Lake Mead lies along the eastern edge of the Basin and Range province, an area characterized by alternating northward trending mountain ranges and intervening wide basins with low relief. The Colorado River has cut deep canyons through the mountain ranges where the historic river channel is bordered by steep, high-relief walls composed primarily of Precambrian to Tertiary Igneous and metamorphic rocks.
In the adjacent basins, the pre-impoundment Colorado River valley is wide with low banks and flanked largely by the Muddy Creek Formation, Tertiary aged muddy sandstone, or Quaternary alluvial deposits. Following the completion of Hoover Dam in 1936, the waters of the Colorado River began backing up, filling the narrow canyons and flooding the adjacent basins as the waters rose. The resulting morphology of the new reservoir, thus, reflected its basin and range heritage.