Channel network extension by logging roads in two basins, western Cascades, Oregon

Based on field surveys and analysis of road networks using a geographic information system (GIS), we assessed the hydrologic integration of an extensive logging-road network with the stream network in two adjacent 62 and 119 km(2) basins in the western Cascades of Oregon. Detailed surveys of road drainage for 20 percent of the 350 km road network revealed two hydrologic flow paths that link roads to stream channels: roadside ditches draining to streams (35 percent of the 436 culverts examined), and roadside ditches draining to culverts with gullies incised below their outlets (23 percent of culverts). Gully incision is significantly more likely below culverts on steep (greater than or equal to 40 percent) slopes with longer than average contributing ditch length. Fifty-seven percent of the surveyed road length is connected to the stream network by these surface flowpaths, increasing drainage density by 21 to 50 percent, depending on which road segments are assumed to be connected to streams. We propose a conceptual model to describe the hydrologic function of roads based on two effects: (1) a volumetric effect, increasing the volume of water available for quickflow and (2) a timing effect, altering now-routing efficiency through extensions to the drainage network. This study examines the second of these two effects. Future work must quantify discharge along mad segments connected to the stream network in order to more fully explain road impacts on basin hydrology.