Use of stable isotope tracers as a diagnostic tool for determining connectedness of road runoff to stream peak flows in disturbed forest environments.

Forest disturbance, and in particular forest road construction can cause changes in hydrological processes emergent at the catchment scale. The magnitude of the impacts of forest roads on stream peak flows and chronic sedimentation is perceived to be directly proportional to the degree that roads are connected to the stream system. Scientific studies to date that have examined the degree to which the roadside ditch acts as an extension of the stream system have been highly equivocal. The engineering prescription to "fix" the perceived impacts of roads is to isolate the road system from the stream system. This is generally carried out by upgrading the drainage system for the forest road by either installing more drainage structures, cross drain culverts, or simply isolating live stream crossing culverts using water bars or rolling dips. At this time there is no simple, universally applicable method to determine the degree of connectivity of a road system to the stream. This paper presents some preliminary research using stable isotope tracer approaches as a diagnostic tool for quantifying connectedness of road runoff to stream peak flows. We argue that time source hydrograph separation may be a tool for determining whether or not engineering improvements have "worked" and how much current road systems in any given watershed influence stream peak flow. We present data for a rainstorm in the Winter of 2003 from nested sub-catchments and road ditches at the Oak Creek Watershed in the McDonald/Dunn Research Forest, Western Oregon.