Okay I've done some more detailed research into the actual books. The results are: "Aarhus River": 8 books, "Aarhus" (e.g. "the river of Aarhus", etc.) 9 books. Whilst that is not overwhelming in favour of "Aarhus (river)" (and I wasn't able to look at every instance of Aarhus as a river) it certainly doesn't justify "Aarhus River", especially in light of the other criteria.