God's response to the choices of the people arrives as a massive, destructive flood. This disaster stands in sharp contrast to the quiet waters of the local stream that the people had rejected. Because the people of Judah disrespected their local king and celebrated with the enemies of the Davidic dynasty, God brings upon them a cruel and vastly superior military force [מלבי״ם, רד״ק, ביאור שטיינזלץ]. This punishment is a direct consequence of the conspirators' actions in Judah [רד״ק, אבן עזרא].
To illustrate this coming destruction, the prophet uses the imagery of a great river, pointing to the Euphrates or Tigris, representing the king of Assyria and his sweeping, energetic empire [מלבי״ם, ביאור שטיינזלץ]. The waters possess two distinct qualities: their might reflects the sheer power and roaring noise of the army, while their abundance highlights the massive number of soldiers [מלבי״ם, מצודת דוד]. Accompanying the king is all his glory, which is an essential part of the poetic imagery [שד״ל]. This glory refers to his multitude of troops and the vast wealth he gathered from conquering other nations, giving the Assyrian king absolute confidence to wage war fearlessly and cover the land with his forces [מצודת דוד, רד״ק]. Within this imagery, the king of Assyria himself is compared to the mighty waters, while his accumulated glory and wealth are likened to the abundant waters [מלבי״ם].
As the flood progresses, the water rises over its usual channels—the strong pathways where it normally flows [רש״י, מצודת ציון, אבן עזרא, מלבי״ם]. The image of the water ascending hints at the Assyrian army marching up to the Land of Israel, which sits at a higher topographical elevation than surrounding lands [רד״ק]. Typically, a river only overflows when its natural channels are blocked. However, the Assyrian army will be so massive that even as it spreads out to fight multiple nations at once and fills all its regular pathways, it will continue to swell, overflow, and flood the land of Judah [מלבי״ם].
At the peak of this disaster, the river surges over its high, upright banks that normally contain the water like a gutter [רש״י, מצודת ציון, רד״ק]. The waters swell until they completely breach these natural boundaries [רש״י]. In this metaphor, the high riverbanks symbolize the fortified cities of Judah. The overwhelming Assyrian army will wash over and conquer every single one of these strongholds, leaving only Jerusalem untouched [רד״ק].