Humans have long harnessed the kinetic energy of falling water—but it was the development of modern construction methods that allowed for the rise of megadams around the globe. Large-scale hydropower is lauded as a greenhouse gas–free energy source, but it effectively kills wild rivers, dramatically altering ecosystem structure and function to generate electricity.

Like windpower, hydropower has a long history of use around the world. Ancient societies used watermills for grinding grain and other mechanical needs. Hydropower is now the largest and lowest-cost source of renewable energy in the world, with some 777 gigawatts of installed capacity. China’s Three Gorges Dam is the single largest electricity-generating facility in the world, producing 20 gigawatts of power—more than 20 times the size of the average coal power plant.

The first hydroelectric dams were installed in the United States in the late 1800s; today hydropower accounts for 6 percent of all U.S. electricity generation and 60 percent of the electricity production from all renewable resources. Hydropower is considered one of the least-polluting energy sources because of its low greenhouse gas emissions, but it does have serious ecological impacts. Damming a river can completely alter the natural ecosystem by flooding the upstream portion and altering flow rates and natural silt deposition downstream. The resulting habitat fragmentation, loss of water quality, and changes in species diversity may put increased pressure on vulnerable species. In the Pacific Northwest, for example, the vast network of hydroelectric dams installed on the Columbia River system in the twentieth century decimated regional salmon populations, wiping out entire runs and endangering many others.

The future growth potential of hydropower in most developed countries is limited. More than 45,000 large dams already degrade rivers across the Earth. Since much of the hydropower infrastructure in the United States is old, there are efficiency improvements that can be realized by upgrading dams, but most major rivers that have potential for producing electricity are already dammed. Small-scale hydropower (“micro-hydro”) and so-called “run of the river” technologies that generate power without dams or impoundments can be ecologically benign and a useful part of regional distributed energy efforts; their total potential generating capacity, however, is modest. Other emerging hydropower technologies, including tidal and wave power, have not yet proven commercially viable and are far inferior (in terms of cost and power generation) when compared to traditional hydropower.