Similar to wind power and biomass, hydroelectric power begins with the sun, which drives the water cycle. Heat from the sun causes water to evaporate, and this water returns to the ground as precipitation. As the water flows towards the ocean through lakes and rivers, its kinetic energy can be captured in dams and converted into electrical power. Dams typically create a reservoir of stored water and pass it through a turbine to generate power as needed. As the most common form of renewable energy, hydropower supplies around 20% of the world’s electricity and around 34% of renewable energy in the United States.

Pros:

Operating a hydroelectric facility generates minimal carbon dioxide emissions.

Reservoirs allow energy be easily stored and used as needed.

Hydroelectric facilities are relatively inexpensive to operate.

Many rivers in the U.S. are suitable for hydropower, especially in the West.

As long as rivers are flowing, hydroelectric power will not run out.

Cons:

Reservoirs for hydroelectric power often require large amounts of flooding, destroying riparian zones and other ecosystems as well as human communities.

Dams cause sediment build-ups in reservoirs as water passes through but leaves dirt and clay behind.

When water goes through dams, it releases methane, a greenhouse gas formed as organic matter decomposes in reservoirs. By some estimates, these emissions could contribute more to climate change than comparable burning of fossil fuels.

Hydroelectric plants require a high initial investment, although usually not as high as other forms of renewable energy.

Dams depend on water flow, so they generate less energy in times of drought.

Most of the potential hydropower in the U.S. is already being used, leaving little room for growth.

Other sources of information on biomass:

U.S. Department of Energy - Hydropower Basics

Union of Concerned Scientists - How Hydroelectric Energy Works

National Geographic - Hydropower

National Atlas - Hydropower

Resource Maps:

Idaho National Laboratory - State Resource Assessment

National Atlas - Hydropower Map