In order to have comparable values and usable data across the state of Germany, the Federal and State Water Authorities agreed in December 1970 to create a unified system for hydrological work on certain important rivers and their above-ground catchment areas and to issue them with index numbers.
Lakes are integrated as a part of the watercourse formed by their main tributary and their outlet.
For coastal regions, the scheme of numbering was altered in different ways by different states: Some waterbodies indexed under this classification system have un-indexed headstreams or lateral tributaries in the form of very small streams or ditches.
If such an unclassified waterbody is relevant for the water management of the region, it may be given a number within the local classification system of the regional Wasser- und Bodenverband (association for water and ground management).
If a river has subsidiary watercourses, an additional figure is allocated to its index number for each further branch.
In practice, catchment numbering for water sources below the level of streams is not used.