The accuracy of simulations water level regimes has roughness coefficient a major impact. The aim of this work is to verify the impact of the channel roughness coefficient values on the water level regimes of the lower Váh River. Observed section, water structure Selice - Komárno is influenced by river tributaries of Small Danube, relocation of Nitra River and by the confluence of the Váh and Danube River in Komárno. It is important to know the most accurate navigable water levels in order to minimize error in detection the parameters of the waterway. The calculations include water level regimes for several discharge combinations of the Váh and Danube River with changing of the channel roughness value. For calculation program HEC-RAS was used.