The aim of this study was to investigate the variation of flow conditions along the Gabčíkovo -Topoľníky channel (Žitný Island) on the base of Manning´s roughness coefficient value. This coefficient is not easy to determine and its value is varying constantly during the growing season, especially in a lowland stream with aquatic vegetation occurrence. Vegetation impedes the water flow and may increase flood risks. Thus, determining the effect of aquatic vegetation on flow conditions in streams

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... s very important for estimation of hydrodynamics in natural streams. Measurements performed during growing season at the Gabčíkovo -Topoľníky channel stream were used for an evaluation of vegetation impact on flow conditions. The variations of roughness coefficients of Gabčíkovo -Topoľníky channel are presented in Manning's equation, and the results reveal that the n value increases with the decreasing of flow depth. Manning's coefficient value found in this study is in the range of 0.020 to 0.079. The outcomes of this study can be concluded that the variation of Manning's coefficient value is influenced by the cross-section profile characteristic, flow depth, slope of channel, and especially quantity of submerged vegetations in the channel.