In 1969, the U.S. Army Corps of Engineers finished construction of the Red Rock Dam on the Des Moines River in Marion County, Iowa. One of thousands of U.S. dams built that decade, its purpose was to moderate seasonal flooding, allowing the Corps to release the million-and-a-half acre feet of snowmelt it impounded each spring at will. And for more than 50 years, aside from providing locals with a reservoir in which to fish and go boating, that’s all it did.