Modeling temporal trends in bedload transport in gravel-bed streams using hierarchical mixed-effects models

•We use hierarchical mixed effects to model bedload transport in streams.•We found that sediment storage decreases over time.•Mixed effect models are useful predictive tools in fluvial morphology.

In this paper, we used a bedload transport data set collected at North Fork Caspar Creek, California, to examine temporal variation in sediment transport rate over a 7-year period. Using a hierarchical mixed-effects model, we examined across and within-event variation to determine whether the bedload-shear stress relation trends over time. The relation between bedload transport and shear stress was modeled using log(Qb) = α + β*log(τ) + ε, where α and β are constants and ε is an error term. Depending on the length of observation, α and β can vary over several orders of magnitude, making modeling of transport based on flow challenging and highly inaccurate. We found a higher order yearly relation between bedload and shear stress, indicating systematic changes to the system over time. In the absence of significant additions to the system, α decreases roughly linearly over time, while β does not show any trend. From the systematic decline in α, we infer changes to sediment availability in the stream over time. Mixed-effects models have the potential to be a useful predictive tool in fluvial geomorphology, as they are more powerful at detecting trends in sediment transport rates than individual linear regressions.