Discharge derived from rating curves are used to link the forecast future with the reality present. However, uncertainties in the rating curve may result in a poor estimate of present reality. The authors investigate the effect of rating curve uncertainty on flood forecast performance and conclude that models with uncertainty have low sensitivity to threshold exceedances used for flood warning. Well-defined stage-discharge curves based on a large range of flow observations (i.e. curves with low uncertainty) are recommended for operational flood forecasting sites.