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Surficial Geology and Distribution of Post- Impoundment Sediment of the Western Part of Lake Mead Based on a Sidescan Sonar

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Surficial Geology and Distribution of Post- Impoundment Sediment of the Western Part of Lake Mead Based on a Sidescan Sonar USGS science fora changing world Surficial Geology and Distribution of Post- Impoundment Sediment of the Western Part of Lake Mead Based on a Sidescan Sonar and High-Resolution Seismic-Reflection Survey David C. Twichell1, VeeAnn A. Cross1, Mark J. Rudin2, and Kenneth F. Parolski1 U.S. Geological Survey, Woods Hole, MA Dept. Health Physics, University of Nevada at Las Vegas, Las Vegas, NV 114*44' 114*43* 36*4* IW44* !MS43' Open-File Report 99-581 Prepared in cooperation with Department of Health Physics, University of Nevada at Las Vegas December, 1999 U. S. Department of the Interior U. S. Geological Survey SURFICIAL GEOLOGY AND DISTRIBUTION OF POST-IMPOUNDMENT SEDIMENT OF THE WESTERN PART OF LAKE MEAD BASED ON A SIDESCAN SONAR AND HIGH-RESOLUTION SEISMIC-REFLECTION SURVEY David C. Twichell1, VeeAnn A. Cross1 , Mark J. Rudin2, and Kenneth F. Parolski 1 1 U.S. Geological Survey, Woods Hole, MA 02543 2 Dept. Health Physics, Univ. of Nevada at Las Vegas, Las Vegas, NV ABSTRACT Sidescan sonar imagery and high-resolution seismic-reflection profiles were collected in Las Vegas Bay and Boulder Basin of Lake Mead to determine the surficial geology as well as the distribution and thickness of sediment that has accumulated in these areas of the lake since the completion of Hoover Dam in 1935 (Gould, 1951). Results indicate that the accumulation of post-impoundment sediment is restricted to the original Colorado River bed which runs down the axis of Boulder Basin from Boulder Canyon to Hoover Dam, and the old Las Vegas Creek bed that bisects Las Vegas Bay. The sediment cover along the original Colorado River bed is continuous and is typically greater than 10-m thick throughout much of its length with the thickness in some areas exceeding 35 meters. The flat-lying nature of the deposits suggests that they are the result of turbidity currents that flow the length of the lake. The sediment cover in Las Vegas Bay is much thinner (rarely exceeding 2 m in thickness) and more discontinuous. The source for these sediments presumably is Las Vegas Wash and a series of other ephemeral washes that empty into this part of the lake. The presence of sediments along the entire length of the Las Vegas Creek bed suggests that turbidity currents probably are active here as well, and that sediment has been transported from these streams at least 10 km down the axis of this valley to where it enters Boulder Basin. Alluvial deposits and rock outcrops are still exposed on large parts of the lake floor. INTRODUCTION Lake Mead is a large interstate reservoir located in the Mojave Desert of southeastern Nevada and northwestern Arizona (Fig. 1). It was impounded in 1935 by the construction of Hoover Dam and is one of a series of multi-purpose reservoirs on the Colorado River. The lake extends 183 km from the mouth of the Grand Canyon to Black Canyon, the site of Hoover Dam, and is surrounded by Precambrian through Tertiary aged volcanic and intrusive rocks as well as Tertiary sedimentary strata and Quaternary alluvial deposits (Fig. 2). It provides water for residential, commercial, industrial, recreational, and other non-agricultural users in communities across the southwestern U.S. The Colorado River, via discharges from Lake Powell, supplies approximately 98% of the annual inflow to Lake Mead (Preissler et al, 1998, Water Resources Data, Arizona, Water Year 1998, 1998). The remainder is derived from the Virgin and Moapa Rivers which discharge into the Overton Arm, the Las Vegas Wash which discharges into 114°52' 114°50' 114°48 114042360 10' 114°40' 114°38' 114°3ff 36°10' 36°8' 36°8' 36°6' 36°6' 36°4' 36°4 36°2' 36°2' 36°OQ 114°54' 114°52' 114°50' 114°48' 114°46' 36°00' 114°44' 114°42' 114°40' 114°38' 114°36' Figure 1. Location map showing the morphology of the western part of Lake Mead and the surrounding area. Morphology is shown as a shaded- relief image that was derived from combining a digital elevation model of the surrounding landscape (USGS, 1999) with a gridded version of the bathymetry collected during this survey. Green line is the approximate location of the shoreline (360 m contour), and names are places referred to in the text. Inset shows the location of the study area (yellow box). 