Deuteronomy 3:16

And unto the Reubenites and unto the Gadites I gave from Gilead even unto the river Arnon half the valley, and the border even unto the river Jabbok, which is the border of the children of Ammon;

From Gilead - i:e., not the mountainous region, but the town Ramoth-gilead.

Even unto the river Arnon half the valley. The word "valley" signifies a wady, either filled with water or dry, as the Arnon is in summer; and thus the proper rendering of the passage will be-`even to the half or middle of the river Arnon' (cf. Joshua 12:2). This prudent arrangement of the boundaries was evidently made to prevent all disputes between the adjacent tribes about the exclusive right to the water.