Channel Response and Reservoir Delta Evolution From Source to Sink Following an Extreme Flood

Disturbances such as floods, fires, and mass movements introduce large volumes of sediment into fluvial systems. Tracking the movement of disturbance-generated sediment provides insight into the processes of fluvial system response. Few studies document a post-disturbance response that explicitly links channel change to evolution of a reservoir delta. We exploit the tight coupling in a sediment source-to-sink analysis of flood deposits over 6 years. Within the first year after the flood, the channel incised 1.5 m and widened 10 m. Limited channel morphologic change thereafter is attributed to bed armoring. Morphodynamic modeling of the channel predicted transport of Radio-Frequency Identification-tagged clasts from which we infer a discharge threshold of >14 m(3)/s for future geomorphic adjustments. Post-flood discharges and reservoir drawdowns deposited 3 times the volume of sediment in the delta than was deposited during the flood and prograded the delta 200 m. Overall system response followed a characteristic sediment decay flux. Despite the sediment flux decline, comparable rates of delta progradation occurred due to reservoir drawdowns that shifted the sediment source from the channel to incision of the delta. Thus, the delta evolved via distinct processes based on sediment source and reservoir level: (a) aggradation of the delta topset and progradation of the Gilbert-type clinoform when sediment is sourced from the channel during years when reservoir level fluctuated seasonally; and (b) delta progradation through incision of the delta topset during reservoir drawdowns. Our results reveal a genetic relationship between post-disturbance channel and delta evolution.