Gewässerkennzahl

The Gewässerkennzahl (GKZ, rarely GWK or GEWKZ) or "waterbody index number/waterbody number" is an identifier with which all watercourses in Germany are numbered, together with their catchments and precipitation areas. It is also referred to as a Gebietskennzahl or "basin number". A Gewässerkennzahl may have up to 13 figures (theoretically even 19). Basins normally are only defined up to seven figures. For a more detailed subdivision, the Gewässerkennzahl my be enlarged by ten more figures. Only that enlarged version is called Fließgewässerkennziffer. The Gewässerkennzahlen are defined by the environment offices of the states.

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. Linked to that was the establishment of the size and boundaries of their catchment areas.

Every waterbody (streams, rivers, canals and ditches, but also lakes and even some bays) and its catchment area was given a waterbody number in such a way that it could be clearly identified. The waterbody numbers are built in hierarchical fashion so that, based on the number, the next river system of the waterbody can be deduced.

At first the course of water has to be defined from source to mouth. Then the four major tributaries (or 'affluents') are identified. They are marked by even figures in downstream sequence, "-2, -4, -6, -8". This way, the (main) course is divided into five sections, which are marked by odd figures, "-1, -3, -5, -7, -9". A number with an even end-digit is the number of a whole watercourse, while a number with an odd end-digit is the number of a section. Lowest sections are always given a nine, even if not all figures between one and nine have been used. In the next step of numbering, each section defined by the first step is dealt in the same way, selecting four major tributaries and marking five sections.

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