Despite frequent use of estimated flood discharges from urban and small rural catchments, the assessment of uncertainty in instantaneous discharge due to inaccurate head measurements is generally not performed. The present work discusses the impact of inaccurate head measurement on observed instantaneous flood discharge. The sources and sizes of uncertainties in head determination are discussed. A simple uncertainty model is developed in order to quantify the uncertainty in error-corrupted instantaneous and mean discharge. It is shown that uncertainty in mean discharge is less affected by uncorrelated head measurement error, while instantaneous discharges are substantially affected. Practical modelling using data from a Norwegian urban gauging station is undertaken. It is shown that the estimated annual maximum instantaneous flood discharge from this watershed has 95% confidence limits with sizes of 17–19% of the estimated discharge for average head measurement accuracy. Furthermore, it is shown that variability in head determination causes only a minor potential for bias in estimated instantaneous flood population statistics and quantiles.