Investigations of runoff production and sedimentation on forest roads

Forest roads constructed in steep mountain landscapes have been associated with a number of effects on hydrologic and geomorphic processes. This research examined the effects of forest roads on the flow of water and sediment in drainage basins in the Cascade range of western Oregon. A study conducted at the hillslope scale (< 0.1 km²) during the 1996 water year examined the factors controlling runoff production on forest roads. Runoff response was related to climatic conditions (storm size and soil moisture) and to the hillslope setting (size of the contributing hillslope, hillslope gradient, and soil depth on contributing hillslopes) on which roads were located. These observations were consistent with a theoretical model of runoff production on steep hillslopes. A study conducted at the large basin scale (181 km²) examined erosion associated with forest roads during the February 1996 flood. Roads functioned both as initiation sites for erosion and as depositional sites, interrupting the flow of water and sediment along hillslopes and in channels. Roads constructed prior to 1960 in midslope and valley floor positions experienced the highest frequency of erosion and deposition, and these impacts were concentrated at elevations below 800 m, where storm precipitation was augmented by snowmelt. Both fluvial and mass wasting processes deposited sediment on roads and eroded sediment from roads, and multiple processes were linked in complex cascades at many sites. Roads were a net source of sediment in the basins studied, although they functioned as both sources and storage sites for sediment, depending upon their location on hillslopes. The results point to the importance of roads in both modifying physical processes and in routing material through drainage basins. The findings have significant implications for the management of roads in forested landscapes.