Key:order:strahler

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order:strahler
Description
The Strahler stream order of a way in the waterway network.
Used on these elements
should not be used on nodesmay be used on waysshould not be used on areasshould not be used on relations (except multipolygon relations)
Requires
Status: in use

order:strahler=* indicates the Strahler stream order, or Strahler number, of a waterway=* such as a river. Stream order is a method to classify waterways based on the relative "largeness" of upstream tributaries, with larger watersheds tending to have a higher Strahler numbers.

Background

Waterway classification is useful for data analysis and filtering. For example, a cartographer may want to show only high-importance streams at low zoom levels. The Strahler method is one of the simpler ways to classify waterways since it relies only on network topology, not flow. While end users could calculate Strahler numbers themselves, this is a compute-intensive task that would require all waterways to be fully mapped and properly connected in OSM. By tagging stream order directly in OSM based on external analyses, all data consumers can benefit from consistent, ready-to-use data.

How to map

Find a reliable source the Strahler number of a river. Tag the value on the waterway=* using order:strahler=*. Values must be integers greater than 0 and generally less than 12. This value need only be tagged on the Role main_stream of the waterway, not on side streams.