Judah's choice to trust Assyria for deliverance from Israel and Syria is likened to choosing the wrong river. Yahweh is likened to the river Shiloah, flowing softly and peacefully (8:6) and the king of Assyria is likened to the Euphrates, strong and mighty, glorious and threatening. This river is the river to choose, the flesh says, if you really want to overcome the banks of another land.
Ah, yes, says the Lord, but can you stop this torrent when it begins to overflow your banks? You will be consumed by that which you worship and put your trust in if it is not Immanuel. And when this happens, even that will be because "Immanuel" (8:8, 10).
The people, by the way, are apparently well represented by their king, wicked and unbelieving as he is. They make the same choice as he.