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Results

The generalized morphology of the lake and the surrounding area is shown in Figure 1. The lake walls are steep in Boulder Canyon and near Hoover Dam. These steep cliffs coincide with exposures of Precambrian gneiss and Cretaceous and Tertiary volcanic and intrusive rocks (Wilson et al, 1969; Stewart and Carlson, 1978) around the margins of the lake (Fig. 2). In Boulder Basin, the gradient of the lake walls is gentler off areas where alluvial deposits and Tertiary-aged sedimentary strata are present. The axis of the basin is flat, and the gradient between Boulder Canyon and Hoover Dam is remarkably gentle. Water depth, based on the bathymetry collected during this survey, on the easternmost line in the axis of Boulder Canyon was 142 m while, near Hoover Dam, 26 km to the southwest, the lake is 147 m deep. Las Vegas Bay has a narrow, steep-sided valley running down its axis that opens into Boulder Basin northeast of Saddle Mountain Fig. 1. Several smaller tributaries feed into this axial valley showing a well-defined dendritic drainage pattern that was cut in the Tertiary sedimentary strata prior to formation of the lake. This morphology characterizes the northern flank of the lake with Swallow, Callville, and Hamblin Bays having dendritic drainage networks in their floors as well. This northern section of the lake presumably is underlain by the Tertiary sedimentary strata that is exposed onshore which has the same distinctive morphology Fig. 2).

The sidescan sonar image shows four distinctive geologic provinces on the lake floor (Fig. 4 and Fig. 5). Two of these provinces, rock outcrop and alluvial deposits, reflect the pre-impoundment geology of the area while the other two, thin and thick sediment show the distribution of deposits that are interpreted to have accumulated in the lake since its formation.

Areas of rock outcrop show on the sidescan image either as high-backscatter or as alternating bands of high and low backscatter. The high-backscatter signature represents steep rock walls that face the sonar, while the alternating high- and low-backscatter bands appear to represent terraced outcrops. The terraces probably are controlled by bedding of the underlying strata. The distribution of these two signatures coincides with the distribution of different rock types surrounding the lake. The areas of rock outcrop associated with uniform high backscatter Fig. 8 closely correspond with the distribution of Precambrian gneiss, Cretaceous through Tertiary volcanic and intrusive rocks around the lake Fig. 2). These rocks are resistant to erosion and form the steep cliffs that surround the lake. These cliffs are prevalent east of Hamblin Bay (especially in Boulder Canyon), in the southwestern part of the lake surrounding Promontory Point, and also off of Saddle Mountain. Areas of alternating bands of high and low backscatter that have an extensive network of valleys cut through them Fig. 9 occur in Las Vegas Bay and along the northern side of Boulder Basin from Las Vegas Bay eastward to Hamblin Bay. North of this part of the lake, the rocks consist of a mix of Tertiary fluvial deposits and tuffs (Stewart and Carlson., 1978) which were extensively eroded by fluvial processes prior to formation of the lake.

Alluvial deposits are the second pre-impoundment geologic province that is still preserved along large sections of the flanks of the lake (Fig. 4, Fig. 5). These deposits show on the sidescan sonar image as areas of uniform moderate backscatter which are dissected by an intricate network of small channels Fig. 10. These channels coalesce downslope, and in some cases form larger braided streams Fig. 10. Two large areas of alluvial deposits occur adjacent to alluvial fan deposits that have been mapped on land Fig. 2). These large areas of alluvial deposits occur along the southeastern side of Boulder Basin and off of Boulder Beach (Fig. 5). Smaller areas of alluvial deposits are also found in Las Vegas Bay and Callville Bay.

The sediment that has accumulated in the lake since impoundment has a uniform low-backscatter signature associated with its surface (Fig. 4). This sediment fill has been divided into two provinces based on the seismic-reflection data; namely thin and thick sediment cover (Fig. 5). Sediment thickness will be discussed more in the description of the seismic data. Because of the continuous coverage of the lake floor provided by the sidescan mosaic, the outline of the post-impoundment deposits can be mapped in more detail than can be derived from the seismic profiles that are spaced several hundred meters apart. The thick sediment fills the former Colorado River channel and laps up against the older rock and alluvial units that are exposed along the flanks of the lake (Fig. 9, Fig. 11). Because of the dramatic contrast in acoustic signatures, the edge of the thick sediment unit is easy to identify. This deposit has an irregular edge with embayments and promontories coinciding with tributary valleys and spurs that were formed prior to the lake being formed (Fig. 5). Along the contact with alluvial deposits, the narrow embayments coincide with small valleys in the surface of the alluvial fans that have been partially filled. Within the area of thick sediment cover, the backscatter signature is not uniform. Discontinuous patches of slightly higher backscatter such as the area south of Callville Bay or at the mouth of the channel that originates in Las Vegas Bay may reflect areas where surface sediments are slightly coarser grained (Fig. 4). This inference awaits confirmation by direct sampling. In addition to these broader patches of moderate backscatter, there are also two parallel bands of moderate backscatter that are most clearly expressed in the area of thick sediments immediately south of Swallow Bay Fig. 11. These two moderate backscatter bands occur on the flanks of an extremely subtle channel that is evident in the seismic profiles that cross it Fig. 11. The seismic profiles suggest that the surface expression of this channel is in the same location as the original Colorado River channel, and that the channel on the lake floor only has about 2-m relief.

