Discharge in river bed can be measured using many direct methods in field research. However, those methods are not enough to estimate flow velocity in any chosen point in river bed cross-section. This research work presents a spline function method, which allows mathematical modeling of flow velocity. This model also served to calculate the wetted perimeter and total discharge to in the cross-section using isotachs and velocity profiles. The modelling method has been implemented under the RIVER program. The model of a spline function approximation served to calculate the flow velocity and to demonstrate the distribution of flow velocities in the Obra river bed, in its lower course near Midzyrzecz (the town in the west of Poland). On the basis of this model it was possible to make detailed analysis of flow velocity changes in the river bed at various water stages. The most important changes, observed during low and high water stages, were differences in spatial extend of maximum flow velocity field in researched cross-sections. One of the reasons of these changes was asymmetrical shape of cross-section profile (in case of cross-section located on meander bend). Another analysis was made to describe flow velocity changes near the bed at different water stages. Presented investigation is essential to describe detailed features of the Obra river regime and needed in further research to determine the changes of bed load transport intensity in various hydrological conditions.
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