Deuteronomy 2:36

Aroer, which is by the brink of the river of Arnon - Aroer stood on the north bank of the river, and was assigned Joshua 13:9, Joshua 13:16 to the tribe of Reuben, of which it formed the most southerly city. The valley of the Arnon is here deep, and the descent to it abrupt. In Roman times it was spanned by a viaduct, the ruins of which still remain, and which was probably built on the lines of the original structure of Mesha 2 Kings 3:5. Aroer here must not be confounded with “Aroer, which is before Rabbah” Joshua 13:25. This latter place was “built,” “i. e.” rebuilt, by the Gadites Numbers 32:34; it belonged to that tribe, and was consequently far to the north of the Arnon. A third Aroer in the tribe of Judah is mentioned in 1 Samuel 30:28.

“The city that is by the river,” literally, “in the midst of the river” (compare Joshua 13:9, Joshua 13:16) is Ar Moab (compare Numbers 21:15 note).