Large Dams: Are they Really Feasible?
When a Dam is constructed, be it for Hydropower or water supply, the destruction is highly visible. But the environmental impacts of a dam stretch much further downstream than the location of the actual dam site.
Alteration of River ChannelThese dams and associated extractive uses of water have altered two-thirds of the world’s major rivers. And around the globe, government officials, development agencies, and industry leaders are proposing and building new dams to meet growing human demands. In fact, nearly 950 new dams are planned. Dams and other structures that alter a river's natural course block the pathways used by Migrating Fish, reduce and rearrange the patterns of flowing water that have choreographed Aquatic Life Cycles for millennia, and change water quality. These changes can have significant effects on the social fabric and economic well-being of people and communities, particularly among those whose livelihoods are closely connected to nature.
Effects on BiosphereA dam also holds back Sediments that would naturally replenish downstream Ecosystems. When a river is deprived of its sediment load, it seeks to recapture it by eroding the downstream river bed and banks (which can undermine bridges and other riverbank structures, as well as riverside woodlands). Riverbeds downstream of dams are typically eroded by several meters within the decade of first closing a dam; the damage can extend for tens or even hundreds of kilometers below a dam. Riverbed deepening (or "incising") will also lower groundwater tables along a river, lowering the water table accessible to plant roots (and to human communities drawing water from wells). Altering the riverbed also reduces habitat for fish that spawn in river bottoms, and for Invertebrates. In aggregate, dammed rivers have also impacted processes in the broader biosphere. Most reservoirs, especially those in the Tropics, are significant contributors to Greenhouse Gas Emissions (a recent study pegged global greenhouse gas emissions from reservoirs on par with that of the aviation industry, about 4% of human-caused GHG emissions). (Courtesy: Internationalrivers.org)