Channel Responses to Fluctuations in Water and Sediment Supply on the Lower Duchesne River, Utah

Channel responses to flow depletions in the lower Duchesne River over the past 100 years have been highly complex, and variable in both space and time. The gravel bed reaches aggraded only when gravel was supplied to the channel through local bank erosion, and degraded only during extreme flood events. In general, sand-bed reaches adjusted to all perturbations with bed-level changes, whereas the gravel-bed reaches adjusted primarily through width changes. No tributaries enter the Duchesne River within the study area, so all reaches have experienced identical changes in stream flow and upstream sediment supply. A 50% reduction in stream flow and an increase in fine sediment supply due to local gully erosion produced distinctly different responses in the gravel-bed and sand-bed portions of the study area. Gravel-bed reaches responded primarily with channel narrowing, whereas bed aggradation and four large-scale avulsions occurred in the sand-bed reaches. These avulsions almost completely replaced a section of sinuous channel about 14 km long with a straighter section about 7 km long. The most upstream avulsion, located near a break in valley slope and the transition from a gravel bed upstream and a sand bed downstream, transformed a sinuous sand-bed reach into a braided gravel-bed reach, and eventually into a meandering gravel-bed reach over a 30-year period. Later, a slight increase in flood magnitudes and durations caused widening and secondary bed aggradation in the gravel-bed reaches, whereas the sand-bed reaches incised and narrowed.