The purpose of the work was to present the results of simulation studies of river runoff carried out with the use of a conceptual model specially developed for this purpose. Different variants of forecasts were developed for the impact of introducing various forms of urbanization on the formation of river runoff progression during flood flows in the small, lowland catchment of upper Mławka river, covering the area of ​​66 km2. For the needs of the research, classification of urbanized areas was made based on their location within the catchment, and the method of discharging rainfall runoff. Particular variants differed from one another as to the location of sealed areas, the degree of urbanization of the catchment, the initial waterlogging of the catchment, as well as the model rainfall assumed for calculations. They provided the foundations for the assessment of the impact of various forms of urbanization of the catchment area on the formation of flood flows, taking into account the catchment’s natural properties, such as the occurrence of variable source areas. The subject of the analysis was not only the total runoff volume, but also its components – surface, subsurface and groundwater runoff. The analysis of the results of river runoff simulation, taking into account the components of this runoff, leads to the conclusion that the total increase of direct runoff volume and its peak value are caused not only by direct supply from sealed surfaces, but also from indirect impact, causing changes in the runoff regime in non-urbanized areas. Sealed and channelled areas located beyond the maximum range of active (variable source) areas, on which direct runoff does not occur in natural conditions, increase the runoff area in the scale of the catchment area as a whole. To compare, the peak flood flow values for variants associated with urbanization and channelling outside the variable source areas are between 20% and 40% higher than in the case of analogous conditions and the sealing of the same area in the area of direct runoff.