HR: 17:30h
AN: H54A-06    [Abstracts]
TI: Sequential Sediment Budgets in an Ungauged Watershed: Redwood Creek, Marin County, California
AU: * Downs, P W
EM: downs@stillwatersci.com
AF: Stillwater Sciences, 2855 Telegraph Ave, #400, Berkeley, CA 94705 United States
AU: Stallman, J
EM: jay@stillwatersci.com
AF: Stillwater Sciences, 850 G Street, Suite K, Arcata, CA 95521 United States
AB: Sediment budgets provide an organizing framework in fluvial geomorphology and have enormous potential in environmental management. A sediment budget approach assisted in developing strategies for restoring Big Lagoon, the wetland ecosystem at the terminus of the 22.7 km² Redwood Creek watershed in Marin County, California. Persistence of a restored lagoon largely depends on the current sediment yield relative to the reference yield prior to European settlement. Process-based, distributed sediment budgets were constructed for several historical time periods to account for accelerated sediment production from contemporary land management practices and legacy factors stemming from past resource exploitation. Sediment production, storage, and transfer were investigated using digital terrain modeling, field reconnaissance to ascertain and validate hillslope processes, mainstem channel surveys and dendrochronology to assess trends in alluvial sediment storage, application of published process rate estimates, use of short-term and prorated stream gauging records, and sediment transport modeling to validate sediment yields into Big Lagoon. Evidence suggests that the Redwood Creek valley bottom aggraded from at least 3,500 B.P., with floodplain wetlands acting as sediment sinks (average annual sediment yield of 34 t km² yr^-1). Channel incision rapidly followed European settlement and intensive hillslope disturbances beginning around 1840 (peak yield 1921-1982 of 324 t km² yr^-1). Mainstem and large tributary valley bottoms became major sediment sources during this time and remain sources despite progressive retirement of most agricultural land use (yield 1981-2000 of 198 t km² yr^-1). Numerous issues related to data availability and resolution limited quantification of some sediment sources and resulted in potential uncertainties in estimates of yield to Big Lagoon. Historical sediment budgets, however, require more than adequate data sources, they require accurate conceptual models of geomorphic processes operating over historical time periods as the basis for correctly interpreting results, while the applicability of sediment budgets over decadal timescales may depend on climate factors influencing the frequency and magnitude of large storm events during the period. Further, legacy factors affecting sediment storage and transfer are critical yet difficult to resolve and often overlooked in sediment budgeting. `Rapid' sediment budgets are desirable components of historically-informed environmental management but are subject to uncertainties relating to data availability, accuracy, resolution, interpretation, and applicability. Future work is focusing on data acquisition necessary for a transparent realization of this potential.
DE: 1625 Geomorphology and weathering (0790, 1824, 1825, 1826, 1886)
SC: Hydrology [H]
MN: Fall Meeting 2005