Most hydroelectric energy is generated by damming rivers and then directing their flow through a large turbine. While this technology is very effective for generating electricity, it has many negative side effects, both environmental and social. Deciding to use this form of power production in a particular area involves determining whether it is overly damaging to the environment and to people's lives.
Stopping the flow of a river and directing its flow through a turbine in a dam is, not surprisingly, very disruptive to the river's natural function. Flooding dynamics are disrupted. While humans may see this as a benefit, many rivers require periodic flooding for the ecological health of their surroundings, and many downstream ecosystems have been damaged or destroyed by dams. The turbidity created by water released from the dam can erode riverbanks. Artificial lakes created by dams on the upstream side drown natural communities and create problems of siltation, and, particularly in tropical climates, the creation of standing water, from which disease can spread.
Many species of fish, most notably salmon, swim up river to reproduce every year as part of their natural life cycle. Dams disrupt this cycle and have badly damaged fish populations where they have been built. Dams now include "fish ladders," which are designed to allow the fish to bypass the dam, but, while these ameliorate the problem, they do not eliminate it, and the natural movement of the fish is still impeded to some extent.
According to the website International Rivers, 60 million people worldwide have been displaced by hydroelectric projects. These people were living in areas that were flooded by enormous hydro dams, which can flood many miles of river upstream. Many of the affected people are very poor and have nowhere else to go. In addition, local customs and networks are disrupted or destroyed when entire villages are submerged.
The production of concrete is a very energy-intensive process and is a major contributor to global warming gases. According to the website of the Hoover Dam Tour Company, the Hoover Dam is comprised of 4.4 million yards of concrete. The cumulative effect of all the concrete used in all the dams in the world brings into question the claim that hydroelectric energy does not contribute to climate change.
Hydroelectric energy is site-specific. Dams have to be built at particular points on particular rivers, where the geography makes them feasible. As a result, extensive power lines often need to be built to transport the power from the dam to where it is used. These power lines use large amounts of resources and energy to build, create linear clearcuts across the landscape that are maintained with pesticides, disrupt habitat, and create power corridors that are susceptible to extreme weather and terrorism.