SEDIMENT MOVEMENT THROUGH RIPARIAN VEGETATION UNDER SIMULATED RAINFALL AND OVERLAND-FLOW

A 2-year rainfall simulation study was conducted to evaluate the effec tiveness of riparian vegetation to reduce sediment movement, Three veg etation height treatments [clipped to the soil surface, 10 cm height, and undisturbed (unclipped)] were evaluated in 2 montane riparian vege tation communities in northern Colorado. One community was a tufted ha irgrass (Deschampsia caespitosa (L.) Beauv.), cinquefoil (Potentilla g racilis Dougl. ex Hook), Kentucky bluegrass (Poa pratensis L.), and se dge (Carex spp.) association. The other community was dominated by bea ked sedge (Carex rostrata Stokes) and water sedge (Carer aquatilis Wah l.). Water was sprayed on plots (3 m x 10 m) 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 th e sediment was derived from an upland sail The second year a fine sili ca sediment was used. Thirty kg of sediment was added to each plot. Th e first of 2 experiments was concerned with movement of sand particles greater than 200 mu m. 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 mu m) contained in sediment traps at 60, 120, and 180 cm d ownslope from the upslope border of the simulator plots. Results of th e first experiment showed a significant increase in sand movement down slope when vegetation was clipped to the son surface compared with und isturbed vegetation. In the second experiment, most significant differ ences in movement for finer particles occurred in the 210 mu m and 10- 30 mu m particle ranges. A smaller percentage of particles in the 210 mu m range was present in sediment traps at ail 3 distances downslope when vegetation was clipped to the soil surface, as these smaller part icles stayed in suspension. Increased vegetation height resulted in a significantly smaller percentage of the 10-30 mu m particle size range present at 120 cm distance. This study showed that additional variabl es (% surface vegetation cover, aboveground biomass, % shrubs, surface roughness coefficient, soil texture of introduced sediment, % bare gr ound, distance downslope, vegetation density, grass spp., and sedge sp p.), besides vegetation height, influenced sediment movement. Land man agers 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.