same on coastline after coastline where dams have stopped a river’s sediments.
Go there and read. More tomorrow.