Debris Flow Associated Sediment Entrainment Thresholds in Small Watersheds in the Central Klamath Mountains, NW California

Five small watersheds, averaging 14 square kilometers, in The Central Klamath Mountains in Northern California will be examined to estimate the streams' ability to entrain debris flow associated sediment. A better understanding of the ability of the streams to deal with an inundation of sediment is imperative to ensure effective watershed management. The debris flow histories of the basins, developed using air photos and field inventories, show that multiple debris flow events occurred between 1944 and 1997 in the watersheds. Debris flows related to fire, roads, timber harvest or natural events may negatively influence water quality and the biological characteristics of a stream system. The debris flow sediment choking the stream channel may not be a permanent impairment of the system, however. Anecdotal evidence indicates that sediment introduced by debris flows in these watersheds was often appreciably removed within a few years. This study will quantify the discharge conditions needed to remove sediment from the stream channel. The bed material, as well as the material making up terraces, will be analyzed to determine the size distribution of the sediment introduced into the system by debris flows. Stream competency calculations will be used to determine the discharge that is required to entrain such sediment. This information will be used to estimate the rate and timing of delivery of debris flow related sediment to larger streams, such as the Klamath and Scott Rivers.