Mapping the floor of Lake Mead (Nevada and Arizona): Preliminary discussion and GIS data release, USGS Open-File Report 03-320

MORPHOLOGY

Figure 1. Map showing the location of the study area and survey tracklines.

Lake Mead lies in the Basin and Range province of southern Nevada and northern Arizona, and is divided into several broad intermountain basins that are separated by narrow, steep-sided canyons where the former Colorado River cut through mountain ranges. The major basins are Gregg and Temple Basins in the eastern part of the lake, Virgin Basin and Overton Arm in the central part, and Boulder Basin in the western part (Fig. 1). These basins are 3-13 km wide and 14-20 km long. The margins of the basins have gentle gradients, commonly constructed from submerged alluvial fans or from Cenozoic sedimentary rocks of the Muddy Creek Formation (Longwell, 1936; 1960).

The canyons separating the basins are much narrower, and have near-vertical walls composed mostly of Precambrian and Paleozoic rocks (Longwell, 1936). The floor of Iceberg Canyon, which lies northeast of Gregg Basin, is 250-500 m wide and is straight (Fig. 1). Virgin Canyon, which separates Temple Basin from Gregg Basin, is 60-400 m wide and is sinuous. There is no canyon separating Virgin Basin from Temple Basin. Boulder Canyon, which separates Boulder Basin from Virgin Basin, is 80-400 m wide and sinuous as well.

Figure 2. Map showing the lake floor gradient from Iceberg Canyon to Boulder Basin.

The axial valley of the pre-impoundment Colorado River is filled with sediment, and the gradient of the present sediment surface is shown in Figure 2 (Twichell and others, 2002; 2003). The lake floor gradient is steepest on the delta front in Iceberg Canyon; however, even here the slope does not exceed 1º. In Gregg Basin the lake-floor gradient has decreased to 0.5-0.9º. Farther west, in Virgin Basin the gradient is 0.4-0.7º, and in Boulder Basin it has decreased to 0.3-0.5º. The gradient along the axial valley progressively decreases from the delta-front to the Hoover Dam except in Boulder Canyon where the lake floor rises 2-3 m as it crosses a landslide deposit emplaced shortly after the lake was filled (Leifson, 1960; Gould, 1960; Fig. 2).

   Title Page / Contents / Discussion / GIS Data / Contacts