Figure 1. Location map showing the morphology of the western part of Lake Mead and the surrounding area. Morphology is shown as a shaded-relief image that was derived from combining a digital elevation model of the surrounding landscape (USGS, 1999) with a gridded version of the bathymetry collected during this survey. Green line is the approximate location of the shoreline (360 m contour), and names are places referred to in the text. Inset shows the location of the study area (yellow box).
Figure 2. Generalized geology of the area surrounding Lake Mead. Geology of Arizona is modified from Wilson et al. (1969), and geology of Nevada is modified from Stewart and Carlson (1978). Alluvial is mostly Quaternary and recent unconsolidated alluvial gravel, sand and silt. K-T Volcanics are mostly Cretaceous and Tertiary andesitic and basaltic flows. T Intrusives are mostly Tertiary aged rhyolitic flows and shallow intrusive rocks including diorite and granite. T Sedimentary are Tertiary aged tuffaceous sedimentary rocks. Precambrian includes gneiss and some areas of undivided schist and granite of Precambrian age.
Figure 3. Tracklines in the Boulder Basin and Las Vegas Bay parts of Lake Mead along which sidescan sonar, subbottom seismic-reflection profiles, bathymetry, and navigation were collected. All four types of data were continuously collected along all of these lines.
Figure 4. Sidescan sonar mosaic of the Boulder Basin and Las Vegas Bay parts of Lake Mead. On this image, strong acoustic returns (high-backscatter) are shown by the white and light gray shades while weak acoustic returns (low-backscatter) are shown by the black and dark gray shades. Solid black areas around the edge of the lake were not imaged by the sidescan system.
Figure 5. Interpretation of the sidescan sonar image from the Boulder Basin and Las Vegas Bay parts of Lake Mead. The lake floor is divided into four general provinces. Alluvial: unconsolidated sand and gravel that was deposited primarily as alluvial fans prior to the formation of the lake. Rock: areas of exposed rock outcrop, and, like alluvial deposits, indicates areas of the lake floor that have not been covered by sediment (or only have a minimal sediment cover) since the lake formed. Thick sediment: sediment that has accumulated since impoundment that is more than 2-m thick. Thin sediment: sediment that has accumulated since impoundment that is less than 2-m thick, and often is too thin to be resolved by the seismic system used to map sediment thickness (ie. less than 0.5 m thick).
Figure 6. Enlarged section of a seismic profile showing the different features identified on the profiles. The numbers across the top of the profile represent the shot or trace number (shots here are 1.0 seconds apart). The depth on the left side of the profile is in seconds, and on the right depth has been converted to meters assuming a constant speed of sound of 1500 m/sec. The lake surface is at the top of the profile. Acoustic cross-talk, or interference, between the sidescan sonar and subbottom profiler shows in the water column. The lake floor is the first reflector (yellow), and the reflector representing the pre-impoundment surface is the deepest reflector (red). Four reflectors within the sediment that has accumulated since impoundment show as flat-lying horizons that pinch out against the pre-impoundment surface.
Figure 7. Thickness of sediment, in meters, that has accumulated in Boulder Basin and Las Vegas Bay since impoundment of Lake Mead. Sediment is thickest in the floor of Boulder Basin where it commonly exceeds 20 m, is thin in Las Vegas Bay where it does not exceed 4 m in thickness, is limited to the axial valley, and is absent from the walls of much of the lake. A shaded-relief image of the lake floor and surrounding landscape shows the strong control that the lake floor morphology has on sediment distribution.
Figure 8. Sidescan sonar image showing the steep, smooth rock walls in the Narrows (see inset map for figure location) with the post-impoundment sediment lapping up against these cliffs. The seismic profile (location is the heavy red line on the sidescan image) shows the thick sediment covering the floor of the lake and lapping up against the steep rock walls. Note that the surface of the sediment is flat and that no sediment drape is apparent on the adjacent rock faces. On the seismic profile, 0.02 sec. two-way travel time is approximately 15-m.
Figure 9. Sidescan sonar image of part of the terraced rock outcrop that characterized much of the northern side of the lake (location in inset map). Terraces show as alternating bands of high and low backscatter on the sidescan while uniform low-backscatter along southern part of image is post-impoundment sediment. Small tributary valleys with low-backscatter floors have thin sediment cover. Seismic profile ( heavy red line on sidescan image) shows the gentler gradient and terraced nature of this side of the lake as well as post- impoundment sediment onlapping and burying this older surface. On the profile, 0.02 sec. Two-way travel time is approximately 15 m.
Figure 10. Sidescan sonar image showing part of a submerged alluvial fan (location in inset map). The fine, downslope-trending, low backscatter lineations represent the floors of a network of small channels that sculpt the surface of this alluvial fan. One of the larger channels appears to be a braided stream. Seismic profile shows that these channels mostly have less than 5-m relief (0.2 sec. TWT is approximately 15 m), and that there is no sediment cover on the alluvial fan.
Figure 11. Central part of Boulder Basin (location in inset map) showing a subtle channel expressed on the lake floor in the sidescan mosaic as well as on two seismic profiles. Profile locations shown on sidescan mosaic. Profiles show that the channel has less than 2-m relief. Note also that sediment filling the basin floor is remarkably flat. The sediment, where it pinches out along the edge of the basin, is less than 5 m shallower than it is in the channel axis.
Figure 12. Sidescan sonar image from Las Vegas Bay (location in inset) showing the low-backscatter sediment cover in the narrow valley floor and the high-backscatter signature from the rock faces adjacent to the floor. Sediment cover is thin as patches of high-backscatter rock outcrop are present in the floor of the valley. Seismic profiles (locations shown on sidescan image) show the flat-floored nature of the valley and that sediment here is less than 2 m thick. Note rock outcrop on line 41 that also shows on the sidescan image.
Figure 13. Four seismic profiles showing the acoustic stratigraphy of the sediments filling the lake. A: profile form Boulder Canyon showing flat-lying sediments abutting the steep canyon walls. B: profile immediately south of Callville Bay showing the channel and pinching out of sediment against the gentle northern slope. C: profile south of Swallow Bay showing four reflectors in the post-impoundment section. Note that all of them are nearly horizontal and that the deeper ones are not laterally as extensive as the shallower ones. D: profile in the western part of Boulder Basin showing that the channel is still present here. Profile locations shown on inset map.
Figure 14. Sidescan sonar image of part of an alluvial fan surface in the western part of Boulder Basin (location shown in inset). The preservation of a road (about 15-m wide), trail and possibly foundations indicates that little reworking has taken place on this part of the lake floor since the area was flooded. Note also that the alluvial deposits are truncated by the road (end of arrows) which indicated that they have not been active since the lake was filled.
Figure 15. Sidescan sonar image from the southwestern side of Boulder Basin showing the transition from a continuous sand sheet to the channelled alluvial fan surface. The continuous sand sheet occurs in water less than about 18 m deep and probably represents reworking of the alluvial fan deposits by waves. The 18-m depth of this transition probably represents the maximum lowering of the lake since it initially was filled.
Figure 16. Sidescan sonar image and seismic profiles of the end of the valley in Las Vegas Bay where it opens into Boulder Basin. Sidescan image shows a narrow channel floor bounded by steep rock walls where it exits Las Vegas Bay and ends in Boulder Basin. The presence of sediment covering the valley floor on profile A at a depth shallower than the floor of Boulder Basin (138 m) suggests sediment is passing down this channel from the Las Vegas Wash area, but the thin sediment cover and absence of a channel or fan-like deposit where it opens onto Boulder Basin (profiles B and C) suggests the valley is not a major sediment source to the Boulder Basin area.