Predicting joint frequency distributions of depth and velocity for instream habitat assessment



Stream ecosystem structure and function are strongly influenced by patterns of velocity and depth. Simple methods for predicting the spatial distributions of these two variables, as functions of one-dimensional reach and discharge characteristics, have been recently reported in the literature. These studies have provided valuable insight into the fundamental factors influencing stream behaviour and represent a practical alternative to multi-dimensional hydrodynamic models. However, these previous studies have handled velocity and depth separately, while there is evidence that meso-habitats and stream biota are associated with distinct combinations of the two variables. Therefore, we used survey data from 92 stream reaches in New Zealand to develop a model for the joint distribution of depth and velocity. We found that, for each reach, the bivariate distribution of relative velocity and relative depth could be described by a mixture of two end-member distributions, one bivariate normal and the other bivariate lognormal, each with fixed parameters. The relative contribution of each shape for a particular reach at a particular discharge could then be related to the reach mean Froude number, the reach mean relative roughness, and the ratio of the survey discharge to the mean discharge. As these inputs can be readily estimated for changed channel morphology, our model should provide a useful approach for linking river rehabilitation strategies to hydraulics and ecology. Copyright © 2007 John Wiley & Sons, Ltd.