Partnering the steam turbines were the hydroelectric turbogenerators in dams. The physical principle of the turbine was first applied by the ancient Greeks, who used water wheels to convert spinning to power—in this case a millstone turning to grind grain. Hydroelectric projects apply the same principle to create electric power on a grand scale. In the most basic terms, the power in a given amount of water comes from its flowing action as it falls; the more water falling or the greater the fall, the more power. As demonstrated by the many hydroelectric projects that began in the 1930s, that meant building taller dams or building dams across larger rivers—or both.

The two biggest dams—and by far the largest construction projects of their day—were the Grand Coulee Dam on the Columbia River in Washington and the Hoover Dam on the Nevada-Arizona border on the upper reaches of the Colorado River. Like the dams cropping up on other rivers, these would harness more power for the nation while also providing irrigation and flood control. The Grand Coulee, more than four times wider than the Hoover to span the mighty Columbia River, far outstrips the Hoover as a hydroelectric plant. It generates more than 6.5 million kilowatts with its 24 turbines, compared to just over 2 million kilowatts from the Hoover's 17 turbines. Completed in 1942, the Grand Coulee still ranks as the third-largest producer of hydroelectric power in the world.

In some regions, engineers have combined multiple sources of power to fuel growing needs. The Tennessee Valley Authority, another New Deal project, today manages numerous small dams, 11 steam turbine power plants, and two nuclear power plants, altogether producing 125 billion kilowatt-hours of electricity a year.

New engineering challenges have continued to arise, among them how to transmit electricity at higher and higher voltages for maximum efficiency. Improvements in both materials and systems have brought transmission voltages up from the 220 volts of the 1880s to the 765,000 volts of today. And still the search goes on for new and better ways to harness energy from sources that now include everything from nuclear reactors to the wind, the Sun, and even the geothermal energy of Earth itself. Wind farms, with scores of sleek, narrow-bladed, computer-controlled wind turbines, have become increasingly productive; improvements in efficiency have brought the cost of wind-produced electricity down significantly in the past 15 years. IOUs are also devoting more research dollars to improving solar power. Photovoltaic cells that generate electricity directly are becoming more efficient, but engineers are also working on other innovative approaches, including a technique known as solar thermal, in which arrays of mirrored parabola-shaped collectors focus sunbeams to heat oil to as high as 750°F to drive steam turbines.