This paper deals with the assessment of hydrological predictions driven by probabilistic hybrid rainfall forecasts obtained by merging numerical weather prediction and radar-based rainfall forecasts. Deterministic and ensemble hybrid rainfall forecasts have been utilized to provide river flow predictions on a catchment study located in the UK. A novel assessment methodology has been developed and implemented for the purpose of comparing deterministic and ensemble river flow predictions. The assessment methodology provides a measure of the degree of underestimation/overestimation characterizing the forecasts in comparison with a series of normalized reference thresholds. The analysis of the results obtained by implementing this evaluation methodology shows that the overall performance of the deterministic forecasts is very similar to that of the ensemble forecasts, with a slightly larger degree of underestimation affecting the ensemble forecasts in comparison with the deterministic forecasts. Copyright © 2012 John Wiley & Sons, Ltd.