Large (or mega-) dams (over 100 m high) have been used all over the world for flood protection and water resource management. The construction of the colossal Hoover Dam in the 1930s heralded the start of an economic and political craze for large dams that continues to the present day. They were believed to be the solution to water supply or flood problems, or as a ‘green’ source of power. However, the environmental (and in some cases socio-economic) problems soon became apparent.
The environmental impacts of large dams on river systems are well documented. These impacts include:
Siltation of the reservoir.
Clearwater erosion immediately downstream of the dam.
Loss of downstream transport of nutrients and (in some societies, beneficial) annual flood on the floodplain downstream.
Decrease in discharge downstream, particularly summer (low) baseflows.
Unnatural flow regime of river due to block release of water.
Release of cold, poorly oxygenated reservoir water affects fish.
Increased disease associated with water bodies.
Socio-economic impacts of large dams include:
Loss of fertile land beneath reservoir.
Hydroelectric power may be used by corporate industry or by distant urban areas rather than by the local inhabitants.
As a result of environmnetal considerations, fluvial geomorphologists and some dam engineers have switched to constructing strategically-placed small dams. However, some countries and dam engineers have failed to heed these lessons and have continued to pursue the construction of large dams.
So are Big Dams (over 100 m high) still the way forward in terms of flood protection and water resource management?