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Step pools are a common bed morphology in boulder-rich gravel streams, but predicting how mountainous landscapes will respond to environmental perturbations such as climate-related hydrological changes requires a better understanding of channel morphodynamics and factors that influence bed stability. Flume experiments exploring bed stabilization demonstrate feedbacks among surface roughness, coarse grain clustering, and surface grain size. Clustering is quantified by using a novel normalization of Ripley's K statistic designed for use in power law functions. At 95% confidence, many but not all beds stabilized with coarse grains becoming more clustered than complete spatial randomness. The clustering statistic predicts hydraulic roughness better than D-84 does (the diameter at which 84% of grains are smaller), suggesting that the spatial organization of the bed can be a stronger control than grain size on flow hydraulics. Initial conditions affect the degree of clustering at stability, indicating sensitivity to history.

Plain Language Summary It remains difficult to predict what effect large floods or other environmental changes might have on mountain river channels and ecosystems. Will a section of a river channel remain stable and essentially unchanged, or could large amounts of erosion or deposition occur, possibly destroying roads and other infrastructures, or harming habitats for fish and other organisms? Channel changes are hard to predict because of feedbacks among many variables, including the size of gravel and boulders in the channel and how that sediment is arranged on the bed. To understand these feedbacks, laboratory experiments were conducted in an idealized river. The results show that the stability of river beds depends in part on how clustered (i.e., close together) the largest sediment grains are (typically boulders). Sediment size and the roughness of the river bed also influence how stable the river bottom becomes. A possible application of these results is to better be able to "read" sediment size and river bed stability from measurements of river bed elevations along channels.