Knots in the Strings: Do Small-Scale River Features Shape Catchment-Scale Fluxes?

Field evidence suggests that reach-scale heterogeneities in river corridors can strongly influence catchment-scale dynamics including material fluxes and biogeochemical transformations. However, spatial effects and the emergence of processes are not commonly incorporated into catchment-scale hydrological and biogeochemical models. We differentiate river reaches as strings—relatively simple, homogeneous reaches with limited lateral and vertical connectivity—or knots associated with bifurcations, confluences, and obstructions, which are spatially and temporally heterogeneous reaches in a river network. We explore how knots affect reach-scale processes including flow attenuation, enhanced vertical and lateral connectivity, and augmented solute retention and uptake. We discuss how the simplifications associated with common models might affect both understanding river corridors and river networks, and management designed to increase resilience to natural hazards. We emphasize the need to better understand how small-scale heterogeneities cumulatively influence catchment-scale dynamics. Case studies from Scotland and Germany illustrate the effects of knots and the need to capture knot-related nonlinearities in hydrological and biogeochemical modeling. We highlight data challenges in related modeling, including: the availability, quality, and resolution of the source data that map knots and strings; the dependence of processes on the physical structure of the river network and how river corridor reaches are connected in multiple dimensions that may be impossible to measure directly; and the need for interdisciplinary efforts to develop integrated, high-resolution spatial datasets and co-located, high-frequency functional data across both time and space. We end by suggesting how to incorporate knots in large-domain models. This article is categorized under: Science of Water > Hydrological Processes Science of Water > Water and Environmental Change Water and Life > Conservation, Management, and Awareness.