Glen Canyon Dam, Arizona

Construction of this dam on the Colorado River, upstream from the Grand Canyon, started in 1956 and created Lake Powell when it was finished in 1963. The lake flooded a 186-mile long valley, forming the second largest man-made lake in the country (after Lake Mead, 200 miles downstream), a water storage reservoir meant to, among other things, control flooding downstream. The dam is 710 feet high above bedrock (making it the second highest concrete-arch dam in the U.S.), and is 1,560 feet across.