Numbers 21:13
13 From there they set out and kcamped on the other side of the Arnon, which is in the wilderness that extends from the border of the Amorites, for the Arnon is the border of Moab, between Moab and the Amorites.
Numbers 22:36
36 When Balak heard that Balaam had come, he went out to meet him at the city of Moab, on the border formed by the Arnon, at the extremity of the border.