RUNOFF AND SEDIMENT YIELD FROM RAINFALL SIMULATIONS ON MANCOS SHALE HILLSLOPES, WESTERN COLORADO

Mancos Shale regions of the Western United States have been identified as a source of sediment, salinity, and much of the selenium transported by the Colorado River, and management strategies to mitigate the effects of different land uses on erosion and runoff are being studied. Rainfall simulations were made to quantify runoff, sediment concentration, sediment yield, runoff salinity, and trace elements from weathered Mancos hillslopes in the Gunnison Gorge National Conservation Area near Montrose, Colorado. Rainfall simulations were made on dry, 1 m2 plots with slopes ranging from 22 to 38 degrees on varying aspects. Vegetation cover in the plots ranged from 0 to 25 percent. Paired experimental plots represented hillslopes both undisturbed and disturbed by an off-road motorcycle immediately before the simulation. A rainfall rate approximating the 100-year, 1-hour storm at Montrose (38 mm) was applied for 90 minutes on 17 plots.

Runoff hydrographs, normalized for the actual amount of applied rainfall, displayed great variability in response time and magnitude within and between the two land-use categories. Normalized runoff ranged from 0.3 to 15 L/m2/hr (mean 6.0 L/m2/hr) from undisturbed plots and 0.4 to 18 L/m2/hr (mean 5.8 L/m2/hr) from plots disturbed by motorcycle tracks. Mean sediment concentrations ranged from 0.3 to 55 g/L (mean 26 g/L) from undisturbed plots and 0.7 to 76 g/L (mean 24 g/L) from disturbed plots. Sediment in the runoff was mostly silt- and clay-size material (finer than 0.063 mm). However, silt- and clay-size material made up a larger percentage of the sediment eroded from disturbed plots in the early, middle, and late runoff stages (95, 94, 94 percent) compared to undisturbed plots where the percent silt- and clay-size material was smaller and decreased in the late runoff stage (90, 91, and 86 percent). Normalized sediment yields ranged from 0.001 to 8 Mg/ha/hr (mean 2.5 Mg/ha/hr) from undisturbed plots and 0.003 to 8 Mg/ha/hr (mean 2.5 Mg/ha/hr) from disturbed plots. These data indicate that motorcycle disturbance creates no difference in runoff or sediment yield at the small-plot scale. Plot slope and aspect alone do not account for variability in runoff or sediment yield. Soil chemistry, physical and biological soil crusts, and vegetation, are being evaluated as additional controlling variables.