Gewässerkennzahl

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.

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:

• Lower Saxony left the rule of using only odd numbers for sections. Nevertheless, bay as dependent waterbodies got even numbers.
• Mecklenburg-Vorpommern stuck to using even numbers for important rivers and odd numbers for – here coastal – sections in between. But the course of the coast was defined in a way, that in two straits different third figures were used for their two banks.

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 by within the local classification system of the regional Wasser- und Bodenverband (association for water and ground management).

The numbers for a watercourse and its catchment area are thus identical. If a river has subsidiary watercourses, an additional figure is allocated to its index number for each further branch. So in theory even a rivulet could be allocated its own catchment. In practice, catchment numbering for water sources below the level of streams are not used.

Catchment areas are thus distinguished by a number with up to a maximum of seven digits. Watercourse index numbers, by contrast, may have up to 13 digits in order to be able to classify all their tributaries and headstreams; although in practice only 10 digits are used.

The first digit of the number indicates which major river basin the waterbody belongs to, as follows:

• 1 Danube
• 2 Rhine
• 3 Ems
• 4 Weser
• 5 Elbe
• 6 Oder
• 9 Coastal region

The second and subsequent digits of the index number represent further subdivisions of the river and its catchment area.

The Heusiepen stream in Remscheid has waterbody number 27366462. This can be decoded as follows:

Main stem system / catchment area Rhine (2)

→ River system / catchment area Wupper (2-736)

→ River system / catchment area Morsbach (2-736-6)

→ River system / catchment area Gelpe (2-736-6-4)

→ River system / catchment area Saalbach (2-736-6-4-6)

→ Stream without defined catchment area Heusiepen (2-736-6-4-6-2)

Here some three-figure numbers are not listed.