The big idea
A dam holds back a river so the water stands deep behind it. Water pushes harder the deeper it is, so a dam is thickest at the bottom. It holds by its own weight (gravity), by curving into the canyon walls (arch), by leaning on props (buttresses), or by sheer bulk of earth and rock (embankment). Let the deep water fall through a pipe and spin a turbine, and it makes electricity: the power is the weight of water each second times the height it falls, less what is lost on the way. Spillways let floods pass safely over or around the dam.
Memory trick: “Turn the river, build in blocks, cool the concrete, fill the lake, drop the water, spin the wheel.”
A bit of history: People have dammed rivers for thousands of years. Sadd el-Kafara in Egypt, from about 2600 B.C., washed away before it was finished. The Roman dam at Proserpina in Spain still holds water. In 1848 James B. Francis improved the water turbine at Lowell, Massachusetts, and turbines of his kind are still widely used. Hoover Dam was built from 1931 to 1936, Grand Coulee's first powerhouse was finished in 1941, Itaipu's first unit ran in 1984, and the Three Gorges Dam was finished in 2006, with all its main units running by 2012.
Every dam is a line drawn across moving water. The Bible speaks of the sea held back the same way: And said, Hitherto shalt thou come, but no further: and here shall thy proud waves be stayed?
(Job 38:11, KJV)