Sediment Dynamics and Bed Stability in Step‐Pool Streams: Insights From 18 Years of Field Observations

Abstract

Step‐pools are a common morphology in gravel‐bed streams with gradients between 3% and 30%, where coarse sediment and wood are present. Recent experimental work has offered insight on the formation of step‐pools, flow resistance, and interactions between sediment transport and channel morphology. However, field observations are sparse and often limited by short observation periods. Using an extensive 18‐year data set, we examine controls on sediment mobility, step stability, and sediment storage in East Creek, a step‐pool reach in British Columbia. Event bedload yield correlated with peak flow and ranged over two orders of magnitude. For most events, bedload grain‐size was finer than the grain size distribution of the sediment found in pools. Fractional sediment mobility was independent of the flow magnitude and controlled by sediment supply. The large stones that comprise the steps were generally mobile during large events (>10‐year recurrence interval), although the largest ones were not. Despite the movement of large stones, the skeleton of the steps remained in place even during an event with a near 125‐year recurrence interval. Temporal trends in sediment storage in pools show that most events cause negligible changes in sediment storage. Our field observations indicate that the step‐pool morphology of East Creek is stable, as the morphology and shape of the steps persisted under a wide range of flows. The overall stability of the reach is confirmed by observed trends in bedload yield and particle mobility.

Effects of Channel Width Variations on Turbulent Flow Structures in the Presence of Three-Dimensional Pool-Riffle

“…In another field study, researchers considered a reach of Babolroud river with width variation and found that the vertical components of the velocity near the pool bed are oriented downward, and upwards near the water surface, while in the riffle part, these components are oriented towards the bed in gravel-bed rivers, emphasizing the effect of width changes, flow non-uniformity, and changes in input sediments on the morphodynamics of the pool-riffle sequence [21]. Hassan et al (2023) [22] determined the sediment mobility, step stability, and sediment storage in a step-pool reach in British Columbia in Canada. They revealed that sediment mobility is not dependent on the flow magnitude but is controlled by sediment supply [22].…”

“…Hassan et al (2023) [22] determined the sediment mobility, step stability, and sediment storage in a step-pool reach in British Columbia in Canada. They revealed that sediment mobility is not dependent on the flow magnitude but is controlled by sediment supply [22]. Based on their research, the amount of width and wavelength change were known as significant factors affecting the sequence of pools and riffles.…”

Evaluating the Effect of Morphologic Units on Fractional Sediment Mobility and Bedload Transport in a Small Pool‐Riffle Reach

“…A detailed description of bedload measurement procedures in East Creek can be found in Hassan, Saletti, et al. (2022). Because collecting bedload samples was not always possible after each flow event due to weather conditions, only GSDs that were associated to single events were used for comparison with simulated bedload GSDs.…”

“…Though many authors have studied sediment transport processes and bed stability in East Creek (Cienciala & Hassan, 2013; Hassan et al., 2021, 2022b, 2022c; McDowell et al., 2021; McDowell & Hassan, 2020; Papangelakis & Hassan, 2016; Wlodarczyk et al., 2023), this study is the first to use a sediment transport model that accounts for spatial variations in fractional sediment mobility.…”