Charles Ellicott Commentary Isaiah 8:7

"now therefore, behold, the Lord bringeth up upon them the waters of the River, strong and many, [even] the king of Assyria and all his glory: and it shall come up over all its channels, and go over all its banks;" — Isaiah 8:7 (ASV)

The waters of the river ... — “The river” is, as elsewhere (Joshua 24:2, 14), the Euphrates. It is used here in two ways:

1. As the symbol of the Assyrian monarchy, just as Shiloah had been the symbol of the monarchy of Judah;
2. And as the symbol of the Assyrian armies that were to pour down like that river during its floods.

The “channels” and “banks” describe the intended course of that army as invading Syria and Israel; but it was to overflow those banks and sweep over Judah.

In the former case, the kingdoms were to be utterly submerged by the violence of the current. In Judah, it was to reach only to the neck; i.e., it was not to cause such an utter destruction.

Jeremiah (Jeremiah 47:2) reproduces this image.