2. Hydroelectric, nuclear, and biomass
The sources comprising the first wave of clean energy are still some of the most prevalent today.
Renewables make up about 12 percent of electricity generation in the US, but of that hydroelectric dams (shown above as teal dots) contribute 56 percent. Most of those dams were built before the mid-1970s, as part of ambitious federal infrastructure efforts like the Hoover Dam in Nevada.
The orange blocks above show the top 20 percent of the most productive counties in terms of tons of potential biomass output. That includes gas emissions from landfills or organic waste from farming and timber industries. Together they make up about 12 percent of renewable electricity generation.
There are 104 operable commercial nuclear reactors at 65 nuclear power plants (purple dots above) in the US, generating about 20 percent of the nation's electricity. Most definitions don't include nuclear as "renewable," since it relies on exhaustible uranium for fuel. It is "clean" in that it produces no carbon emissions, but nuclear waste and radioactive leaks can pose environmental threats.
Sources: Nuclear Research Council, US Army Corps of Engineers, National Renewable Energy Laboratory, Saxum, Energy Information Administration