Enhanced TIFF Sidescan-Sonar Mosaic 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）。该湖于1935年因胡佛水坝（Hoover Dam）的修建完成蓄水，是科罗拉多河（Colorado River）上一系列多功能水库中的一员。湖体全长183公里，从大峡谷口延伸至胡佛水坝所在的黑峡谷（Black Canyon），可为美国西南部多个社区的住宅、商业、工业、娱乐及其他非农业用水用户供水。针对米德湖的研究已较为广泛，但绝大多数相关研究均聚焦于测定湖水、沉积物及生物群落中的人为污染物水平，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（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，针对湖内沉积物及其沉积过程的研究寥寥无几（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，在全湖范围内采集了侧扫声呐（sidescan-sonar）影像与高分辨率地震反射剖面（high-resolution seismic-reflection profiles）数据。此类数据可用于详细绘制湖底表层地质图，以及胡佛水坝建成后在湖内沉积的沉积物的分布与厚度。研究结果表明，蓄水后的沉积物堆积主要局限于如今已被湖水淹没的古河道与古溪床区域，而湖岸周边几乎无蓄水后沉积物分布。原科罗拉多河河床沿线的沉积物盖层连续分布，大部分河段的沉积物厚度通常超过10米；部分区域的沉积物厚度甚至超过35米，而较小的支流山谷内的沉积物充填厚度通常不足4米。在未被蓄水后沉积物覆盖的古河道区域，蓄水前的冲积层（alluvial deposits）与岩石露头（rock outcrops）仍裸露于湖底。