Predicting downstream hydraulic geometry: A test of rational regime theory

[1] The classical equations of hydraulic geometry are purely empirical, but the widespread similarity of the scaling (downstream) form of them suggests that they express some important underlying regularities in the morphology of stream channels through the drainage network. A successful physical theory of river regime must be able to reproduce and explain this regularity. In this paper we test the regime theory of Eaton et al. (2004) using selected data of hydraulic geometry. We first use data from environments in which bank strength presumably does not vary greatly, such as in anabranched channel systems and deltas. Regime models parameterized by assuming uniform relative bank strength plausibly describe the observed bankfull channel geometries in these systems. We then test a modified bank strength formulation for vegetated gravel bed rivers against downstream hydraulic geometry data sets in which relative bank strength is supposed to vary with channel scale. Assuming a uniform effective cohesion due to riparian vegetation, the regime model is again able to reproduce details of the channel geometry. Both analyses show that the classical hydraulic geometry represents only an approximation of the variation of channel form. If we have confidence in the theory, we may infer information about bank strength and bed material transport. The pattern of variation in these quantities, as well as discharge, through the drainage system lends approximate regularity to stream channel scaling that is summarized in the empirical relations.