Uncertainties in discharge determination may have serious consequences for hydrological modelling and resulting discharge predictions used for flood forecasting, climate change impact assessment and reservoir operation. The aim of this study is to quantify the effect of discharge errors on parameters and performance of a conceptual hydrological model for discharge prediction applied to two catchments. Six error sources in discharge determination are considered: random measurement errors without autocorrelation; random measurement errors with autocorrelation; systematic relative measurement errors; systematic absolute measurement errors; hysteresis in the discharge–water level relation and effects of an outdated discharge–water level relation. Assuming realistic magnitudes for each error source, results show that systematic errors and an outdated discharge–water level relation have a considerable influence on model performance, while other error sources have a small to negligible effect. The effects of errors on parameters are large if the effects on model performance are large as well and vice versa. Parameters controlling the water balance are influenced by systematic errors and parameters related to the shape of the hydrograph are influenced by random errors. Large effects of discharge errors on model performance and parameters should be taken into account when using discharge predictions for flood forecasting and impact assessment.