COMPARISON OF HOLOCENE TERRACE DEPOSITS WITH ACTIVE DEPOSITIONAL PROCESSES IN THE BEAR CREEK SUB-BASIN, LOWER EEL WATERSHED, HUMBOLDT COUNTY, CALIFORNIA

This study evaluates the current and historical depositional processes in the Bear Creek sub-basin in Southern Humboldt County, California. The Bear Creek sub-basin covers an area approximately 8 square miles and drains to the Lower Eel River approximately 8 miles upstream of Scotia, CA. The stream network is deeply incised with steep slopes draining directly to the stream channels throughout the sub-basin. Rapid influx of sediment during the 1997 wet season led state and federal regulatory agencies to list Bear Creek as impaired. The enlargement of the main stem channel of Bear Creek removed much of the alder-dominated riparian vegetation within the valley bottom. Sediment deposition rapidly filled the enlarged channel to an elevation varying between 4 and 9 feet above the pre-event streambed elevation. The primary depositional reach is approximately 3 miles long. Within this reach, the channel is closely constrained by hill slopes to a stream valley approximately 150 feet wide. The active channel width varies from 20 to 30 percent of the valley floor and meanders across the valley bottom occasionally eroding into the adjacent hill slopes. This depositional surface associated with the 1997 wet season is preserved as a terraced landscape throughout this reach. Four distinct sets of terraces were observed within this reach. An older terrace associated with the 1964 wet season was observed at an elevation roughly 3 feet higher than the 1997 terrace. The oldest observed terrace includes gravel deposits with wood debris carbon dated at approximately 4500 ybp. Wood debris samples were collected from the deposit near the base of 22 feet thick vertical streambank exposures.

Recent sediment volumes are comparable to historical sediment volumes interpreted from the stratigraphic record exposed in the oldest terraces. At least three separate events distinguished in the stratigraphic record indicate historic sediment volumes greater than recent deposits. Physical recovery of the system appears to be rapid as evidenced by the re-established longitudinal stream profile to approximate pre-event levels within 1 to 2 years. Fish habitat structures installed between 1983 and 1993 are now exposed and functioning within the active streambed.