A theory of the resistance to flow in rivers with large-scale roughness is developed and a resistance equation is constructed using data from the upper River Tees, England. The theoretical analysis shows that, in channels with large values of relative roughness, resistance to flow depends mainly on the size, shape, spacing, and size distribution of the boundary roughness elements. Channel geometry is important only in its effect on the drag of the elements. This theory is illustrated using data from field sites at which roughness shape and size distribution were similar and roughness spacing could be directly related to relative roughness. In the resistance equation (which is specific to the field sites) the resistance coefficient varies with just relative roughness and a parameter of channel geometry.