Although hydroelectric power stations create power from a reusable resource, there are some concerns about their impact on water. They alter the flow and temperature regimes that destroy fish spawning areas, handicap fish migration, kill fish in turbines and dry out wetlands. They can also capture sediment and nutrients behind dams, which can reduce the fertility of the waters downstream and may also increase erosion of river banks. For instance dams have reduced the sediment carried into Lake Geneva by some 50 %.
Climate change could also make many hydroelectric power plants less reliable in future as water availability changes. While some plants in northern Europe could generate more power, hydroelectric dams in Bulgaria, Portugal, Spain, Turkey and Ukraine could reduce output by 20-50 % because of declining rainfall.
Source: State of the Environment Report No 1/2005 "The European environment - State and outlook 2005" (published 29 Nov 2005)