Isaiah 8:7 - Ellicott's Commentary On The Whole Bible

The waters of the river... — “The river” is, as elsewhere (Joshua 24:2; Joshua 24:14), the Euphrates; here used (1) as the symbol of the Assyrian monarchy, as Shiloah had been of that of Judah, and (2) of the Assyrian armies that were to pour down like that river in the time of its inundations. 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 as by the violence of the current. In Judah, it was to reach only “to the neck,” i.e., was not to work out so utter a destruction. Jeremiah (Jeremiah 47:2) reproduces the image.