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USGS Sediment Studies in Lake Mead

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  Lake-Floor Mapping

Sediment carried by the Colorado River originally was transported to the Gulf of California. Upon completion of the Hoover Dam however, much of the river's sediment was deposited in the quiet waters of Lake Mead. Geophysical data are being used to determine the amount of sediment that has been deposited in the lake and its processes of deposition.

For a larger view of Figure 1, click here.

Figure 1: Sidescan sonar imagery

Sidescan sonar imagery provides a detailed view of the lake floor. Maps derived from this imagery are used to identify the different types of substrate exposed on the lake floor. The top panel of Figure 1 is of part of the sidescan sonar image, which shows three distinctive substrate types. Rock outcrop and alluvial fans are original substrate types that existed in this area prior to formation of the lake. The post-impoundment sediment (material that has accumulated since formation of the lake) has a weak reflective surface. In this example, sediment fills a meander bend in the original Colorado River while no sediment appears to cover the rock or alluvial fan that flank the original river channel.

The thickness of the post-impoundment sediment is measured using seismic profiling techniques. The bottom panel of Figure 1 shows a seismic profile that was collected along the red line superimposed on the sidescan image. The seismic profile provides a cross section of the lake. The first dark return is the lake floor itself. Sound penetrates the floor where sediment is present, which measures sediment thickness. On this profile, post-impoundment sediment is only found in the deepest part of the lake where it fills the original channel of the Colorado River. The deepest point on this profile is 135m, and the sediment is approximately 18 m thick at its thickest point.

 

Interestingly, these sediments are not uniformly distributed throughout the lake. Instead, they are concentrated in the deepest parts of the lake along the valleys cut by rivers that originally flowed through this area. Figure 2 shows the distribution of post-impoundment sediments in Lake Mead. These sediments form a continuous cover along the entire extent of the original Colorado River valley from the eastern extremity of the lake to the Hoover Dam in the west. Sediment filling the original Colorado River valley is thickest to the east at the mouth of the Colorado River.

Figure 2: Map showing distribution of post-impoundment sediments

For a larger view of Figure 2, click here.

Here sediment is nearly 70 m thick. It thins to 15-25 m in the central third of the lake, and then gradually increases in thickness in the western third of the lake. Near the Hoover Dam, sediment reaches 30 m in thickness. The maximum thickness in the western part of the lake is 45 m. In the Overton Arm, sediment covers the floor of the original Virgin River channel, but here the sediment is only 1-4 m thick. The thinner sediment cover reflects the smaller sediment load carried by the Virgin River in comparison to the Colorado River.

The distribution of sediment in Lake Mead indicates that the Colorado River is the primary source of sediment to the lake. The presence of sediment along the entire 100 km length of the lake indicates unique sediment dispersal. The geometry of the sedimentary deposit suggests that it is the result of 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.

After entering the lake, the denser river water sinks and flows along the lake floor. The flows probably move fastest in the eastern part of the lake where the lake floor slope is steeper. Here, the muddy part of the flow will remain in suspension, while the sand in the flows settles. Farther to the west, where the lake floor less steep, the flows will slow down and sediment will become gradually finer as it settles out of suspension. The finest sediment is deposited near the Hoover Dam, over 100 km from where the Colorado River enters the lake.

 

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This page last modified on Monday, 14-Jan-2013 04:50:59 EST