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Lake Mead Mapping Completed

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research-equipped houseboat at the dock
Research vessel: Houseboat rigged with SIS-1000, boomer, and fathometer was distinctive and drew curious crowds when we were docked.
Lake Mead is one of the largest reservoirs in the US, and some of the classic work on density flows was completed here in the early 1950s. Since then remarkably few publications have appeared on sedimentation and sedimentary processes in the lake. Presently the reservoir is one of the primary suppliers of water to residential, industrial and agricultural users in the southwestern US. Consequently, there is a growing concern about water quality. Because of uncertainty as to the continued importance of density flows in distributing sediment and possibly pollutants in the lake, a systematic mapping was undertaken of the entire lake. Sidescan-sonar imagery and seismic-reflection profiles were collected to map the geology of the lake floor and the thickness and distribution of sediment that has accumulated since completion of the Hoover Dam in 1935.

This field program was started in 1999 when the western third of the lake (nearest Hoover Dam) was surveyed, and the remainder of the lake was surveyed this April. VeeAnn Cross, Chuck Worley, and Ken Parolski (WHFC) mobilized a houseboat for the 4-week field program while Dave Twichell (WHFC) met with scientists from the Bureau of Reclamation, National Park Service, and WRD to discuss previous work and future plans. Mark Rudin, University of Nevada at Las Vegas (UNLV) joined VeeAnn, Ken, and Dave for the field program. Despite days with 40-50 kt winds, engine failures, and a seemingly unending array of equipment problems, a spectacular data set was collected.

sidescan sonar image of a portion of the bottom of Lake Mead
Bottom structures: Sidescan image showing alluvial fans that existed prior to formation of Lake Mead and that remain preserved on the sides of the lake. Sandy sediments that were deposited since impoundment cover the deepest part of the lake. Note channels in the surface of these sandy deposits.

Density flows seem to be the primary mechanism of sediment transport in Lake Mead. Post-impoundment sediment has a nearly flat surface and is limited to the deepest part of the lake, whereas the flanks of the lake are sediment free (or at least less than 20 cm thick). Away from the delta at the mouth of the Colorado River, these sediments are as much as 45 m thick, and several reflectors can be traced throughout the gas-free part of the deposit in the seismic profiles. The lake is divided into several basins separated by narrow canyons. This morphology has many analogies to turbidite pathways on the continental slope in the northern Gulf of Mexico. The sidescan imagery shows a downslope progression from sandy delta deposits through a zone of channelized sand deposition to muddy deposits. The subbottom profiles provide a unique opportunity to map the deposits generated by density flows and show how their geometry is influenced by basin morphology. Future work includes coring, which will be conducted in collaboration with scientists from UNLV to see if there is any truth to the story.

Related Web Sites
USGS Sediment Studies in Lake Mead
U.S. Geological Survey (USGS)

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Channel Islands Cruise

Lake Mead Mapping

Research New Underwater Microscope System

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