Euphrates — From Jordan and the wilderness beyond it unto Euphrates. Or, of the wilderness, which lies towards or reacheth to the river Euphrates, namely, the great wilderness of Kedemoth, Deuteronomy 2:26, which was extended far and wide towards Euphrates: for that was the eastern border of Reuben's possession, and not Euphrates, to which their habitation never reached. Multiplied — Which forced them to enlarge their habitation as far as they could towards Euphrates.
Related Commentaries of 1 Chronicles 5:9
1 Chronicles 5:9
9 And eastward he inhabited unto the entering in of the wilderness from the river Euphrates: because their cattle were multiplied in the land of Gilead.