Reservoir Sedimentation Related to Wildfire in the Sierra Nevada Mountains of California

Science Center Objects

Reservoirs naturally fill with sediment transported from the upstream watershed. As reservoirs fill with sediment, the amount of available water decreases for functions such as water supply, irrigation, flood control, and hydropower. In the Sierra Nevada Mountains of California, a large number of reservoirs are significantly affected by reservoir sedimentation (Minear and Kondolf 2009). Factors that affect reservoir sedimentation include: sediment yield of the watershed, the age of the reservoir, the ability of an individual reservoir to trap sediment (trap efficiency), and the sediment trapped by other dams in the watershed, among other factors (Morris and Fan 1998). Wildfires can greatly increase reservoir sedimentation rates, primarily by increasing the sediment yield of the watershed. In Southern California, wildfires have been shown to increase sediment yield rates by 5-120 times the non-wildfire sediment yield rate (Rice 1982, Doehring 1968, and Krammes 1969). For the Sierra Nevada Mountains of California, there are very few studies on the effects of wildfire on sediment yield, particularly for large watersheds.

Location map of study area in California

Location map of study area in California, USA. Study area is shown in red (Sierra Nevada Mountains), yellow dots are dams regulated by the state Division of Safety of Dams.

The Sierra Nevada Conservancy (SNC), a state-funded organization, would like to better understand reservoir sedimentation and in particular, the sediment-related effects of wildfire in the Sierra Nevada Mountains of California (Figure 1). This knowledge will be increasingly important in the future because climate models project an increase in the frequency, magnitude and range of wildfires in the Sierra Nevada (Westerling and Bryant 2008). A better understanding of these processes will allow the SNC to help federal, state and local water managers plan for better watershed management and to better assess the costs associated with minimizing reservoir sedimentation and the effects of wildfires.

This study has two phases: the first will consist of the collection of existing data and updating a regional reservoir sedimentation model to locate reservoirs effected by fire. The second phase is a future phase, not covered by this proposal, for which detailed field studies will be conducted at to-be-determined reservoirs that are affected by fire. Phase 1, which is the phase covered in this proposal, has four objectives and associated tasks: 1. Compile existing reservoir sedimentation records; 2. Develop a regional GIS-based reservoir sedimentation model for the Sierra Nevada Mountains; 3. Locate fire-affected reservoirs for future detailed field studies; 4. Develop a proposal for future field-based research to determine wildfire-related rates of sediment yield at those reservoirs determined in Objective 3.

The results of this study will culminate in a better understanding of reservoir sedimentation and sediment yields related to wildfire, which directly coincides with USGS Science Strategies, addressing 1. Understanding Ecosystems and Predicting Ecosystem Change, 2. A Water Census of the United States, and 3. Climate Variability and Change (USGS 2007). In addition, the scientific knowledge gained from this study and the connections with local cooperators will be beneficial to Center scientists for future work in the Sierra Nevada. The matching funds requested in this proposal supports the Water Resource Area Priorities and the Cooperative Water Program mission by providing reliable, impartial and timely information needed to understand water resources through shared efforts and funding with State and local partners.