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Abstract Streambed biogeochemical processes strongly influence riverine water quality and gaseous emissions. These processes depend largely on flow paths through the hyporheic zone (HZ), the streambed volume saturated with stream water. Boulders and other macroroughness elements are known to induce hyporheic flows in gravel‐bed streams. However, data quantifying the impact of these elements on hyporheic chemistry are lacking. We demonstrate that, in gravel‐bed rivers, the amount of dissolved oxygen (DO) in the bed depends chiefly on changes in bed shape, or morphology, such as the formation of scour and depositional areas, caused by the boulders, among other factors. The study was conducted by comparing DO distributions across different bed states and hydraulic conditions. Our experimental facility replicates conditions observed in natural gravel‐bed streams. We instrumented a section in the bed with DO sensors. Results generally indicate that boulder placement on planar beds has some effects, which are significant at high base flows, on increasing hyporheic oxygen amount compared to the planar case without boulders. Conversely, boulder‐induced morphological changes noticeably and significantly increase the amount of oxygen in the HZ, with the increase depending on sediment inputs during flood flows able to mobilize the sediment. Therefore, streambeds of natural, plane‐bed streams may have deeper oxic zones than previously thought because the presence of boulders and the occurrence of flood flows with varying sediment inputs induce streambed variations among these elements. 

Abstract Grain force‐balance models utilize grain protrusion and in‐situ resistance force data to evaluate the likely distributions of gravel‐bed sediment entrainment thresholds, specifically dimensionless critical shear stress (τ*_c). These methods can give insight into the spatial variability of particle mobilities both within a channel, and between different gravel‐beds, but are yet to be evaluated across multiple sites with varying texture and fabric. We evaluate two published force‐balance approaches: (a) a Monte Carlo style sampling approach using grain size and topography distributions from field measurements; and (b) an automated point cloud segmentation and analysis approach with an updated set of force‐balance equations, Pro+. We compare the workflows, assumptions and inputs for each approach, apply them to an extensive UK‐wide data set comprising 45 upland riverbeds, and evaluate the estimatedτ*_cdistributions. We find that mobility thresholds estimated from both methods are variable, with medianτ*_cranging from 0.05 to 0.15, and are consistent with published values of approximately 0.02–0.1. Uncertainties in grain sampling strategy or point cloud segmentation quality lead to markedly different grain size distributions between approaches, but their resulting influences onτ*_cdistributions are small relative to the range of estimatedτ*_c. Sensitivity analyses onτ*_cdistributions for grain‐size fractions also show that bed mobilities are sensitive to the roughness height of the velocity profile. We highlight uncertainties associated with these approaches, suggest areas for further targeted comparisons between methods, and provide guidance for the application of grain force‐balance models for estimating entrainment thresholds and bed stability in gravel‐bed rivers.