Forest roads and geomorphic process interactions, Cascade Range, Oregon

A major flood in February 1996 triggered more than 100 geomorphic features affecting forest roads in a 181 km(2) study area in the western Cascade Ran ge, Oregon. Eight types of features, including mass movements and fluvial f eatures, were mapped, measured and analysed using geographic information sy stems and sediment budgets for the road network. Although roads functioned as both production and depositional sites for mass movements and fluvial pr ocesses, the net effect of roads was an increase in basin-wide sediment pro duction. Debris slides from mobilized road fills were the dominant process of sediment production from roads. Road-related sedimentation features were concentrated in a portion of the study area that experienced a rain-on-sno w event during the storm and was characterized by the oldest roads and stee p slopes underlain by unstable, highly weathered bedrock. The downslope inc rease in frequency of features and volumes of sediment produced, combined w ith the downslope increase in relative frequency of fluvial over mass-wasti ng processes, suggests that during an extreme storm event, a road network m ay have major impacts on stream channels far removed from initiation sites. Overall this study indicated that the nature of geomorphic processes influ enced by roads is strongly conditioned by road location and construction pr actices, basin geology and storm characteristics. Published in 2001 by John Wiley & Sons, Ltd.