Erosion and Sediment Delivery From Unpaved Roads and Off-Highway Vehicle (OHV) Trails in the Upper South Platte River Watershed, Colorado

Unpaved roads are often the dominant source of sediment in forested areas, and they are of particular concern in the Upper South Platte River (USPR) watershed because this is the primary source of drinking water for Denver, has a high-value fishery, and has a high density of roads and off-highway vehicle (OHV) trails. The goal of this project is to quantify sediment production and delivery from unpaved roads and OHV trails, as there are no data on these sources in the USPR watershed. Since summer 2001 we have been measuring rainfall, sediment production, and segment characteristics from up to 20 road segments, and in August 2005 we began making similar measurements on OHV segments. Sediment delivery is being assessed by detailed surveys of selected roads and OHV trails. Summer rainstorms larger than 10 mm typically produce sediment from each road and OHV segment while undisturbed areas generally produce no surface runoff. The mean annual sediment production from unpaved roads has ranged from 0.4 to 6.7 kg m-2 yr-1, and this variation is largely due to differences in the amount and intensity of summer precipitation. In summer 2006 the mean sediment production from OHV trails was 18.4 kg m-2, or more than 5 times the mean value from unpaved roads. A survey of 17.3 km of unpaved roads showed that 14% of the total road length was connected to the stream network; initial surveys on 3 km of OHV trails indicate a similar degree of connectivity. The overall road density in the study area is about 1.1 km km-2, so unpaved roads are contributing about 1.3 Mg km-2 yr-1 of sediment to the stream network. The results suggest that unpaved roads and OHV trails may be the largest chronic sediment source in the Upper South Platte River watershed.