114°54' 114°38' 114°36' 36°10' 36°8' 36°8" 36°6' 36°6' 36°4' 36°4' Generalized Geology Allivial K-T Volcanics 36°2' Precambrian 36°2' T Intrusives T Sedimentary 6 Kilometers Coast 36°00' 114°54 114°52' 114°50' 114°48 114°46' 114D44' 114°42' 36°00' 114°40' 114°38' 114°36' Figure 2. Generalized geology of the area surrounding Lake Mead. Geology of Arizona is modified from Wilson et al. (1969), and geology of Nevada is modified from Stewart and Carlson (1978). Alluvial is mostly Quaternary and recent unconsolidated alluvial gravel, sand and silt. K-T Volcanics are mostly Cretaceous and Tertiary andesitic and basaltic flows. T Intrusives are mostly Tertiary aged rhyolitic flows and shallow intrusive rocks including diorite and granite. T Sedimentary are Tertiary aged tuffaceous sedimentary rocks. Precambrian includes gneiss and some areas of undivided schist and granite of Precambrian age. Las Vegas Bay, and a number of desert washes which surround the lake. Outflows from Lake Mead include the Colorado River (below Hoover Dam) and water diversions located at Saddle Mountain (Fig. 1) and in the Overton Arm. These diversions provide water for municipal and industrial uses for the greater Las Vegas Metropolitan area and Moapa Valley, respectively. The impact of these inflows on the ecosystem of Lake Mead has been well documented (Pressler, et al., 1998; Bevans et al, 1996; Bevans et al., 1998; Covay and Leiker, 1998; LaBounty and Horn, 1997; Paulson, 1981). Much of the work has been conducted in Las Vegas Bay and Boulder Basin of Lake Mead (Fig. 1) due to the proximity to the potable water intakes at Saddle Mountain. A majority of the studies involved determining levels of anthropogenic contaminants such as synthetic organic compounds, heavy metals and dissolved ions, furans/dioxins, and nutrient loading in lake water, sediment, and biota. However, little work has focused on the transportation and deposition of sediments in Las Vegas Bay and Boulder Basin. These sediments are known to be largely silts that have been transported along the lake floor by turbidity currents active in the original Colorado River bed (Gould, 1951). By 1947, in excess of 30 m of sediment had accumulated behind Hoover Dam (U.S. Bureau of Reclamation, 1947; Gould, 1951; Vetter, 1953). In 1965, the Glen Canyon Dam was built on the Colorado River approximately 300 km upstream of Lake Mead (Lucchita and Leopold, 1999). The effect of this dam on sedimentation in Lake Mead is not well understood. The purpose of this exploratory study is to utilize sidescan sonar imagery and high- resolution seismic reflection profiles to determine the surficial geology as well as the distribution and thickness of sediment that has accumulated in Las Vegas Bay and Boulder Basin of Lake Mead. METHODS A two-week sidescan sonar and high-resolution seismic-reflection survey was completed during May 12-26, 1999. Geophysical data were collected along approximately 335 km of survey tracklines (Fig. 3). The grid of survey lines, spaced 500-1000 m apart, provide complete sidescan sonar coverage of the study area except in some of the shallow parts of the lake, and allow a detailed mapping of the thickness and distribution of post-impoundment sediments within the study area. The survey was conducted aboard a 19-m houseboat. A Datasonics Inc. SIS-10001 sidescan sonar and chirp subbottom profiling system was used to collect the sidescan imagery and subbottom profile information. The sound source for the subbottom data was a chirp system with a central frequency of 3.5 kHz. Minimum resolution based on this frequency and the sampling interval used to log the data is approximately 50 cm. The sidescan imagery and subbottom data were logged digitally using an ISIS 1 digital acquisition system developed by Triton-Elics Industries1 . Both data types were collected on all tracklines. Single- beam bathymetry was collected along all the survey tracklines as well. All navigation was done with a P-Code GPS receiver with an estimated accuracy of +/- 10 m. The navigation information is stored every 10 seconds and includes Julian day, hour, minute, The use of trade names is for descriptive purposes only and does not indicate endorsement by the U. S. Geological Survey, nor the University of Nevada, Las Vegas. 114°4236°10' 114°40' 36°10 36°8' 36°8' 36°6 36°4' 36°2' Explanation Survey tracklines 114052' 114°50' 114048' 114°46' 36°00' 114044' 114°42' 114°40' 114038' 114036' Figure 3. Tracklines in the Boulder Basin and Las Vegas Bay parts of Lake Mead along which sidescan sonar, subbottom seismic-reflection profiles, bathymetry, and navigation were collected. All four types of data were continuously collected along all of these lines. second, longitude, latitude, and depth. The time recorded with the navigation information is in UTC. During the field operation, the sidescan sonar data were processed and a digital mosaic was completed. The sidescan imagery was demultiplexed, corrected for slant- range distortions and signal attenuation using techniques summarized by Danforth et al (1991). These strips of image data were then mapped into their proper geographic location using software described by Paskevich (1996). Once the individual strips were mapped, they were combined together into a composite digital mosaic using a remote sensing software package developed by PCI (Anonymous, 1992).
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