Sediment movement through riparian vegetation under simulated rainfall and overland flow

A 2-year rainfall simulation study was conducted to evaluate the effectiveness of riparian vegetation to reduce sediment movement. Three vegetation height treatments [clipped to the soil surface, 10 cm height, and undisturbed (unclipped)] were evaluated in 2 montane riparian vegetation communities in northern Colorado. One community was a tufted hairgrass (Deschampsia caespitosa (L.) Beauv.), cinquefoil (Potentilla gracilis Dougl. ex Hook), Kentucky bluegrass (Poa pratensis L.), and sedge (Carex spp.) association. The other community was dominated by beaked sedge (Carex rostrata Stokes) and water sedge (Carex aquatilis Wahl.). Water was sprayed on plots (3 m X 10 cm) at a rate of 60 mm hour-1 with a rainfall simulator, while overland flow containing sediment was introduced at the upper end of the plots at a rate of 25 mm hour-1. Two sediment sources were evaluated. In the first year's studies the sediment was derived from an upland soil. The second year a fine silica sediment was used. Thirty kg of sediment was added to each plot. The first of 2 experiments was concerned with movement of sand particles greater than 200 micrometers. The second experiment was designed to evaluate the quantity of 5 particle size classes (2-10, 10-30, 30-50, 50-100, and 100-200 micrometers) contained in sediment traps at 60, 120, and 180 cm downslope from the upslope border of the simulator plots. Results of the first experiment showed a significant increase in sand movement downslope when vegetation was clipped to the soil surface compared with undisturbed vegetation. In the second experiment, most significant differences in movement for finer particles occurred in the 2-10 micrometers and 10-30 micrometers particle ranges. A smaller percentage of particles in the 2-10 micrometers range was present in sediment traps at all 3 distances downslope when vegetation was clipped to the soil surface, as these smaller particles stayed in suspension. Increased vegetation height resulted in a significantly smaller percentage of the 10-30 micrometers particle size range present at 120 cm distance. This study showed that additional variables (% surface vegetation cover, aboveground biomass, % shrubs, surface roughness coefficient, soil texture of introduced sediment, % bare ground, distance downslope, vegetation density, grass spp., and sedge spp.), besides vegetation height, influenced sediment movement. Land managers should understand that when they manage ecosystems for a single factor, such as vegetation height, they cannot address complex issues such as sediment particle detachment, movement and filtration.