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Lake Mead Mapping Completed
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Research vessel: Houseboat rigged with SIS-1000,
boomer, and fathometer was distinctive and drew curious crowds when we were docked.
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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.
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.
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June 2001
in this issue:
cover story:
Great Blue Hole of Belize
Channel Islands Cruise
Lake Mead Mapping
New Underwater Microscope System
Hurricane Display
Reston Open House
WHFC Outreach
Monterey Open House
School-to-Work Partnership
Acadiana Migratory Bird Day
SWICA-M³
Global Assessment of Geologically-Sourced Methane
Methane Hydrates
Metadata Workshop
Sue Hunt—Recycling
Coastal Stewardship
GIS 2001: Logan
GIS 2001: Massachusetts Bay
WHFC Visitors
Northeast Earthquake Hazards Map
June Publications List
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U. S. Department of the Interior | U.S. Geological Survey
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