Short and long-term changes to bed mobility and bed composition under altered sediment regimes

Altered flow and sediment transport regimes from impoundment can result in significant channel bed composition changes that exacerbate the geomorphic and ecological effects of flow regulation. Using long-term discharge and cross-sectional data in combination with a two-fraction sediment transport model, we assess changes in the downstream bed of two flow-regulated rivers with equivalent dam-induced changes in flow but opposite changes in sediment flux. Supply limitation has led to incision and armoring in one case while supply excess has led to aggradation and embeddedness in the other. Under limited sediment supply, bed elevation variability decreases soon after impoundment, while excess sediment supply results in a decades-long gradual decrease in both bed elevation variability and depth of incision. Although the balance of sediment supply and transport differs between dam management styles, in both cases both the immediate and more gradual changes can be explained within the framework of a two-fraction sediment transport model. Our results demonstrate the importance of considering bed composition on sediment transport predictions and the development of management strategies for ecosystem maintenance.