The field data used in this essay were assembled during the botanical inventory of old river-beds by a number of colleagues from the “Stichting tot Onderzoek van Levensgemeenschappen” (Foundation for Biocenological Research), namely Mrs. A. J. Quene-Boterenbrood (1954), Mrs. W. A. E. van Donselaar-ten Bokkel Huinink (1955), Mr. P. J. Schroevers (1954), Mr. J. van Donselaar (1955 and 1956), Mr. L. G. Kop (1956) and E. E. van der Voo (1954-1956). The observations were obtained from 125 old riverbeds of the Maas, Rhine, Waal and the Guelders IJssel (see survey fist page 257). On the basis of this investigation the degree of frequency of the plant species was examined in order to select those species which proved to be sufficiently frequent to be correlated with certain environmental factors. This led to the provisional choice of twelve species. Five among those were waterplants: Nymphoides peltata (Gmel.) O. Kuntze, Nuphar luteum (L.) Sm., Polygonum amphibium L. fo. natans, Potamogeton natans L., and P. lucens L. Six river-bank plants were also chosen, namely Typha angustifolia L., T. latifolia L., Equisetum fluviatile L., Ranunculus lingua L., Glyceria maxima (Hartm.) Holmb. and Phragmites communis Trin. Later, a few more species were added, e.g. Stratiotes aloides L., Sparganium erectum L., and Oenanthe aquatica (L.) Poir, because these proved to be important environmental indicators as well. In addition, one or two of secondary importance are also discussed.