We consider catchment hydrologic response as if determined from the two independent processes of hillslope and channel network transport, operating across all the hillslopes and stream channels forming a catchment. This raises a number of interesting questions. Which process response characterizes a catchment's response? What governs the importance of one process over another? Is catchment response linear and time invariant, despite inherent nonlinearity in the runoff transport processes? Such questions, fundamental to an understanding of the nature of catchment response, are examined in this paper. We use a theoretical framework provided by geomorphology-based models of runoff routing to examine the hydrologic response over a range of catchment sizes. We demonstrate the effects of catchment size upon the relative roles of hillslope processes, channel routing, and network geomorphology in the hydrologic response of natural catchments. Motivated by the concept of geomorphological dispersion, we parameterize these processes in terms of catchment wave celerity and a catchment dispersion coefficient. We show that these catchment parameters vary with catchment size and provide insights into the nonlinearity of catchment response and the relative roles of the constituent processes. ¿ American Geophysical Union 1995