Lake Mead, formed by the Hoover Dam on the Colorado River, is the largest reservoir in the U.S. and supplies drinking water to millions of people in California, Arizona, Nevada, and part of Mexico.

Correction 6/18/25, 12:47 p.m. ET: This article was updated to correct the year Lake Mead was first filled plus the states and number of people served by the Hoover Dam's hydroelectric power.

Both Lake Mead, which sits behind Hoover Dam, and the Colorado River, get their water from snowmelt that runs off of the Rocky Mountains, as well as other Colorado Plateau ridges. But during a ...