FLUVIAL GEOMORPHOLOGY OF REDWOOD CREEK, HUMBOLDT COUNTY, CALIFORNIA

Redwood Creek is an elongate coastal watershed, which runs north-northwest to the Pacific Ocean in northern California. The 108-km (67-mi) mainstem channel trends parallel to the regional structural grain. A series of large 20+-year storms in 1955, 1964, 1972, and 1975 contributed sediment to the stream network, causing aggradation of streambeds. Previous studies by staff of Redwood National and State Parks showed active channel sediment moved downstream at one-half to one mile per year and elevated sediment left tributaries within 5-10 years after a major input of sediment, unless dammed by debris. A moderate 12-year storm in 1996/1997 contibuted sediment into some streams.

Redwood Creek has been recognized as impaired due to excess sediment loading and awaits the development and implementation of a Total Maximum Daily Load. State agencies, including the California Geological Survey (CGS), studied Redwood Creek to determine the status of the watershed. The results were based on extensive existing information and new data collected by the state agencies.

CGS modeled channel gradients with a 10-meter Digital Elevation Model (DEM) to map source, transport and response reaches. The mainstem channel is dominated by low-gradient response reaches; tributaries are dominated by steeper source and transport reaches.

CGS mapped channel features from aerial photographs taken in 1984 (1:32,000) and 2000 (1:24,000). We identified stable stream features, such as point bars and vegetated gravel bars, and unstable features indicative of stream disturbance. The unstable channel features included lateral and mid-channel bars, tributary fans, and eroding banks. CGS used the lengths of the unstable features to analyze the distribution of stream disturbance. The results of our air-photo study and analyses correlate positively with results from previous field studies.

Between photo years 1984 and 2000, stream disturbance in the watershed decreased in length. Most features disappeared from tributary channels, at our mapping scale, and disturbance decreased in the mainstem channel. However, in the lower 10.5 km (6 mi) of Redwood Creek, disturbance increased from 5.6 km (3.5 mi) in photo year 1984 to 10.5 km (6.5 mi) in photo year 2000, due to the buildup of multiple gravel bars in a braided reach of the mainstem channel.