Table 1. Summary of areal extend and sediment volume associated with different parts of the post-impoundment deposit.
The following chart is listed according to
(1)FEATURE,
(2)AREA: meters squared,
(3)VOLUME: meters to the third power:
Study Area: 17,511,270 and no volume given
Rock Outcrop: 12,426,033 and no volume given
Total Sediment: 5,105,340 and 5,726,743
1997 Deltas - Total: 301,070 and 1,844,160
Las Vegas Delta: 181,311 and 1,702,399
Gypsum Delta: 101,458 and 138,819
Government Delta: 18,301 and 2,942
1968 Deltas - Total: 521,329 and 772,996
Las Vegas Delta: 294,828 and 562,660
Gypsum Delta: 163,515 and 126,154
Government Delta: 143,873 and 87,285
Prodelta - Total Since 1935: 4,584,011 and 3,109,587
Figure 1. Maps showing location of Lake Mead in the southwestern United States, Boulder Basin at the western end of the lake, and the study area in the northwestern corner of Las Vegas Bay.
Figure 2. Survey track lines along which sidescan sonar and subbottom seismic-reflection profiles were collected. Red lines are tracks along which data were collected in 2000 and cyan lines are tracks run in 1999. Core locations are shown by black triangles. Yellow lettered lines are locations of profiles shown in Figure 13.
Figure 3. Sidescan sonar image of the northwestern part of Las Vegas Bay. 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 black and dark gray shades. The imagery in the submerged parts of Las Vegas, Gypsum, and Government Washes was collected in 2000, and the imagery covering the deeper part of Las Vegas Bay was collected in 1999. Boxes show locations of Figures 9 and Figures 11. Orthophotograph showing the land area surrounding Las Vegas Bay was compiled by the EROS Data Center (USGS, 1999).
Figure 4. Six different surficial geologic units were identified on the sidescan imagery and their distributions are shown here.
Figure 5. Seismic profile showing the lake floor, the preimpoundment surface, and the post-impoundment sediment (red). In some areas such as on the left side of this profile there is no sediment cover on the lake floor.
Figure 6. Thickness of sediment that has accumulated in Las Vegas Bay since impoundment of Lake Mead. Sediment thicknesses are in meters. Note that the scale is not in even increments. The thickest sediment is in the delta at the mouth of Las Vegas Wash, but note the tongues of moderately thick sediment beyond the deltas off Las Vegas and Gypsum Washes. In the remainder of Las Vegas Bay sediment is mostly less than 1.5 m thick and is limited to the floors of submerged stream beds.
Figure 7. Shaded-relief with 10-m contours generated from a digital elevation model (U.S.G.S., 1999) based on preimpoundment topography. Contours are relative to lake level in June, 2000 which was 268 m. The original stream beds, the steep walls to these valleys, and the rough terrain along the flanks of Las Vegas Bay are all shown in this image. The deltas at the mouths of Las Vegas, Gypsum, and Government Washes are morphologic features that have formed since impoundment.
Figure 8. Four profiles from the delta and prodelta area off the mouth of Las Vegas Wash. Profiles A and B are strike lines, one on top of the delta (A) and one beyond the edge of the delta. The approximate depth of the preimpoundment valley floor is shown on profile A. Profiles C and D are dip lines that show the flat delta top, the steep delta front, and flat prodelta area. Note the topset bed that is buried by less than 1 m of sediment on the delta top, and faint foreset reflectors below the topset bed.
Figure 9. Orthophotograph taken in 1996 when the lake was about 4-m lower than when this survey was conducted showing a stream and subaerial part of the delta. B. Sidescan sonar image superimposed on orthophotograph for reference showing the same stream bed that now is submerged. The red line marks the location of the seismic profile shown in C, and the red arrow marks the transition from delta top to delta front. Box outlines area shown in Figure 10.
Figure 10. Sidescan image of the delta front part of the delta off Las Vegas Wash. The headwall scarp is located on the uppermost part of the delta front, and the slump deposits cover the lower part of the delta front. The former stream bed that was shown in Figure 9 can be seen extending to the edge of the delta. Figure location is shown in Figure 9.
Figure 11. Sidescan mosaic (top panel) showing the distribution of the three facies that occur beyond the delta front. High-backscatter sand tends to occur nearest the delta front while moderate backscatter sandy muds and low-backscatter muds in general occur further from the delta. The rock outcrops in the low-backscatter mud area suggest that the mud cover is thin and discontinuous. The floats for Las Vegas Bay Marina and the floating break water show on the image as well. Red line shows location of seismic profile in lower panel. Seismic profile shows the 2-4 m thick tongue of sediment extending beyond the delta front. Box shows location of Figure 12.
Figure 12. The delta front of the delta off Gypsum Wash shows at the northern edge of the image. Extending southward from the delta front is a narrow channel that leads to a high-backscatter sand deposit in 20-25 m water depth. This sand deposit appears to be the surficial expression of an older delta that probably formed between 1964-1968 when the lake was about 20 m lower than at present. The seismic profile (location shown by red line on sidescan image) shows the delta deposit and the tongue of thicker sediment underlying the area of high-backscatter sands.
Figure 13. Seismic profiles showing different examples of the post-impoundment sediment fill. A and B show places where the sediment is thicker and has some internal reflectors. C shows the thin sediment cover blanketing only the floor of the submerged valley, and D shows foreset beds in what is interpreted to be a delta off Government Wash that formed in 1968. Profile locations shown in Figure 2.
Figure 14. Bar graph showing the relative proportion of sediment deposited in different parts of Las Vegas Bay. The deltas have been broken into two parts: the 1968 (light gray part of the bars) and 1997 (dark gray part of the bars) deltas. The delta in Las Vegas Wash is by far the largest of the three deltas, but its volume is not as great as that of the fine-grained prodelta deposits. Prodelta deposits include all sediment, excluding the deltas, that has accumulated in this part of the lake since impoundment.
Figure 15. Graph showing monthly elevation of Lake Mead since 1935. The dotted lines show three lake elevations significant to this study. Present refers to the time of the survey (June, 2000), and the other two lines show the elevation of the lake during two periods of delta formation.