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Research Projects Sediment-Related Road Effects on Stream Channel Networks in an Eastern Sierra Nevada Watershed PI: Tom Bullard Project Period: 2000 - 2002 Funding: State of California Lahontan Regional Water Quality Control Board Keywords: stream channel networks, Sierra Nevada watershed, sediment, road effects, Squaw Creek, Lake Tahoe |
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Project Description Problem Definition: Squaw Creek, a small (21.1 km2), subalpine watershed located approximately 9.6 km northwest of Lake Tahoe, California between the towns of Tahoe City and Truckee. Squaw Creek consists of a main, low-gradient trunk that is formed from the confluence of two primary forks in the north and south subwatersheds, as well as numerous first through third order streams feeding into the main channels. Current land uses include an expanded ski resort with year-round recreational opportunities, a golf course, commercial equestrian operations, sports fields, a network of hiking and bicycling trails, logging roads, residential development, and ski resort maintenance access roads. The stream system is listed as an impaired waterway for excessive non-point source sedimentation under section 303(d) of the Clean Water Act because of its high sediment production and turbidity levels relative to other tributary watersheds to the Lower Truckee River. The principal intent of the study was to gain a greater understanding of the geomorphic processes influencing sediment movement through a subalpine catchment that could assist land managers in identifying sediment sources and applying appropriate measures to control sediment entering the stream system. Approach: We conducted a sediment source assessment that focused on three primary objectives: 1) a watershed scale geomorphic process-response analysis, 2) qualitative and quantitative analyses of hillslope and in-stream erosion and storage, and 3) an assessment of the influence of land use and cover on sediment supply and transport processes. The watershed was evaluated from a geomorphic perspective to identify and characterize sources of sediment, including influences of road networks on sediment delivery to the stream network. A qualitative and quantitative assessment of the spatial and temporal variability of sediment transport was completed followed by historic and present-day erosion susceptibility models created by compiling field and morphometric data to aid in the assessment of land use and land cover influences on sediment sources. The models were developed utilizing a Geographic Information System (GIS), field investigations, and modified published methodologies to identify and characterize natural and anthropogenic influences on sediment production, transport, and storage in the watershed. Roads in the watershed contribute to sediment production by concentrating runoff, thereby increasing sediment load to the stream network. Most unimproved (dirt) roads connect either directly or indirectly with streams and, therefore, act as extensions of stream networks by effectively increasing watershed drainage density and subsequently sediment loads to streams. In the South Fork subwatershed of Squaw Creek, road connectivity may increase the effective drainage density by a factor of 2 or more. |
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