What is "gewasserkennzahl"

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.