Revisiting the drivers of at-a-station hydraulic geometry in stream reaches

Abstract Knowledge of temporal variations in river hydraulic characteristics (i.e., width, depth, velocity) across stream networks is a key element for catchment management because hydraulics influence physical habitats and biodiversity, water temperature, nutrient fluxes, sediment transport and associated ecosystem services. Reach scale at-a-station hydraulic geometry describes the variation of mean water depth and width with discharge within a stream reach. The ability to understand and predict the form of these hydraulic geometry relationships across river networks therefore represents a powerful tool for river managers. Although variations of these relationships between rivers have been widely documented, they are still largely unexplained. This study provides an analysis of a unique data set of hydraulic geometry collected in 492 stream reaches in France. Relationships between the hydraulic geometry parameters and many variables describing catchment- and reach-scale characteristics of reaches were analyzed using stepwise linear regression and discriminant analysis. Results show that width-discharge relationships across reaches are well predicted by longitudinal and lateral descriptors of reaches: the ratio of median to bankfull width, the median Froude number, the width to depth ratio, the active channel relative to the catchment area and the fluvial pattern (braided or not). Fluvial pattern, reach averaged active channel width and bankfull width can be accurately estimated using aerial imagery, making application of our models over river networks possible. Fewer relationships significantly explained depth-discharge relationships, despite a weak influence of reach Froude number, flow resistance (i.e., relative grain size, relative bedform submergence) and width to depth ratio. Unavailable descriptors of channel resistance would probably improve these models (e.g., describing bank stability, spill and form resistance). Despite the uncertainties of hydraulic geometry relationships, this study shows that predictions of hydraulic geometry exponents across river networks are possible, offering a large range of potential applications.