Vegetation growth drastically influences water levels and flow patterns in lowland rivers. As soon as plants start to grow in spring, the flow resistance and by this the upstream water levels increase to reach a maximum in early summer. The universities of Ghent and Antwerp are carrying out research in the river Aa (Belgium) and the Biebrza river (Poland) in order to better understand the natural phenomena involved and to come to a more accurate determination of the different parameters involved. Regular field measurement campaigns allow to determine the variation of the roughness coefficient (Manningcoefficient n) as a function of time, type and density of vegetation, hydraulic parameters etc. and to evaluate flow and geomorphology patterns in the vicinity of vegetation patches. During these measurement campaigns, velocity and discharge measurements are carried out in multiple cross-sections in presence of vegetation using both an electromagnetic device and a hydrometric propeller.