The use of water-powered wheels to ease human labor dates to at least 200 B.C., when ancient Greeks and Romans learned they could harness a stream’s power to help grind grain and perform other tasks.

Until medieval times, the power was gained simply by putting a wheel into a running stream, but that left mill operators subject to the whims of nature.

“During storms, the wheels got ripped out and during low flows, the wheels didn’t even go into the water,” said Bob Walter, an associate professor in the Department of Earth and Environment at Franklin and Marshall College. “You were limited by optimum stream conditions. Putting in a dam allowed you to run the stream year-round, provided it wasn’t frozen over.”

Damming created a pool of water, from which a steady stream could be diverted to the water wheel. It was a technology that thrived for centuries, and was brought to the New World by the earliest settlers; by the 1840 census the United States had nearly 65,000 operating mills. The United States wasn’t alone; Walter said that France had more than 80,000 mills in the 1700s.

They performed a number of tasks. Grist mills ground grain into flour, boring mills bored holes in metal to make rifle barrels, gunpowder mills made gunpowder, sawmills cut logs into boards, oil mills ground grains into oil, fulling mills produced cloth—and those were just some of the uses.

Demand for dams, and the mills they powered, was intense. “They actually dammed streams from head to mouth so they could extract the greatest amount of power possible,” said Dorothy Merritts, a professor in the Department of Earth and Environment at Franklin and Marshall. “Unfortunately, every one of those reservoirs, as far as we can tell, filled with sediment and buried the original landscape.”

Most settlers, though, probably had little idea that their dams—often just 8 to 10 feet high—were dramatically altering the valley landscape. Even then, it was likely a price they were willing to pay. “There is no one to blame,” Walter said. “At least no one living to blame. It was our ancestors that caused this to happen, and at the time they did it, it was for the economic benefit of the region.”