So, you are DMing a forgotten realms campaign. In the realms, the lower planes are (hypothetically, but we will assume it’s true) strung along the river stix, like beads on a string, also, mount celestia has an infinite sea of holy water around it. Think about what would happen if some one opened a portal from the sea of holy water to the bottom of the river stix (between the lower planes, so nobody would find it).

Did your PCs decide to do this, or is this a hypothetical scenario. Either way, it is up to the DMs (or your) discretion. Maybe you can't open planar portals on the River Styx (since it is its own method of interplanar travel. Maybe the River Styx ran ran into the Abyss and poured down its endless layers. Maybe the excess water formed a new branch of the river, the River Acheron (or something). Maybe the planar portal won't let water pass through it. Maybe it nullified the effects of the River Styx and make it lose it's cleaning properties, ad becomes a tourist location for divine beings. There are many possibilities.

This is just hypothetical. Those are great ideas!