SEDIMENT ACCUMULATIONS ON THE BOTTOM OF LAKE MEAD: A 70-YEAR RECORD OF DEPOSITIONAL PROCESSES

Lake Mead formed after completion of Hoover Dam in 1935, and high-resolution seismic profiles and short cores (3-5m length) have allowed us to map the distribution and stratigraphy of sediment deposited since impoundment. Continuous records of lake level and river discharge throughout the lake’s history help explain the stratigraphic development. Post-impoundment sediment is limited to the floors of the drowned Colorado River and tributary valleys, and is absent from the remainder of the lake floor. Maximum sediment thickness exceeds 80 m on the delta, which formed where the Colorado River enters the eastern end of the lake. Sediment thins to 20 m at the toe of the delta, and reaches a thickness of 15-35 m thick along the remainder of the drowned Colorado River channel to Hoover Dam. Tributary valleys have a thinner sediment cover indicating the Colorado River has been the primary sediment source. Cores recovered fine silt interrupted by graded beds containing as much as 30% sand that were deposited from turbidity currents, which flowed the full length of the lake. Seismic stratigraphy shows three intervals of high amplitude reflections in the distal western part of the lake, which are attributed to periods of coarser-grained sedimentation. Deposition of the youngest of these coarser intervals occurred after completion of Glen Canyon Dam in 1964. At that time river discharge was greatly diminished, and subsequent coarse sediment input is tied to lowered lake level resulting in erosion of the subaerially exposed delta and westward transport of this coarser material by turbidity currents.