From there they traveled and encamped on the other side of the Arnon River that is in the desert, that extends from the border of the Amorites; for the Arnon is the border of Moab between Moab and the Amorites. Bible

“the other side of the Arnon River. Israel was traveling from south to north, so the “other side” of the Arnon was the north side.

Commentary for: Numbers 21:13