To appreciate how essential dams were in the nineteenth century, simply look at the 1840 U.S. Census: It found that almost every river had a dam, and many rivers had dozens. In total, the twenty-six states that made up the United States at the time had around 65,000 dams. With a population of only 17 million at that time, the United States had one dam for every 261 people.
– Martin Doyle, The Source: How Rivers Made America and America Remade its Rivers. Via Marginal Revolution.