RESPONSE OF PHYSICAL STREAM HABITAT TO HYDROLOGIC DISTURBANCE, BEAR CREEK, OZARK PLATEAUS, ARKANSAS

An understanding of the geomorphic role of floods is a prerequisite for predicting the potential effects of human-induced hydrologic disturbances on physical aquatic habitat. By surveying 31 cross sections four times during June 2001-June 2002, we documented the patterns of channel change associated with floods on a 1.1 km reach of Bear Creek, a gravel-bed tributary to the Buffalo National River, Arkansas. The study period included seven floods with peak discharges exceeding 185 cubic meters per second (cms), an approximate 1-year recurrence interval flood. As a result, total change per unit channel length was about 4 cubic meters/meter; given Bear Creek’s 238 square kilometer basin size, this is a large amount of annual geomorphic change relative to change measured on other Ozarks streams. Minimal channel change (near the limit of detection) occurred during a 6-month period that was dominated by low flow (Q < 0.6 cms) and that included two small floods with peak discharges of 17-23 cms. A resurvey following the largest flood (460 cms, estimated recurrence 2-4 years) showed channel change consistent with a velocity reversal (greater velocities in pools than in riffles), including substantial erosion in pools and deposition dominantly in riffles. In contrast, a subsequent group of intermediate-sized floods (195-250 cms) deposited sediment in pools and eroded riffles. The study period floods affected riffle and pool habitats differently, but in a way that is consistent with the velocity-reversal hypothesis. This information relating aquatic habitat response to floods is of interest to land managers because it provides baseline rates of change from which to evaluate the significance of projected human disturbances in the drainage basin.