For a rectangular free overfall, the end depth ratio (EDR) has been well connected with the main parameters using dimensional analysis. Using experimental results on three different roughnesses, it has been found that the EDR can be predicted from the curve Delleur, et al., for smooth channels, if the relative roughness ks/ydac (in which ks is the Nikuradse equivalent sand roughness and yc is the critical depth) is less than about 0.1. If ks/yc is greater than 0.1, then the EDR is reduced by the roughness and this effect has been correlated using the experimental results. A method has been proposed to predict the discharge from the measured end depth for a rough rectangular free overfall.