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

Sediment distribution
and thickness

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.

Figure 9. Isopach map of sediment thickness in Lake Mead.

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

Figure 10A. Seismic reflection profile showing gas in the sediment.>

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).

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