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.
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.
[an error occurred while processing this directive]