The Generation and Fate of Road-Surface Sediment in Forested Watersheds in Southwestern Washington

Abstract

Studies conducted in southwestern Washington, examining the generation of sediment on gravel road surfaces, its transport to small streams and its fate after entering flowing waters, are reviewed. The amount of sediment produced on an hourly basis from a road segment was related to traffic rate. Accumulated material flushed rapidly from the road surface with precipitation, leading to a decrease in sediment concentration in the ditch with time during a storm. Depth of road ballast and type of surfacing material also influenced sediment generation. The majority of the sediment produced by these roads was finer than 0.004 mm. Steeper roads produced a higher proportion of coarser particles. Approximately 34% of surveyed road drainage points entered streams, mainly first- or second-order channels. Thus, the delivery of road sediment to larger streams often depended on its transport through these smaller channels. Small streams temporarily retained a high proportion of road sediment input to them. Coarser particles were retained at a higher rate than finer material. Due to the fine particle size of road sediment delivered to streams, little effect of this material on the composition of streambed gravel could be discerned. The results of these studies provide guidelines for management of those roads where sediment has the highest potential to impact aquatic resources. For. Sci 35(2):453-468.