STRATIGRAPHIC EVOLUTION OF LAS VEGAS BAY, LAKE MEAD, NEVADA: 1935 – 2002, PART 1 – THE SUBMERGED PORTION

Lake Mead, NV and AZ is the largest reservoir on the Colorado River. Because the reservoir’s history is well documented through historic records and recent geological surveys, it provides a unique mid-scale laboratory to study the effects the fluctuations in lake level, sediment supply, and basin physiography, have had on stratigraphic evolution of the basin.

Fifteen chirp seismic lines, two sidescan sonar surveys, sixteen sediment vibra-cores, laser particle size analysis and 137Cs analyses have been used to determine the distribution and temporal relationships of the sediment within the Las Vegas Bay (LVB) region of Lake Mead. Sidescan sonar imagery shows that sediment distribution is limited to the floors of the valleys in LVB. Seismic profiles show that accumulations beyond the deltas are less than 4 m thick. Several beds within the cores have been dated using 137Cs analyses and are correlated between cores to document the depositional response of LVB to changes in lake level and sediment input. The evolution of the basin can be divided into two generalized periods. The early period (1935-1964) is characterized by a low input of fine-grained sediment and dramatic seasonal fluctuations in lake level. The latter period (1965-2002) is characterized by larger sediment input, coarser material, and longer period changes in lake level.

Sediment supply appears to have been the largest factor governing the temporal filling of the basin. While dramatic changes in lake level occurred during the early history of the reservoir, they are not well expressed in the deeper portions of the basin because of a low sediment supply at this time. Changes in lake level are more clearly preserved during the latter part of the basin’s history because of higher sedimentation rates and more varied sediment types.

Physiography has played a significant role in controlling the path sediment laden underflow currents travel through the study area. Sand is observed on the basin floor where there are bends in the main channel, and where breaks in slope occur where lateral tributaries debouch into the main channel axis. Two fan complexes have been identified at the base of tributaries to LVB in seismic profiles, and a single muddy meander channel within a relatively flat portion of LVB has been identified using sidescan sonar imagery.