Nowadays many surface waters in The Netherlands tend to become ecologically uniform with the same mediocre quality. A differentiated approach to water management is necessary to stop this process of impoverishment of aquatic ecosystems. In The Netherlands the provincial authorities represent the appropriate level at which this differentiated approach to water management can be put into practice. Such an approach has been realized by the Department of Water Management of the Province of Overijssel. As a part of this approach the project 'Ecological characterization of surface waters in the province of Overijssel (EKOO)' was formulated in 1981.The aims of the EKOO-project are to develop a regional ecological characterization of surface waters based on macrofauna composition and to reach a better understanding of the variety and the structure of the macrofauna communities present in the waters of the province of Overijssel. The project thus provides knowledge of aquatic ecosystems on a regional scale and a basis for the development of water management policies. An additional aim of this study is to develop the typological approach used in water management.The typological approach used can be interpreted as an integration of the zonal concept and the continuum concept. Taxon combinations will be described as types. Within a limited range of environmental conditions, these types are representative of communities, but at the same time they together form a continuum. Due to hierarchical relations between major environmental variables (master factors) and evolutionary and historical factors, a type can only be described within a biogeographical region.The study was designed as a qualitative survey but organized as much as possible along quantitative lines. Macrofauna composition (taxon composition and abundance) was chosen as a basic parameter. About 70 variables that were considered physically, chemically or biologically relevant, were measured at each sampling site. In total 664 sites were sampled, distributed over ...