Effects of Road Treatments on Road-Stream Connectivity

Unpaved roads area major source of sediment, particularly in forested watersheds. Road maintenance and rehabilitation treatments are often intended to reduce road sediment production and delivery, but there are almost no data on how these different treatments affect road-stream connectivity. This paper reports on short- and longer-term changes to road-stream connectivity resulting from different road treatments. The data were collected in the Sierra Nevada and Cascade mountains in California by pre- and post-treatment surveys (short-term changes) and comparing connectivity among different road treatments (longer-term changes). In the southern Sierra Nevada grading and waterbar installation tripled road sediment production and increased the proportion of road segments with sediment plumes, but these actions did not significantly alter mean sediment plume length or the pre-treatment hydrologic connectivity of 27%. Roads with at least 20% gravel cover did not have significantly shorter sediment plumes or lower hydrologic connectivity. On the Lassen National Forest in northeastern California, road-stream connectivity was largely eliminated by outsloping and the installation of additional cross-drains, and was reduced from 23% to 13% by installing additional drainage dips. Comparisons between road treatments indicate that outsloping resulted in the lowest road-stream connectivity, followed by installing additional cross-drains, road closings, and rocking combined with additional drainage dips. For untreated road segments the amount of road-stream connectivity varied substantially over time, presumably in response to the interannual variations in the amount and intensity of precipitation. These results show that road-stream connectivity can be reduced by treatments that reduce the amount of concentrated runoff. Rocking and road closures also may reduce road-stream connectivity by decreasing the erodibility of the road surface and the amount of surface runoff. More costly treatments, such as road decommissioning, may be unable to fully disconnect roads that are in close proximity to streams. By collecting and compiling these types of data, we can better predict road-stream connectivity and the potential effectiveness of different road treatments.