A severe restriction to dilution discharge measurements is that they are to be applied to steady flow conditions only. Usinga computational model, dilution discharge measurenents by constant-rate injection are simulated during unsteady flow conditions caused by Gaussian flood waves propagating down a rectangular uniform channel. The computational results are evaluated in order to provide a measured a discharge measurement's accuracy. The main source causing discrepancies between the actual and measured discharges is the difference in propagation velocities of the tracer cloud and the flood wave. Correction procedures are recommended, since maximum inaccuracies can easily be larger by an order of magnitude than inaccuracies during steady-state conditions