The thin sediment cover is not as extensive as the thick sediment cover, and is limited to the floors of the tributary valleys in the axes of Las Vegas, Swallow, Callville, and Hamblin Bay (Fig. 5). The seismic profiles show that sediment cover in these valley floors is patchy (especially in Swallow, Callville and Hamblin Bays) and rarely exceeds 2-m in thickness. The uniform nature of the low-backscatter signature from these valley floors suggests that, at most, only a thin veneer of sediment has accumulated in these areas since impoundment Fig. 12. The fact that many of the seismic profiles in these areas show no sediment cover suggests that it is too thin to be resolved by the seismic system Fig. 12. Therefore it is probably less than about 0.5 m thick. High-backscatter targets in places along this channel floor are probably boulders, and their presence further supports thin sediments that have not completely covered the pre-existing geology Fig. 12.

Seismic-reflection profiles were used to map the thickness of sediment that has accumulated on the lake floor (Fig. 6, Fig. 7). Sediment that is interpreted to have accumulated since formation of the lake shows as an unit with some continuous internal reflectors on these subbottom profiles (Fig. 6, Fig. 8, Fig. 11, Fig. 13). The top of this transparent unit is flat, and it fills the deepest parts of the valleys in the lake floor (Fig. 8, Fig.13A). This transparent unit rests on top of a high-amplitude reflector that merges with the present lake floor where the transparent unit pinches out (Fig. 9, Fig. 11, Fig. 13B). Where the transparent unit pinches out consistently coincides with the transition from sediment cover to rock or alluvial deposits on the sidescan imagery (Fig. 8, Fig. 9, Fig. 11). Although cores are not yet available to confirm this interpretation, the close correlation of the sediment distribution based on the seismic profiles with that derived from the sidescan imagery further supports the interpretation of this transparent unit being the post-impoundment sedimentary section. The post-impoundment sedimentary unit sometimes has as many as four discontinuous reflectors within it Fig. 6. The origin of these reflectors is unknown, and will require cores for the explanation.

The sediment thickness map shows that post-impoundment sediment is limited to a relatively small part of the lake floor Fig. 7. It is thickest and most continuous in the deep, flat central part of Boulder Basin where water depths exceed 125 m. The sediment in Las Vegas Bay and Callville Bay by contrast is thin and discontinuous. Much of the lake floor has no post-impoundment sediment covering it at all Fig. 7. Sediment in the floor of Boulder Basin locally exceeds 30 m in thickness, but in most of this area it is less than 20 m thick. It is thickest over the channel of the former Colorado River (Fig. 8, Fig. 9, Fig. 11, Fig. 13D). Although rare, there were a few places along the axis of the former Colorado River where the post-impoundment sediments were too thick to be penetrated by the chirp subbottom profiler. In these areas, sediment generally exceeded 35-m in thickness. In the narrow parts of the lake, specifically near Hoover Dam and in Boulder Canyon, the sediment thins abruptly against the steep rock walls that flank these parts of the lake, but is a fairly constant thickness along the axis of the channel. Exceptions to the thick sediment cover do occur locally in Boulder Canyon where it thins to less than 4 m thick Fig. 7. In the central part of Boulder Basin, where the flanks of lake floor have gentler gradients, the extent of post-impoundment sediment is broader, and it thins gradually rather than abruptly at its edges (Fig. 7, Fig. 9, Fig. 11, Fig. 13B). Here, large areas of the deposit are less than 10 m thick.

Four reflectors have been identified in this post-impoundment deposit (Fig. 6, Fig. 13C). These reflectors all are flat-lying. The youngest one is 1-2 m below the lake floor and the older ones are 4-5 m, 7-8 m, and 18-19 m below the lake floor respectively (Fig. 13. The oldest reflector, because of the shape of the basin has a more limited extent than the younger ones. The horizontal nature of these reflectors results in the oldest reflector being limited to the central part of the basin while the younger ones have a broader extent (Fig. 13. No places were observed where these older reflectors were exposed on the lake floor. A shallow channel, with less than 2-m relief, is present on the floor of Boulder Basin Fig. 11. This channel can also be seen in each of the reflectors in the subsurface which suggests that this channel has been present throughout the filling of the lake (Fig. 11, Fig. 13).
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