Deuteronomy 3:16, 17

But to the Reubenites and the Gadites I gave the territory extending from Gilead down to the Arnon Gorge (the middle of the gorge being the border) and out to the Jabbok River, which is the border of the Ammonites.

Its western border was the Jordan in the Arabah, from Kinnereth to the Sea of the Arabah (that is, the Dead Sea), below the slopes of Pisgah.