2007 Annual Report
1a.Objectives (from AD-416)
To determine rates and volumes of fine-sediment transport to Lake Tahoe from distributed and stream-channel sources.
1b.Approach (from AD-416)
Radionuclides (137Cs, 210Pb) and stable isotopes (13N and 15C) will be used to fingerprint fine-grained sediment sources. Suspended sediment will be sampled at three watersheds during the snowmelt runoff season (May to June). The upland sources of sediment will be investigated before and after stream sampling and may include, but not be limited to, construction sites, dirt roads, fields, pasture, forest, channel bed and banks, and floodplains. Sampling of upland sediment sources will occur in late summer. Cesium-137 can also be used to inventory upland erosion rates using a mass budget approach. Samples for the erosion mass budget will be collected during the upland-source sampling period. All sediment collected for fingerprinting analyses will have the sand removed (> 0.63 mm). Radionuclides will be analyzed at the USDA-ARS, Beltsville Agricultural Research Center, MD, Hydrology and Remote Sensing Lab facilities under the direction of Jerry Ritchie, Soil Scientist. Stable isotopes will be analyzed at the U.S. Geological Survey Stable Isotope Lab in Reston, VA under the supervision of Ty Coplen, Research Hydrologist. When all data have been analyzed by appropriate laboratories, discriminant analysis will be used to determine if the fingerprints are capable of discriminating between sources. A multivariate mixing model will subsequently be used to estimate the relative contributions of each source. The mixing model developed for the Chesapeake Bay Sediment Studies by Jurate Landwehr, U.S. Geological Survey National Research Program, Research Stochastic Hydrologist, will be used. Erosion yields from the Cesium mass budget analysis will be quantified using existing models. Research in fine-grained sediment tracking will be led by Dr. Allen Gellis, USGS, Maryland District, working in cooperation with the ARS, NSL. Using the methodology described above in combination with fine-grained sediment loadings data, the ARS will provide estimates of the contribution of total and fine suspended sediment resulting from in-stream erosional processes for the three watersheds. These watersheds include: Upper Truckee River, Blackwood Creek, and Third Creek.
This report serves to document research conducted under a Reimbursable Cooperative Agreement between ARS and the University of California, Davis. Additional details of research can be found in the report for the in-house project 6408-13000-017-00D, "Integrated Assessment and Analysis of Physical Landscape Processes that Impact the Management of Agricultural Watersheds." Results of research that estimate fine-sediment contributions to Lake Tahoe from 63 of its contributing watersheds was submitted to a peer-reviewed journal for publication. Results represent the first basin-wide study aimed at determining total, fine-sediment loadings to Lake Tahoe as well as determining contributions from streambank erosion. The bank-stability and toe-erosion model was used in the three largest contributing streams to determine potential sediment-load reductions that could be realized using various bank-stabilization measures. Results show that bank-toe erosion can be halted or minimized by the placement of rock along the bank toe, leading to an average reduction in loads of 85% from streambank erosion. The effects of planting of vegetation or decreasing channel slope by placement of additional meanders was found to be moderately effective in reducing loads but, are not as cost effective. A peer-reviewed journal article is being readied for submission. The results of this research are critical for action agencies responsible for developing strategies for improving the clarity of the waters of Lake Tahoe which have seen a consistent reduction over the past 40 years.