Item

Abstract

Direct and indirect impacts of climate change have been increasing fine sediment content in gravel-bed rivers. Excess amounts of intra-gravel fine sediment can reduce bed permeability, decrease dissolved oxygen availability, and enhance streambed levels of sediment-associated nutrients – potentially degrading the health of aquatic ecosystems. Yet, mechanisms of fine sediment ingress to gravel streambeds remain poorly understood, and in-situ field assessments of such mechanisms are especially limited. Understanding ingress mechanisms is vital to make fine sediment transport models for management purposes more assertive in their simulations. Field assessments often rely on the deployment of two different designs of fine sediment traps (ingress baskets) to infer the proportions of vertical and horizontal infiltration mechanisms. Such paired assessments rely on the deployment of vertical-horizontal baskets (permeable-walled and lidless) paired with either vertical (impermeable-walled and lidless) or horizontal baskets (permeable-walled with an impermeable lid). However, the accuracy of this approach is debatable. Here, we evaluate three designs of ingress baskets deployed in triplicate at four sites in a gravel-bed river of a 10 km reach. Ingress masses and rates were measured during seven deployment periods at a range of flow conditions. A total of 252 baskets were deployed. Preliminary assessments indicate that paired comparisons might lead to erroneous estimates of ingress mechanisms, which is in accordance with observations made elsewhere. Limitations of each basket design and the bias that might be introduced through commonly used paired assessments are discussed to advance knowledge on the assessment of ingress mechanisms.