The River is accessible from the Grotto.
The name gives it away. It's an underground river. When you first enter, you'll have some ground to stand on, but that quickly gives way to wall to wall water. At the end of the twisting river is a portal to the Maelstrom.
Expect to find lots Eels and Crocodiles here. Traversing this area as a vampire is nearly impossible, but without night vision it's dangerous too. You can try bringing GLOW or coming as a Light Mage.
There's really no reason to visit the River until you're ready to do the extended late game.