photograph by dherrera_96 via Flickr
Hoover Dam, NV and AZ: Begun in June 1933 and dedicated September 1935 (two years ahead of schedule), this concrete arch-gravity storage dam is among the largest and earliest of the Bureau of Reclamation's massive multiple-purpose dams. By providing electric power, flood control, and irrigation water, this dam made increased levels of population and agricultural production in large areas of the Southwest feasible, affecting not only lands near the river, but also urban centers such as Los Angeles.