USGS Sediment Studies in Lake Mead
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
For a larger view of Figure 1, click
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
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.
For a larger view of Figure 2, click
|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.