Jamieson, Fausset and Brown on 2 Samuel 24:5

5. they passed over Jordan--This census was taken first in the eastern parts of the Hebrew kingdom; and it would seem that Joab was accompanied by a military force, either to aid in this troublesome work, or to overawe the people who might display reluctance or opposition.

the river of Gad--"Wady" would be a better term. It extends over a course estimated at about sixty miles, which, though in summer almost constantly dry, exhibits very evident traces of being swept over by an impetuous torrent in winter (see De 2:36).