In this study, we assess the results of the application of the deterministic hydrological model Hydrograph for calculating the maximum discharge of various recurrence frequencies at the ungauged mountain catchment of the Khemchik River-Khemchik village (the Tyva Republic, Russia, area 1750 km²). We have assessed the parameters of the model for three main landscapes of the study area. Based on streamflow modelling at two gauged catchments, we have verified the parameters and algorithms of the hydrological mode land the results of modelling are satisfactory. The simulations of streamflow with daily time step for the Khemchik River-Khemchik village was conducted for the period between 1966 and 2012 using meteorological observational data from the Teeli meteorological station. Based on the simulated maximum discharge series, we have constructed the frequency distribution curves and compared with the data on calculations according to the standard methodology of frequency analysis provided in SP 33-101-2003. The modelling-based maximum instantaneous discharges for the frequency interval between 99 and 0.7% are 1.3-5 times higher than the values estimated by the standard methodology. The Ministry of Emergency Situations of the Tyva Republic indirectly confirms the plausibility of model calculations, evidencing the regular flooding of the Khemchik village, which is not predicted by the values obtained with the standard methods.