BEDROCK CONTROLS ON WATERSHED DEVELOPMENT IN SOUTH DAKOTA

Geomorphic relations for 16 South Dakota watersheds encompassing the state have been completed as part of a larger project involving peak-flow estimates and process controls of current fluvial regimes. Field measures were collected from 62 sites distributed among the 16 watersheds and included width, depth, slope, drainage area, and substrate characteristics which were used to classify streams using the Rosgen system and the Schumm channel evolution model. Statistical measures were developed that describe peak-flow estimations that reflect the current stage of geomorphic development. These variables were used to develop hydraulic geometry relations for each watershed that were subsequently utilized for assessing process controls and landform evolution. Results indicate that physiographic location dictate watershed response as a function of geologic bedrock type. Statewide, channel bottom sediment size was observed to decrease as drainage area increased, although large scatter in the data prohibit more than generalizations as to process controls. Broad-scale divisions based on 3 primary physiographic regions (Black Hills, Western Prairie, and Glacial Till) reveal bedrock type contributes significantly to runoff volume, and thus, watershed evolution. Fractured crystalline rock and thick Paleozoic and Mesozoic rock in the Black Hills region and glacial till deposits in the eastern part of the state have relatively higher infiltration rates that result in a slower rate of increase in peak discharge. Comparatively, the western prairie region is underlain by Cretaceous shale that has lower infiltration resulting in a higher rate of increase in peak discharge. Streams in this region are characteristically entrenched and unstable. Results from the study indicate a wide latitude in stream morphologies exists across the state controlled primarily by bedrock type.