Jamieson, Fausset and Brown on 1 Chronicles 12:15

15. These are they that went over Jordan in the first month--that is, in spring, when the swollen river generally fills up the banks of its channel (see on Jos 3:14; Jos 4:19; Jos 5:10).

they put to flight all them of the valleys--This was probably done at the time of their separating themselves and their purpose being discovered, they had to cut their passage through the opposing adherents of Saul, both on the eastern and western banks. The impossibility of taking the fords at such a time, and the violent rapidity of the current, make this crossing of the Jordan--in whatever way these Gadites accomplished it--a remarkable feat.