The most important use of the slope-area method is for the indirect determination of flood discharge, normally after the flood has passed. It may also be used, however, where the flow is affected by backwater. The method consists of the estimation of three basic factors. First, the area of the average cross-section in a longitudinal reach of channel of known length second, the slope of the water surface or the slope of the energy gradient in the same reach of channel and third, the character of the streambed so that a suitable roughness factor may be chosen.