I was in Neraeos' Temple and heard this:

Zaphar, the Old Man of the River yells, "Lord! Something strange is
happening!" ... "The water, it grows warm!" ... "The tear is... it's growing!"

So it appears that there is a tear somewhere, perhaps that the sludge is coming through. And it's warm, so it may be somewhere below ground. Closing the tear may cure this.

I was told secondhand that Ephialta, Keeper of the Flame that represents the lifeforce of the Zaphar said that if the stuff wasn't removed from the river, she and the flame would die. The flame itself is nearly quenched.

Penned by my hand on the 6th of Aeguary, in the year 394 AF.