Who is this that cometh up as a flood - The vast concourse of people is here represented as a river: for instance, the Jordan, suddenly swollen with the rains in harvest, rolling its waters along, and overflowing the whole country. A fine image to represent the incursions of vast armies carrying all before them. Such was the army of Pharaoh-necho in its march to Carchemish.
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.