In Jeremiah 46:3-6 we saw only a mighty army marshalling for battle, and its hasty flight. In Jeremiah 46:7-12 the prophet tells us at whose defeat we have been present.
A flood - the Nile. The metaphor describing the advance of the Egyptian army is naturally drawn from the annual overflow of their own sacred stream.
Whose waters are moved ... - literally, his waters toss to and fro as the rivers, the natural branches of the Nile in Lower Egypt.