HR: 14:25h
AN: H32F-03    [PDF]
TI: Detecting Cumulative Effects on Low-gradient Streams in the Central Sierra Nevada, California
AU: * MacDonald, L H
EM: leemac@cnr.colostate.edu
AF: Dept. of Forest, Rangeland, and Watershed Stewardship, Colorado State University, Fort Collins, CO 80523 United States
AU: Brown, N E
EM: brune@cnr.colostate.edu
AF: Dept. of Geosciences, Colorado State University, Fort Collins, CO 80523 United States
AU: Coe, D B
EM: dcoe@nooksack-tribe.org
AF: Nooksack Indian Tribe, 5016 Deming Road, Deming, WA 98244 United States
AB: Cumulative watershed effects (CWEs) result from multiple activities over time and space. The assessment of CWEs in the Sierra Nevada is severely limited by the lack of field data on the effects of a given action, the lack of models to predict the effects of multiple actions at the watershed scale, and the limited data relating stream channel conditions to measured or predicted changes in sediment yields. Since 1999 we have been measuring hillslope-scale sediment production rates from roads, timber harvest, wildfires, and minimally-disturbed areas. From these and other data we are developing models to predict sediment production rates from different land uses. The next, more difficult step in developing a reliable CWE model is to compare predicted changes in catchment-scale sediment production rates to stream channel conditions. Channel conditions were measured in 28 pool-riffle reaches in the American and Cosumnes river basins. Contributing areas ranged from 2.9 to 70 km2, and the elevations of the reaches ranged from 1200 to 1800 m. The basins were selected to encompass a wide range in the amount of natural and anthropogenic disturbance. Variables used to characterize the amount of disturbance include road density, number of road crossings, modeled road sediment production, percent forest harvest, and the percent burned by wildfire. The data collected in each reach included gradient, channel dimensions, grain size, amount of large woody debris, and pool size and sediment infill. Preliminary results indicate that basins with more than 20% of the area in granitic lithologies have significantly more fine sediment coverage in pools (r2=0.35; p=0.0002). The fine sediment coverage in pools also was correlated with the amount of large woody debris. After removing the variability due to lithology and basin area, there was a significant, positive correlation between the estimated sediment production from unpaved roads and residual pool infill volume (r2=0.14; p=0.02). Other than these relationships, there appear to be no significant correlations between the measured channel characteristics and the amount of disturbance. The limited number of significant correlations may be a consequence of a record flood event in early 1997. This event may have effectively "reset" the stream channels, resulting in a lower detectability of cumulative watershed effects.
DE: 1803 Anthropogenic effects
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1899 General or miscellaneous
SC: Hydrology [H]
MN: 2003 Fall Meeting