This paper applies a continuous simulation approach to the estimation of flood frequency for a dam site in a large catchment (1186 km2) in the Czech Republic. The models used allow for the simulation of both high intensity and low intensity rainfall events, and snowmelt events, over subcatchments in contributing to the flood frequency distribution. The methodology is implemented within a Generalised Likelihood Uncertainty Estimation framework that allows for uncertainty in the model parameters and for the realisation effect in reproducing the apparent statistics of potential flood events represented by the short series of observations. A fuzzy rules method is used to evaluate each model run, based on the available observations of peak discharges, flow duration curves and snow water equivalents. This yields a combined likelihood measure that is used to weight the flood predictions for each behavioural parameter set. The cumulative distribution for flood peaks for any chosen probability of exceedence over all behavioural models can then be estimated. This can be used to assess the risk of a potential flood peak (or duration or volume) within a risk based dam safety assessment.