Relative Effects of Lithology on Fine Sediment Deposition in the Coast Range of Oregon, U.S.A.

Watershed managers in the Pacific Northwest need to understand the natural variability of streams before setting up monitoring programs and classification systems that measure and categorize anthropogenic impacts to channels. Of particular concern to watershed management is the amount of fine sediment (<6mm diameter), that moves through and remains on the bed of streams. One of the variables affecting the amount of fine sediment delivered to a stream is the lithology underlying the watershed. The relative pool volume filled by fine sediment is indicative of the supply of fine sediment to a channel. 20 streams in the Coast Range of Oregon were selected to study the variability of fine sediment production associated with the lithology. Field work occurred during the low flow period of July and August, 2002. Rock types studied were marine volcanics and fine grained sandstones. These were selected because (1) these are the two predominant lithologic types that exist in the Coast Range, (2) both rock types are expected to produce fine grained sediment and, (3) the sandstone is known to be less resistant and the volcanics to be more resistant. We hypothesize that the sandstone will produce more fine sediment than the volcanics and that this sediment will be measureable as greater deposition in the pools. The study watersheds were all 2nd to 3rd order, dominated by rain runoff, and were underlain by a minimum of 95% volcanics or sandstone. Channel gradient was held as close to 1% as possible. The channel type selected was "steep pool-riffle," and most of these streams were influenced to some degree by bedrock. Pools were selected to minimize the influence of large woody debris. Relative pool volumes were measured in two pools per channel using a modified V-star technique, Fine sediment was sampled for later sieving, 3 Surber samples of macroinvertebrates were collected per site, and both the thalweg profile and a cross section of a typical high flow were surveyed. Preliminary results using t-tests and multiple linear regression models show that the 10 streams draining resistant marine volcanics had significantly lower total amounts of fine sediment in pools than the 10 streams draining less resistant sandstone. Part of this difference results from the difference in weathering rates between the rock types, but it is apparent that even small deviations in the gradient of a stream are also a significant variable in sediment deposition in the pools.