Vegetation growth drastically influences water levels and flow patterns in lowland rivers. Rise of the water level can have consequences for the surrounding areas and varying velocity patterns can influence the morphology of the river. The universities of Ghent and Antwerp are carrying out research in the river Aa, the Grote Kaliebeek and the Desselse Nete (Antwerp, Belgium) in order to better understand the natural phenomena involved and to come to a more accurate determination of the different parameters. Field measurements are carried out to collect data on discharge, stage and amount and variation of aquatic weed growth. The variation of the friction factor (Manning coefficient), influenced by the varying amount of vegetation, is determined as a function of time and distance. Following a seasonal cycle, flow resistance increases with plant growth in spring, reaches a maximum in early summer and then decreases slowly to the minimum in winter. Knowledge of the variation of the biomass in the time will lead to appropriate use of roughness coefficients in modelling surface flow in rivers.