SEDIMENT FILTRATION IN A MONTANE RIPARIAN ZONE UNDER SIMULATED RAINFALL

A 2 year study was conducted to evaluate the effectiveness of riparian vegetation to filter sediment from overland water now. Three vegetati on height treatments: clipped to the soil surface, clipped to a 10 cm height, and undisturbed were evaluated in 2 montane riparian vegetatio n communities (grass and sedge) in northern Colorado. Water was spraye d on 2 macro-plots (3 m x 10 m) and 2 micro-plots (0.6 m x 2 m) simult aneously at a rate of 60 mm hr(-1) with a rotating boom rainfall simul ator. Overland now containing sediment was introduced at the upper end of the plots at a rate of 25 mm hr(-1) to simulate runoff and sedimen t transport from an upland area, Two sediment sources were used, a san dy loam soil and a ground silica sediment (loam). Thirty kg of sedimen t were added to each macro-plot and id kg of sediment were introduced to each micro-plot (10 Mg ha(-1)). Sediment yields, at the downslope e nd of the plot, were greater when the finer silica sediment was introd uced into overland flow as compared with sediment derived from the san dy loam soil, As expected, the small micro-plots yielded more sediment and were often more sensitive to community and treatment differences than larger plots. We believe this resulted from the shorter travel di stance. However, sediment filtration treatment effects were usually si milar for both plot sizes. Sediment yields, measured at the outlet of the plots, did not decrease, or increase, as vegetation heights increa sed. Accurate prediction of sediment filtration from shallow flow in r iparian zones required consideration of a combination of vegetation an d soil surface characteristics.