peer reviewed ; In september 1983, the nine weirs regulating the flow of the river Meuse between Givet (France) and Namur (Belgium) were kept fully open for technical purposes. The water level therefore dropped, allowing the sampling of benthic organisms and the mapping of the different kinds of banks. For each bank type, the density of unionids mussels was measured. Silt and fine gravel bottoms are the preferred habitats of these mussels. In these natural habitats, the mean biomass is estimated at more than 1.8 tonnes/ha. In pebbles this value is near 1 tonne/ha whereas in the stony blocks and on rocky bars it falls to 165 kg/ha. Man made banks are poor biotopes: 297 kg mussels/ha on old stoneworks and only 65 kg/ha on recent ones. When the filtration rate is considered, is can be shown that, at the time this study was under taken, the unionid mussels living on the Meuse banks filtered more than 300 litres water/sec. This rate will drop to 27 litres/sec within only a few years if the designed hydraulic works are carried out. This study emphasises the negative effects of these works on the self-purification capacity of the river.