A sign at the Hoover Dam on the border of Arizona and Nevada, acknowledging the nearly 5 years it took the Bureau of Reclamation to construct the dam between its ground breaking in 1931 and its dedication in 1935. Hoover Dam is a concrete arch-gravity dam in the Black Canyon of the Colorado River. When full it is the largest dam by volume in the United States. The dam is currently at the center of concerns about drought and climate change in the desert southwest as the in flow of water from the Colorado River decreases and the volume of water in the dam drops, impacting both power generation, and a water source relied upon for agriculture and domestic consumption by more than 40 million Americans.