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Quantifying sediment transport rates in mountainous streams is important for hazard prediction, stream restoration, and landscape evolution. While much of the channel network has steep bed slopes, little is known about the mechanisms of sediment transport for bed slopes between 10% < S < 30%, where both fluvial transport and debris flows occur. To explore these slopes, we performed experiments in a 12-m-long sediment recirculating flume with a nearly uniform gravel bed. At 20% and 30% bed gradients, we observed a 4-to-10 particle-diameter thick, highly concentrated sheetflow layer between the static bed below and the more dilute bedload layer above. Sheetflow thickness increased with steeper bed slopes, and particle velocities increased with bed shear velocity. Sheetflows occurred at Shields stresses close to the predicted bedload-to-debris flow transition, suggesting a change of behavior from bedload to sheetflow to debris flow as the bed steepens.

Plain Language Summary Sediment transport within mountain rivers controls their shape, supplies sediment downstream for aquatic habitat, and can be a major hazard to life and infrastructure. Sediment tends to move by river processes where channel slopes are relatively gentle and by debris flows at very steep gradients; however, the mode of transport is not understood for the range of bed slopes in between. To address this knowledge gap, we performed laboratory experiments in a steep, 12-m-long flume to determine when and where river processes transition to debris flows. Surprisingly, we found a distinct third mode of transport developed called sheetflow. Sheetflows are dense granular slurries that are a hybrid between traditional river transport and debris flows. They occur on lower gradient sandy beds under high shear stresses but have not been previously documented in steep mountain streams. We map the parameter space where sheetflows occur, and quantify controls on their thickness and particle velocities. Our results indicate that there is a continuum of behavior as a channel bed steepens, from river to sheetflow to debris flow processes, which has significant implications for predicting sediment fluxes and channel bed morphology in mountain streams.