An empirical formula relating river discharge (Q) to channel dimensions and water surface slope. Q = AC√(rS), where A is the cross-sectional area of the river, C is the Chezy discharge coefficient, r is the hydraulic radius, and S is the slope of the water surface. This formula is useful for extending river-flow rating curves. The formula was devised by the French hydrologist Antoine Chézy (1718–98).