The conveyance behaviour of overbank flow in meandering two-stage channel systems is of great practical importance in flood estimation and the design of two-stage channels. Methods of conveyance prediction are inadequate because of poor understanding of the energy dissipation processes. This paper examines the effect of the influential parameters for the case involving straight floodbanks and a floodplain without crossfall in terms of Darcy-Weisbach resistance coefficients using the Prandtl-von Karman resistance relationship and treating the system as a whole. Two flow domains are defined, in the first of which both viscosity and roughness influence flow resistance and in the second, only roughness. A design approach is presented for predicting the conveyance capacity in each of these domains based on small-scale data obtained at the Universities of Aberdeen and Glasgow and large-scale data from the UK Flood Channel Facility. The approach was found to give sound predictions of conveyance capacity when applied to other cases, including field data for the River Roding. The floodplain roughness which, in field cases, varies seasonally and is difficult to estimate, is shown to be an important agent of energy loss.