EVOLUTION AND PROCESS CONTROLS OF SOUTH DAKOTA RIVERS

Pleistocene glacial activity has had a profound impact on the development and evolution of South Dakota's fluvial systems. The dichotomy observed in stream orientation, large-scale entrenchment and terrace development can be traced directly to the effects of isostatic loading and subsidence of the eastern half of the state. The understanding of stream behavior and channel evolution has direct implications in the development and management of watersheds. Stream restoration and urban planning have recently received tremendous attention resulting in the need for a greater understanding of stream behavior and channel evolution. A complete understanding of fluvial systems can only be accomplished in combination with knowledge of the geologic history of the system and the mechanics of active stream formation. The goal of this project was to determine the processes that control stream evolution and to incorporate the finding with a regional understanding of the fluvial system. Assessments were made at 63 individual stream segments in 34 streams and rivers within the state to determine Rosgen and Schumm classifications, hydraulic geometry and flood frequency. A GIS database in conjunction with multiple linear regressions and stream flow statistics were used to develop regional equations characterizing watershed stream behavior. Estimated bankfull flows were compared to bankfull flows calculated from field data to determine the accuracy of the estimated return periods for bank forming flows. Bankfull elevations were found not to correlate with the 1.5 recurrence interval discharges typically associated with bank forming flow and show that many of the streams have not reached quasi-equilibrium. Glacial development was shown to have a large-scale effect on stream development, impacting regional orientation and morphology, whereas geology was found to have the primary effect on local stream development. Recent entrenchment and disequilibrium can be attributed to human activities. Results of this and future analyses can than used by engineers and planners to properly and efficiently manage watersheds.