The addition is part of the nation's biggest hydropower expansion since the 1980s. Utilities are proposing more than 70 projects that would boost U.S. hydroelectric capacity by at least 11,000 megawatts, or 11%, over the next decade, according to MWH, a hydro engineering firm, and Hydro Review magazine.

"You're getting good, clean energy," says Linda Church Ciocci, head of the National Hydropower Association. "It's domestic, it's affordable, it's reliable."

In the early 1900s, hydropower was the dominant source of the country's electricity generation, a status solidified by massive federal projects such as Hoover Dam in the Southwest and Grand Coulee Dam in Washington state.

As recently as the 1940s, hydropower accounted for 42% of electricity production. But by the latter part of the century, developers had tapped the most mountainous regions — many in the Northwest — whose steep inclines supply the strongest river flows and permit more cost-efficient projects. Hydropower works when falling water spins turbines, which turn generators. Of the 80,000 U.S. dams, only 2,400 have hydro plants. Hydropower today provides 10% of U.S. electricity generation.