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- This category is intended for all waterbodies (i.e. rivers, lakes, canals, marshes, etc.) that form part of the drainage basin of the main river.
- Tributaries are grouped by stream order. The main river heads the list, under "1" are its direct tributaries, under "2" are tributaries of "1" and so on. For example, [[Category:Danube basin|1Zusam]] for the Zusam, a direct tributary of the Danube. "0" is used for branches of a delta, old river branches, backwaters, braided streams, anabranches, and rivers that flow directly into sea.
- Other waterbodies are grouped as follows: "B" for bogs and swamps, "C" for canals and other artificial waterways, "L" for lakes and lochs, "G" for glaciers, "R" for reservoirs and artificial ponds, "S" for springs, and "W" for waterfalls, e.g. [[Category:Elbe basin|LAmselsee]] for the Amselsee.
- River basin categories are also grouped by stream order, into all higher-order basin systems.
N.B. Currently the stream order does not display properly as a technical change was made to the way categories sort which has inadvertently affected stream order syntax (see archived discussion here so that they are now all listed under "0-9". If you have ideas for resolving this (other than reverting the technical change which is now set in stone), please discuss them at Wikipedia talk:WikiProject Rivers.