Oahe Dam on the Missouri River near Pierre, South Dakota, is built of millions of tons of packed earth, rock, and concrete. But a dam has to do more than just hold back water. The Building Oahe Dam - Part Two film, left, from the 1950s, shows the construction of the emergency tunnels used for floodwater release, the massive water intake tower upstream from the dam, and several other structures that really make Oahe work. Notable in the film are shots of the emergency gates tested for the first time in April 1957.
Fifty-four years later, during the Missouri River Flood of 2001, those tunnels and gates would be used for real. The video on the left shows the tunnels and other structures capably moving 150,000 cubic feet per second (CFS) of river water.