Which variables should be used to link invertebrate drift to river hydraulic conditions?

This paper assesses constraints on the use of hydraulic variables in studies aimed at understanding or predicting invertebrate drift responses to increased discharge. A number of flow variables, all commonly used in eco-hydraulic studies, were tested for their fit to drift across a range of discharges. Discharge was manipulated insitu in a small upland river, with drift monitored and hydraulic conditions characterised during each manipulation. Both central (Generalized Additive) and limiting response (Quantile Regression) models indicated better fits of drift to simple and directly measured hydraulic variables than to more complex ones. Different equations yielded markedly different estimates of shear stress, making it difficult to pinpoint ecologically important hydraulic thresholds. Results indicate that river scientists and managers should interpret hydraulic data with care, particularly when parameters such as shear stress are used to help understand the ecological disturbance effects of high flows.