Summary:

The paper presents the computer program RIVER which calculates a flow capacity of river valley with vegetated floodplains. The program was elaborated in the C programming language. For the mean velocity calculation Darcy-Weisbach formula was used. Velocities, friction factors and the components of discharge in the main channel and on the floodplains were calculated using momentum transfer between the main channel and the floodplains were included in the calculation using Pasche method. The calculation of the discharge rate was performed at a selected crosssection in the Lower Biebrza Basin as an example of this program application. The Biebrza River valley is covered by different types of vegetation as grasses, sedges, reeds, shrubbery and trees. Calculation was performed for a flood event, whose water level was collected during field measurements.