Hoover Dam, a concrete arch-gravity dam in Black Canyon on the Colorado River between Nevada and Arizona, was built to control floods, store irrigation water, and generate electricity, creating Lake Mead and serving utilities in Nevada, Arizona, and California. Authorized by the Boulder Canyon Project Act in 1928 after long disputes over the lower Colorado, it was built from 1931 to 1936 during the Great Depression under Six Companies, Inc. and engineer John L. Savage. Completed ahead of schedule at a cost of more than 100 lives, it became both vital infrastructure and a powerful symbol of federal ambition, later also drawing millions of visitors each year.