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The geomorphic evolution and depositional history of the Carson River delta, Lahontan Reservoir, Nevada
AuthorRowland, Joel Carey
Geological Sciences and Engineering
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Since completion in 1915, Lahontan Reservoir has impounded water from the Carson and Truckee Rivers and served as a sink for sediments and trace elements including Hg (mercury), Au (gold), and Ag (silver) derived from mining operations and transported by the Carson River. At the mouth of the Carson River over half of the sediment entering Lahontan Reservoir has been deposited in a delta, referred to as the Carson River deltaic complex, that covers a 10 km length of the reservoir's bed. This study was undertaken to determine the morphology and geomorphic evolution of the Carson River deltaic complex by examining the role of base level, river discharge, channel location, and channel form on delta development. In addition, this study assess the impact of fluvial processes upon the distribution, deposition, and remobilization of Hg across the Carson River deltaic complex. On a seasonal basis the deltaic complex does not adjust either its channel or depositional patterns in response to large fluctuations in base level and river discharge. In response to longer term (5 to 10 year) changes in hydrologic conditions, such as those resulting from drought, the deltaic complex also remains limited in its morphological adjustments. This lack of deltaic response is due to the erodibility and location of both prereservoir and post-dam geologic units, as well as the rate and timing of the fluctuations in hydrologic conditions. The primary response that has occurred is an upreservoir migration of the depositional locus that is due to the continuing aggradation of the main distributary channel. The net result of migration has been the formation of three post-dam depositional deltaic units. Upstream of the deltaic complex reservoir construction has resulted in the development of an anastomosing channel system along a 5 km reach of floodplain.
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