The term hydropower often suggests giant dams capable of transmitting tens of thousands of cubic feet of water per minute. Such dams are responsible for only about six percent of all the electricity produced in the United States today.

Hydropower facilities do not have to be massive buildings. At one time in the United States--and still, in many places around the world--electrical power is generated at low-head facilities, dams where the vertical drop through which water passes is a relatively short distance and/or where water flow is relatively modest. Indeed, the first commercial hydroelectric facility in the world consisted of a waterwheel on the Fox River in Appleton, Wisconsin. The facility, opened in 1882, generated enough electricity to operate lighting systems at two paper mills and one private residence.

Electrical demand grew rapidly in the United States during the early twentieth century, and hydropower supplied much...