Direct and indirect tributaries are grouped in subcategories, for instance the category [[Category:Prut basin]] contains a subcategory [[Category:Tributaries of the Prut]]. The main river heads the list, for example: [[Category:Prut basin| ]] is added to the article "Prut".
Other waterbodies are grouped as follows: "0" is used for branches of a delta, old river branches, backwaters, braided streams, anabranches, and rivers that flow directly into sea. "B" is 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.