CONTROLS ON FLASH-FLOOD MAGNITUDE AND HYDROGRAPH SHAPE, UPPER-BLUE HILLS BADLANDS, UTAH

Although many arid badland channels erode rapidly, channel flows appea r to be rare, raising the issue of the character and frequency of flow s responsible for channel erosion, Using an acoustic stream gauge, we recorded nine flash flood hydrographs over a 3 yr period from the Uppe r Blue Hills badlands, Utah, with maximum discharges up to similar to 0.9 m(3)/s. Flow hydrographs reveal bores and rapid depth changes that are similar to flash floods observed elsewhere. Bore and hydrograph p eak translation velocities are greatest in narrow channel segments, Ra pid runoff generation during short-duration thunderstorms produced com plex hydrographs whose shapes appear to reflect channel network geomet ry, Storm runoff response is highly sensitive to antecedent moisture, which greatly reduces the regolith infiltration capacity, High anteced ent moisture coupled with a relatively low intensity, long-duration ra instorm produced the largest flow event. Estimating flow frequency in this landscape therefore requires knowledge of the distributions of bo th storm sizes and temporal spacing relative to the short time require d for the regolith infiltration capacity to recover following wetting, here roughly 24 hr, Landscape changes can be produced not only by rar e, large rainfall events, but by a broad range of storm size and frequ ency under optimal antecedent moisture conditions.