Natural conditions in the Middle Biebrza Basin were disturbed by building at the turn of the XIXth century of several canals, witch changed the course of the Jegrznia and Ełk rivers. The beds of these rivers become overgrown with plants and lost water in sections where water was overtaken by canals. Furthermore, the canals exerted a draining effect on adjoining areas because of their relatively high gradient of water surface. The participation of particular types of flow in the basin water alimentation has not been fully explained. It was decided to solve the problem by mathematical model - building or adapting an existing model for particular elements of water cycle. The model of flow in the network of channels, overland and subsurface flow have been combined with the model of groundwater flow. In order to examine the influence of water levels in the Jegrznia river, the Ełk River and in the Woźnawiejski Canal on the groundwater levels in the studied region of the Biebrza River a number of simulations runs were carried out. To increase the ground water level in the Middle Biebrza Basin and to limit the draining impact of the Woźnawiejski Canal a series of small weirs have been constructed in the Woźnawiejski Canal. The numerical model was used taking into account the designed cascade of small weirs.