A theory of flow resistance for large-scale roughness is developed, the main points of which are supported by a flume study from which the outline of a process-based resistance equation is determined. Rivers with large-scale roughness have steep slopes and depths of the same order of magnitude as the bed material size. Flow resistance depends on the form drag of the roughness elements and their disposition in the channel. Theoretical considerations show resistance to be a function of Reynolds number, Froude number, roughness geometry, and channel geometry. These processes are examined using the results of flume experiments based on five different roughness beds and a wide range of flows. Semi-empirical analysis supports the theory and allows the development of a theoretical power law resistance equation which accounts for most of the processes. A summary of the data is presented for use in other research studies.