9 To the east they occupied the land up to the edge of the desert that extends to the Euphrates River, because their livestock had increased in Gilead.

Read 1 Chronicles 5:9 Using Other Translations

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.
He also lived to the east as far as the entrance of the desert this side of the Euphrates, because their livestock had multiplied in the land of Gilead.
And since they had so many livestock in the land of Gilead, they spread east toward the edge of the desert that stretches to the Euphrates River.