Charles Ellicott Commentary 1 Chronicles 12:15

"These are they that went over the Jordan in the first month, when it had overflowed all its banks; and they put to flight all them of the valleys, both toward the east, and toward the west." — 1 Chronicles 12:15 (ASV)

When it had overflown. —A proof of their valour. They did not wait till summer had made the Jordan shallow, but crossed it in spring, when perilously swollen with the rains and the melted snows of Lebanon .

In the first month, —March–April; in Hebrew, Abib or Nisan.

Had overflown. —Was filling or brimming over.

And they put to flight all ... the valleys. — Literally, and they made all the valleys flee: that is, their inhabitants, who were hostile to their enterprise, both to the sunrise and the sunset, or on both sides of the river.