Albert Barnes Commentary Numbers 21:13

"From thence they journeyed, and encamped on the other side of the Arnon, which is in the wilderness, that cometh out of the border of the Amorites: for the Arnon is the border of Moab, between Moab and the Amorites." — Numbers 21:13 (ASV)

The Arnon, now the Wady Mojeb, an impetuous torrent, divided the territory which remained to the Moabites from that which the Amorites had wrested from them (Numbers 21:26).