Under natural conditions aquatic macrophyte vegetations are characterised by a large diversity of growth and life forms strongly altering in space and time. This large diversity is often correlated with a large diversity and a high amount of fauna-elements.Recently, a strong increase in macrophyte growth is observed in many lowland rivers in Flanders. The growth of macrophytes strongly influences the discharge capacity of watercourses. The correlation between the biomass and the discharge capacity is very complex and depends highly on the growth form of the macrophytes and on the flow rate.Dense vegetation related with high water levels encourage water managers to a more frequent and more intensive macrophyte cutting, with many ecological consequences for the present biocoenosis. The pattern of macrophyte cutting determines not only the increase in discharge capacity but also the regeneration of the vegetation and the impact on the fauna-elements.A management on a more ecological basis, cutting only a part of the macrophyte vegetation, might be seen as an acceptable compromise between sufficient discharge on the hand and keeping intact larger parts op the macrophyte vegetation (as a habitat for many organisms) on the other.The first aim of the project is a better understanding of the complex relationship between the biomass development of the different growth forms and their resistance on water discharge at different water levels and different current velocities. The second aim is a better understanding of the effect of macrophyte cutting. Two cutting patterns on a more ecological basis will be carried out and the effects on the discharge capacity will be investigated.The field research will be combined with an experimental approach in which both the abundance of some growth forms, and the depth and discharge of the artificial water canal can be fully controled. This interesting combination makes it possible to extrapolate the field results over the whole range of discharges and depths. The roughness coefficient can be calculated for different vegetation types and included in hydraulic models. This will be the basis for a modelling approach to plan weed-cutting schemes in an environmentally sounded way.