Completing the quest Passage Through The Dark River, which can be acquired from Shadow in Henge of Denravi, is the straightforward manner to arrive here. Alternatively, you can travel through Talmark Wilderness (from Divinity Coast) or Stingray Strand (from Fishermen's Haven) to enter Tears of the Fallen. Head southwest from there into the Twin Serpent Lakes. Riverside Province is at the southern end of that area.