Stage-discharge ratings, along with a variety of empirical corrections for the non-unique relations between stage and discharge have been used for nearly a century. None of these approaches has proven consistently successful in general application because of simplifying assumptions to the momentum and continuity equations. The reference condition for these methods has been poorly defined, often leading to a non-unique reference condition. Furthermore, the assumption of uniform progressive flow underlying many methods is possible only for some waves in prismatic channels over a steady, uniform initial condition. In general, downstream backwater, along with the characteristics of the flood wave, can significantly affect both the velocity and attenuation of the flood wave, resulting in significant and varying errors in methods such as the Jones method. An alternative approach is to develop ratings theoretically, omitting relatively negligible terms from the momentum equation to approximate the dynamics of flow independent from characteristics of the flood wave.