In steady flow a well known method to determine a discharge is the method of dilution discharge-measurement : after constant injection of a known rate of tracer material and sufficient mixing over the entire cross-section, the concentration of the tracer material can be used to compute the discharge by way of an explicit relation. Using the same relation during a flood wave (unsteady flow) will result in inaccurate values of the discharge caused by a difference in the respective velocities of propagation between flood wave and tracer cloud. When, however, the dispersion equation is solved back into time with the aid of a Kalman filter, it turns out to be possible to compute with the dilution method accurate values of the discharge during a flood wave.