The reservoir formed by Hoover Dam that is the largest in the United States by capacity is Lake Mead.
Lake Mead lies on the Colorado River along the Arizona–Nevada border, about 24 miles east of Las Vegas. It began filling when Hoover Dam impounded the river in the 1930s, creating a vast reservoir named for Elwood Mead, who led the U.S. Bureau of Reclamation during the dam’s planning era.
At its original maximum capacity, Lake Mead could hold more than 32 million acre-feet of water; sedimentation and changing measurements have reduced that capacity, but it remains the largest U.S. reservoir by total storage capacity. Its water supplies Arizona, California, Nevada, parts of Mexico, and nearly 20 million people.
A common mix-up is Lake Powell, another enormous Colorado River reservoir upstream behind Glen Canyon Dam. Lake Powell can have a larger surface area under some conditions, but Lake Mead is the larger reservoir by capacity—the measure used in this question. Water levels fluctuate substantially with drought, demand, and conservation policy.