Enhanced TIFF Sidescan-Sonar Mosaic East of Virgin Basin - Lake Mead, Nevada: Geographic Coordinates

Lake Mead is a large interstate reservoir located in the Mojave Desert of southeastern Nevada and northwestern Arizona. 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 provides water for residential, commercial, industrial, recreational, and other non-agricultural users in communities across the southwestern United States. Extensive research has been conducted on Lake Mead, but a majority of the studies have 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 (Preissler, et al., 1998; Bevans et al, 1996; Bevans et al., 1998; Covay and Leiker, 1998; LaBounty and Horn, 1997; Paulson, 1981). By contrast, little work has focused on the sediments in the lake and the processes of deposition (Gould, 1951). To address these questions, sidescan-sonar imagery and high-resolution seismic-reflection profiles were collected throughout Lake Mead by the USGS in cooperation with researchers from University of Nevada Las Vegas (UNLV). These data allow a detailed mapping of the surficial geology and the distribution and thickness of sediment that has accumulated in the lake since the completion of Hoover Dam. Results indicate that the accumulation of post-impoundment sediment is primarily restricted to former river and stream beds that are now submerged below the lake while the margins of the lake appear to be devoid of post-impoundment sediment. The sediment cover along the original Colorado River bed is continuous and is typically greater than 10 m thick through much of its length. Sediment thickness in some areas exceeds 35 m while the smaller tributary valleys typically are filled with less than 4 m of sediment. Away from the river beds that are now covered with post-impoundment sediment, pre-impoundment alluvial deposits and rock outcrops are still exposed on the lake floor.米德湖（Lake Mead）是一座大型跨州水库，坐落于内华达州东南部与亚利桑那州西北部的莫哈韦沙漠（Mojave Desert）。该水库于1935年随胡佛水坝（Hoover Dam）的兴建完工而蓄水，是科罗拉多河（Colorado River）上一系列多功能水库中的一员。该湖从大峡谷河口延伸至胡佛水坝所在的黑峡谷，全长183千米，可为美国西南部各州社区的住宅、商业、工业、娱乐及其他非农业用水主体提供水源。学界已针对米德湖开展了大量研究，但绝大多数研究均围绕测定湖体水体、沉积物及生物群落中的人为污染物水平展开，涵盖合成有机化合物、重金属、溶解离子、呋喃/二噁英以及营养盐负荷等指标（Preissler等，1998；Bevans等，1996；Bevans等，1998；Covay与Leiker，1998；LaBounty与Horn，1997；Paulson，1981）。与之形成鲜明对比的是，针对湖体沉积物及其沉积过程的研究少之又少（Gould，1951）。为填补这一研究空白，美国地质调查局（USGS）联合内华达大学拉斯维加斯分校（University of Nevada Las Vegas, UNLV）的研究人员，在米德湖全域采集了侧扫声呐（sidescan-sonar）影像与高分辨率地震反射剖面（high-resolution seismic-reflection profiles）数据。这些数据可用于精准绘制湖床表层地质图，以及胡佛水坝建成后在湖内沉积的沉积物的分布与厚度特征。研究结果显示，蓄水后形成的沉积物主要分布于此前被湖水淹没的古河道与古溪床中，而湖岸带几乎无蓄水后沉积物分布。沿原科罗拉多河河道分布的沉积物覆盖层连续完整，多数河段的沉积物厚度超过10米；部分区域的沉积物厚度甚至超过35米，而规模较小的支流河谷内的沉积物厚度通常不足4米。在未被蓄水后沉积物覆盖的古河道之外，湖床仍裸露着蓄水前的冲积沉积物与岩石露头。