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Hier einen Ausug aus einer Antwort der MAilingliste:

From your screenshot I can now guess, what you mean: There are also other ways
in the map, which are dislpayed as river, although they are only tagged as
boundary AND NOT as river at the same time?

If this is the case, I would guess, that the complete boundary is made up out of
a multipolygon.

In this case the multipolygon tagging seems to be faulty. Either all outer ways
must be tagged the same (this can not be, since one part is tagged as a river,
while other parts shouldn’t be rivers), or the tag values for the multipolygon
must be placed on the relation instead of the outer ways (this is also not done
in the example, since the boundary tag is on the way and not on the relation).

Dies sagt mir als Einsteiger recht wenig. Habe aber trotzdem zwei Boundaries verglichen, wovon eine als river angezeigt wird:

http://www.openstreetmap.org/?lat=50.2292&lon=7.4422&zoom=15

Boundary (way_54643302) südlich Brodenbach → river
Boundary (way_54643298) nördlich Brodenbach → kein river

Das Tagging in den zugehörigen Relationen scheint gleich zu sein, oder?