Albert Barnes Commentary 2 Samuel 24:5

"And they passed over the Jordan, and encamped in Aroer, on the right side of the city that is in the middle of the valley of Gad, and unto Jazer:" — 2 Samuel 24:5 (ASV)

Aroer - Aroer on the Arnon (Deuteronomy 2:36 note). Aroer itself stood on the very edge of the precipitous cliff of the valley; and in the valley beneath, possibly on an island in the stream, stood another city which is alluded to here.

River - Rather, “the valley” (margin). They passed from Aroer, northward to Gad, and so camped at Jazer (see the marginal references), which is on the frontier of Gad and Reuben.