Analysis of the seismic-reflection data indicates that a large volume
of sediment carried by the Colorado River has accumulated in Lake Mead
since impoundment in 1935. The sediment is not uniformly distributed,
but rather is concentrated in the deepest parts of the lake and covers
the floors of the valleys cut by the Colorado River and the other tributary
streams that originally flowed through the area (Twichell and others,
1999; 2001; 2002; 2003). The sediment forms a continuous cover along the
entire length of the pre-impoundment Colorado River valley from the eastern
extremity of the survey just east of Iceberg Canyon to the Hoover Dam
at the west end of the study area. Sediment also covers the floors of
the larger tributary valleys that feed the Colorado River.
Sediment filling the pre-impoundment Colorado River valley is thickest
in the eastern part of Lake Mead at the mouth of the Colorado River.
Here sediment was in excess of 76 m thick (Fig.
9). Unfortunately, in this portion of the lake, gas in the sediment
blanked the seismic signal and precluded subbottom imaging of this part
of the deposit (Fig. 10A).
Neither the chirp, nor lower frequency boomer system, could penetrate the
gas-filled sediment. Here, total sediment thickness was determined by taking
the difference between the present lake floor and the pre-impoundment surface
surveyed prior to construction of the Hoover Dam (Smith and others, 1960).
Farther west, the amount of gas in the sediment was diminished, and the
seismic signal penetrated to the pre-impoundment surface. In the central
third of the lake the
sediment thinned to 15-25 m, and then gradually increased in thickness in
the western third of the lake. These eastern portions and central third
of the lake were surveyed on this 2001 cruise, and these data are contained
on this DVD. Near Hoover Dam, the area surveyed in 1999 (Cross and Twichell,
2003a), sediment reaches 25 m in thickness (Twichell and others, 2001).
Post-impoundment sediment covers the floor of many of the tributaries
to the pre-impoundment Colorado River, but the sediment cover is not nearly
as thick. The thinner sediment cover indicates that these tributaries
have not contributed nearly as much sediment as the Colorado River (Fig.
9). In the Overton Arm, sediment covers the floor of the original
Virgin River channel, but here the sediment is only 1-4 m thick. Sediment
derived from Las Vegas Wash, which drains the Las Vegas metropolitan area,
can be traced along the entire length of the axial valley under Las Vegas
Bay (Twichell and others, 2001). The sediment reaches 12 m thick in the
delta off the mouth of the Wash, and beyond the delta most of the post-impoundment
sediment is less than 2-m thick.
The presence of sediment along the entire 100 km length of the lake,
but only in the deepest part of the lake, suggests sediment dispersal
by density flows that run the full length of the lake. Colorado River
water, at least during floods, has high concentrations of suspended sediment,
which makes it denser than the lake water. As first described by Gould
(1951), this denser river water, upon entering the lake, sinks and flows
along the lake floor. The resulting deposits have a nearly flat surface,
and are limited to the deepest part of the pre-impoundment Colorado River
channel (Fig. 9). These
seismic data have enabled the first detailed mapping of the distribution
and internal structure of this deposit. The sediment distribution in the
tributary valleys to the Colorado River suggests the same processes of
deposition, but at a smaller scale (Twichell and others, 2001).
U.S. Department of the Interior, U.S. Geological Survey
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