R. W. Faas and S. Wartel, “Mud is transported to the Dutch delta area from the Rhine, Waal, Maas and Scheldt rivers (resp. 0,6; 2,5; 0,7 and 0,7 million tons/year). Above, in the Rotterdam Waterway about 3,5 million tons of mud per year are brought in from the sea. The mud of the u”, njg, pp. 211-218, Jan. 1977.