According the United States Geological Survey, hydroelectric power offers clean energy without the emissions created by fossil fuels, but it is very location- and precipitation-dependent and can have major land and ecological effects. Hydroelectric dams can require major changes in natural waterways to function, and the dams may disrupt or displace humans and wildlife populations.
In order to reliably provide electricity, a hydroelectric power plant requires a dam and reservoir. This allows a constant, steady flow of water through the dam for electricity generation. The creation of this reservoir can have impacts both upstream and downstream from the dam. Waterways downstream may suffer from reduced water flow, and wildlife that previously traveled the length of the river will not be able to traverse the hydroelectric plant.
Hydroelectric plants also carry some risk of catastrophic failure. An excess of precipitation or structural failure of the dam can lead to an uncontrolled release of water downstream, flooding areas with potentially dangerous levels of water. The failure of the Banqiao Dam in China led to the deaths of 171,000 people caught in the subsequent flooding.
Under normal operations, the electricity produced by a hydroelectric plant produces no carbon emissions, making it a good substitute for coal or natural gas generation in terms of the facility's impact on the atmosphere.