Summary

Results of a geophysical mapping of the northwestern part of Las Vegas Bay show that post-impoundment sediment is derived from three point sources and is concentrated along the floors of submerged stream beds. Three ephemeral washes discharge into Las Vegas Bay. Of these, Las Vegas Wash has created at least two deltas at its mouth that account for approximately 41% of the total volume of post-impoundment sediment in the study area. The deeper of the deltas off Las Vegas Wash appears to have formed when the lake was about 20 m lower between 1964 and 1968. The second delta formed between 1989 and 1997 when the lake was about 7 m lower than present. The deltas off Gypsum and Government Washes, when combined together only account for about 6% of the total volume. The fine-grained sediment derived from these streams is carried several kilometers into Las Vegas Bay, and perhaps even as far as Boulder Basin apparently by density flows. This fine-grained sediment accounts for nearly 53% of the total volume of post-impoundment sediment deposited in the study area.

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