BEDLOAD TRANSPORT THROUGH POOL-RIFFLE SEQUENCES IN A LARGE GRAVEL-BEDDED RIVER

Riffles and pools are the dominant channel-scale bedform in gravel-bedded rivers; it follows that significant research has been devoted to understanding the mechanisms underlying their formation and maintenance. This research has spawned various explanations for their existence, such as the velocity reversal hypothesis and flow convergence hypothesis, among others. However, evaluation of these hypotheses has been hindered by a lack of bedload measurements in large rivers, owing to the logistical challenges and costs associated with collecting physical samples at sufficient spatial scale and over the range of flows necessary. To address this need, we collected a novel dataset of bedload transport on the Trinity River in northern California, USA, a relatively large gravel-bedded river with an ongoing restoration program. The dataset consists of continuous longitudinal profiles of bedload transport rates, depths, and velocities over a 20-km reach, at three different flows during high-flow releases in May 2016. Bedload transport rates were calculated from “sediment-generated noise” recorded by hydrophones on a cataraft during the floats, and by bank-mounted hydrophones within the reach that were calibrated to physical bedload samples. Depth, velocity, and position were measured with an ADCP and GPS. Over the 20-km reach, we identified 54 individual pool-riffle sequences. Herein we present a variety of analyses of the dataset, including comparisons of velocities, bedload transport rates, and spatial gradients of these within pools versus riffles. We also asses how these parameters vary with flow. These analyses provide insight into the basic mechanisms underlying pool-riffle maintenance and present a good test for previous hypotheses. The dataset also provides valuable information for resource managers, such as identifying the flows that initiate bedload transport in various reaches and documenting the effects of gravel augmentation on transport rates. This study thus exemplifies the intersection among applied science, basic research, and river management.