Mangoro River

Warning Spoilers through Straits of Hell. "The Mangoro begins northeast of the town of Anjozorobe in the Analamanga region, at approximately 1,100 meters. Its primary tributaries are the Onive River (which joins on the right bank at 200km from the river source) and Nosivolo River. Its mouth is at the Indian Ocean [Western Ocean] near the town of Ambodiharina. Owing to its many tributaries, the Mangoro maintains a high flow year round, making it an ideal river for rafting," from Wikipedia's description of the river in our world, https://en.wikipedia.org/wiki/Mangoro_River

In Straits of Hell Chack-Sab-At's mission to contact the Native Lemurians is tied to the shore of the Mangoro River in chapter 37.