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

SIDESCAN-SONAR IMAGERY

Sidescan-sonar imagery has enabled detailed mapping of the surficial geology of the lake floor. The sidescan-sonar imagery is presented such that a strong acoustic signal (backscatter) is white and a weak backscatter signal is black. As a generality, the post-impoundment sediment has a lower-backscatter and more uniform signature than the pre-impoundment surface. The use of sidescan-sonar imagery has allowed mapping the extent of the post-impoundment sediment throughout the lake (Fig. 3). Here we briefly describe some of the findings from the eastern, central and western portions of the lake.

In the eastern portion of the lake, the post-impoundment sediment surface has more varied backscatter strength than in the central and western parts of the lake.

Figure 4. Sidescan-sonar imagery from Iceberg Canyon.

The post-impoundment sediment in Iceberg Canyon has a moderate backscatter signature in some areas and a low-backscatter signature in others. The northern half of the canyon shows numerous pits in the sediment surface, which appear to be gas-escape structures (Fig. 4). The seismic data suggest a high gas content in the sediment of this part of the lake (see sediment distribution and thickness section). The steep walls of Iceberg Canyon show as uniform high-backscatter bands to either side of the post-impoundment sediment.

Gregg Basin has a broader floor than Iceberg Canyon, and the post-impoundment sediment fill is flanked primarily by alluvial fans along the western side of the basin and rock ledges along the eastern side. The post-impoundment sediment fill is 1 km wide in the northern part of the basin and 2-3 km wide in the southern part (Fig. 3). This sediment mostly

Figure 5. Sidescan-sonar imagery from Gregg Basin.

has a moderate backscatter signature except in the southern half of the basin where the central part is moderate backscatter and the edges are low backscatter (Fig. 5). Sediment cores indicate that near-surface sediment in Gregg Basin contains numerous fine and very-fine sand beds that are separated by silt and clay beds (Twichell and others, 2003). The sidescan-sonar imagery also shows slightly sinuous features on the surface of the post-impoundment sediment that can be traced for 3.5-4 km along the southern part of the basin (Fig. 5). These features are 30-50 m wide, have floors that tend to be moderate backscatter, and are flanked by narrow bands of high-backscatter. Along the outside of the bends the high-backscatter areas are commonly broader. These features are interpreted to be channels although they have no bathymetric expression on the seismic profiles that cross them (Twichell and others, 2002). The presence of these channels on the surface of the post-impoundment sediment indicates that they are modern channels forming by subaqueous processes.

In the central part of Lake Mead (from Virgin Canyon to Boulder Canyon) the post-impoundment sediment surface has a low-backscatter signature except in local areas where recent landslide deposits are still exposed on the lake floor. The transition to post-impoundment sediment having a low-backscatter surface occurs at the southern end of Gregg Basin where it enters Virgin Canyon. The post-impoundment sediment cover in Temple Basin

Figure 6. Sidescan-sonar imagery from Temple Basin.

is mostly less than 1 km wide and is broader in Virgin Basin where it reaches widths of 2.5 km. In Overton Arm this sediment cover is broader north of the islands in its center, and only a narrow thread of post-impoundment sediment cover can be traced south of the islands to Virgin Basin. The surface of the post-impoundment sediment in the central part of the lake shows no evidence of channels. Landslide deposits do cover small parts of the sediment surface. One landslide in the eastern part of Temple Basin (Fig. 6) occurred in 1988 when the lake was at its highest level (W. Burke, 2002, personal communication). The fact that it is still exposed on the lake floor indicates that not much sediment has accumulated in this part of the lake since that time.

Figure 7. Sidescan-sonar imagery from Overton Arm.

One other feature of note in the central part of the lake is the town of St. Thomas that now is submerged in the northern part of Overton Arm. The streets and some foundations are still preserved on the lake floor (Fig. 7).

Boulder Basin comprises the western part of Lake Mead The post-impoundment sediment within the basin has a low-backscatter signature in contrast to Gregg Basin. These sediments are surrounded by Quaternary aged alluvial fan deposits and outcrops of older strata (Longwell, 1936; Twichell and others, 1999).

Figure 8. Sidescan-sonar imagery from Boulder Basin.

The channel of the pre-impoundment Colorado River is mimicked on the present lake floor where two moderate backscatter bands correspond to the channel banks (Fig. 8). The preservation of the channel shape on the lake floor is probably due to dewatering and compaction of the very fine-grained sediment deposited in the western part of the lake.

The sidescan-sonar imagery also shows a narrow band of sediment has been deposited on the floor of the axial valley in Las Vegas Bay since the lake filled (Twichell and others, 2001).

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