Feedback between erosional morphology, sediment transport and abrasion in the transient adjustment of fluvial bedrock channels

Abstract

This thesis explores the dependence of bedrock channel incision on coarse sediment supply through laboratory flume experiments and quantitative field observations of evolving channel morphology. Predictions of the saltation-abrasion model (Sklar and Dietrich, 2004) for sediment flux-dependent channel incision guide our efforts although the results are not model dependent. In channels where the dominant incision process is sediment abrasion, sediment transport and erosion are inherently coupled. Flume experiments support the form of sediment flux-dependent relations proposed in the saltation-abrasion model, but also suggest that the model is incomplete: channel bed morphology adjusts due to localized erosion, and bed morphology in turn influences local flow and sediment transport. The experiments document a positive feedback in which bedload is preferentially transported in local topographic lows due to lateral transport and abrasion in turn deepens those topographic lows. Under sediment-starved conditions this positive feedback results in the incision of narrow inner channels. In the Henry Mountains of southeast Utah, we monitored channel flow and resulting bedrock incision over 2.5 years in a human-perturbed bedrock channel where erosion had incised a narrow inner channel in which sediment transport and erosion were focused, similar to the flume experiments. In addition, the flow record shows that the maximum discharge flood was much less erosive than a lower but prolonged flood due to snowmelt flow. Large flash floods can be net depositors of coarse sediment in the channel, mantling the channel bed with coarse sediment and inhibiting erosion.

(cont.) Finally, a quantitative comparison of tributaries in a Henry Mountains river network shows that channels with higher coarse sediment loads maintain steeper channel slopes and are less incised than smaller drainage area tributaries with less coarse sediment but more exposed bedrock. These field observations validate the idea that sediment cover can inhibit channel incision into bedrock, and also suggest that the slope of incising bedrock channels can adjust to transport the sediment load of the channel rather than to incise bedrock. We found in the flume experiments that erosion adjusts the morphology of bedrock channels until a state is reached where channel flow just transports the imposed sediment load, suggesting that the equilibrium state of bedrock channels is a transport-limited condition. Together, the chapters demonstrate how incising channels respond to changes in coarse sediment load, emphasizing the effects of sediment cover in inhibiting incision and morphological adjustments of channel bed topography.