What does Isaiah 8:7 mean?
This passage uses a vivid metaphor to explain what the Lord will cause to happen to his people. King Ahaz and the people of Judah refused to trust in the "waters of Shiloah" to protect them. Shiloah represents the Lord or the people's perception of Him (Isaiah 8:6). The king, Ahaz, refused God's command to ask for a confirming sign (Isaiah 7:10–13), likely because he had already determined to ask the Assyrian forces for aid. Judah did not consider the Lord powerful enough to protect them from their enemies: Israel and Syria.Instead, the people of Judah put their hope and trust in another river, named here as the Euphrates. This river represented the powerful king of Assyria and his mighty armies. That river did succeed in removing the threat of Israel and Syria. The people of Judah rejoiced over the defeat of those nations.
Now, the Lord says that the king of Assyria in all his power, will not be contained. His waters are indeed "mighty and many", referring to the size and ferocity of his armies. No matter what deals may have been made by Ahaz, the Assyrian "river" will overflow its banks and sweep destruction into Judah, after all.
The bottom line is that the people have put their hope in the wrong river. It is powerful, yes, but it is also unpredictable and out of control. The Lord will use that river to bring judgment on His faithless people.