The graph shows lake elevation (in feet) from 1940 through July 31, 2015. The photo, taken on July 10, 2015 at the Hoover Dam, shows the high points and low points of Lake Mead’s water history. The white “bathtub ring” seen on the rocky sides of the reservoir shows the historical high water level in the reservoir. The ring is a coating of minerals, deposited on the rocks while they were covered by water.