In this paper the distribution and ecology of two lamellibranchs, Dreissena polymorpha and Congeria cochleata, in the estuarine area of the rivers Rhine, Meuse and Scheldt in relation to the hydrography of that area, are discussed. Attention is drawn to the fact that in this Dutch estuary area formerly filter-feeding lamellibranchs attached to firm substrates did not occur below mean salinities of about 8-10‰ Cl’. The introduction of Dreissena polymorpha filled the gap below about 0.3‰ Cl’, but between about 1 and 10‰ Cl’ still no such species occur. Sedimentation of mud is thought to be mainly responsible for this phenomenon.