30 Sep 1935: Hoover Dam dedicated

On this day in 1935, President Franklin D. Roosevelt dedicated the Boulder Dam, along the border of Nevada and Arizona. It was renamed the Hoover Dam in 1947.

The dam was built in the Black Canyon, part of the Colorado River. Behind it is Lake Mead.

Because it abuts two states, the dam was built under the Constitution’s commerce clause. Future president Herbert Hoover was Secretary of Commerce in 1922, when the seven states in the river’s basin signed an agreement guaranteeing certain amounts of water to each state. (In this case, water is measured in acre-feet.)

The dam was also the first American project to use more masonry than the Great Pyramid.

Image: aerial view from the Department of Energy, c. 1970