In the previous chapter, Isaiah had delivered the good news that the current apparent threat from Syria and Remaliah would not occur. The two images that Isaiah sets forth are the pleasing and peaceful waters of Shiloah, and the joy in the news that Rezin will not be a threat.
Symbolically, the waters of Shiloah represent Jehovah’s law at the heart of Jerusalem. It is that which has been rejected, as Isaiah has previously indicated. He uses the refusal of the waters here because he will contrast those easy waters of Jehovah’s rule to the “waters of the river, strong and many”, which will be the Assyrian invasion. The peaceful waters of Shiloah create the antithesis of the strong waters of the rivers, and the previous peace in Jerusalem to the destruction that is coming.
Isaiah doesn’t wait for readers to understand the symbolism, however, for he names the king of Assyria directly, and has him “overflow” the land. The extension of the metaphor of the river refers to a river in devastating flood conditions.