(Pollution – Sediment, Erosion, Water Quality)
Sediment is a principal cause of impairment to surface water quality. Erosion is aparticularly important environmental issue in the Upper South Platte River (USPR)watershed of Colorado because it is the primary source of drinking water for Denver, has a high-value fishery, and several stream reaches are impaired by high levels of sediment. Unpaved roads are often considered a dominant source of sediment in forested watersheds, and off-highway vehicle (OHV) trails are another potentially important but largely unquantified sediment source. The objectives of this study were to: (1) quantify sediment production and delivery from forest road and OHV trail segments in the USPR watershed; (2) test the accuracy of WEPP:Road, SEDMODL2, and two empirical models for predicting sediment production from roads and OHV trails; and (3) compare sediment production, sediment delivery, and sediment yields from forest roads and OHV trails. Rainfall, site characteristics, and sediment production were measured on 14-22 native surface road segments from 2001 to 2006, and these data were used to test the accuracy of WEPP:Road and SEDMODL2. Empirical models for predicting storm-based and annual sediment production were developed from the first four years of data; the last two years of data were used for model testing. Similar measurements on 5-10 OHV trail segments from 2005 to 2006 were used to test WEPP:Road and SEDMODL2. Sediment delivery was assessed by detailed surveys along 17 km of roads and 10 km of OHV trails.
In 2006 mean sediment production from the 10 OHV trail segments was 18.5 kgm-2yr-1, or six times the mean value from the 21 road segments. The percentage of OHV trails connected to streams was 24%, or 70% higher than for roads, largely because more OHV trails were in the valley bottoms. None of the models accurately predicted sediment production from roads or OHV trails, but the performance of SEDMODL2 was greatly improved by calibrating the geology and traffic factors to the study area. SEDMODL2 also could be improved by adjusting the slope factor, better accounting for rill density on native surface roads, and making the rainfall factor dependent on rainfall erosivity rather than rainfall depth. WEPP:Road could be improved by making sediment production decrease rather than increase with higher soil rock content, and increasing the effect of a categorical change from no traffic to low traffic.
Road density in the study area is 0.6 km km-2, or three times the density of OHV trails. Multiplying unit area sediment production normalized by summer erosivity times the density, mean active width, and percent connectivity indicates that roads and OHV trails are respectively delivering approximately 1.1 Mg km-2 and 0.8 Mg km-2 of sediment to the stream network per year. Sediment delivery to streams can be reduced by locating roads and OHV trails out of valley bottoms and off steep hillslopes, decreasing segment lengths, and reducing segment slopes.
Welsh, M. (2008).
Department of Forest, Rangeland, and Watershed Stewardship, Thesis.