Sorry, that was a dead link. This one's better:
(German language):
http://www.nomen-online.de/bedeutung/nachname/Rohr-1213.html
Translated in English:
Surname Rohr
Meaning / origin:
Middle High German. Placename rohr =
Rohr = pipe (zelfstandig naamwoord: tube, pipe, tubing, cane, barrel, reed, reeds, oven, wicker, blowpipe)
Schilfrohr = reed
Röhricht for somebody, who lives at or originates from the reeds.
The first existence of the name:
de Ror (about 1288), vomme Rore (about 1328), Ror (about 1373)