Just throwing some ideas for the river and more factions.
The river is called ''the Water-Son of the 3 Sisters''. It his said in the old legends that the river is actualy the sentient spaw of three water hags who all fell pregnant from the god X and gave birth on their way to drown the unwanted kid in the sea, breaking their waters midway thru the desert trip, thus creating the river. The hags are said to have drowned. To this day, the River change its form from time to time, taking the shape of one of his mothers: the Hunchback (when the river gets wide and rise on inhabited land), the Thirsting (when its at his thinnest form, during the dry season) and the Sage (the normal, calm for of the river). During the Hunchback and the rainy season, children must beware of the Banderhobbs. The dry mummies that emerge when the Water-Son is almost dried up are slow and fragile, but can extract water from your body and the dust they release when destroyed can choke a man days after the mummy's final death.
Factions:
The House of Paper: magical archivist, creator of Scroll Mummies (egyptian-theme liches, embalmed in scrolls), collector of lore.
The Erosion: Group of people thinking ruins his the final and purest state of all things. Think the Doom Guard from Planescape. Attack conservationist, hunts undead.
The Fennecs: a guild of thieves that hides in caverns in the desert. Their leader his a blind woman who can ''see'' only with sounds. They use blind beggars as a spy network.
Pilgrims of the Final Dawn: a group of cleric and pilgrim that shelters repentant vampires from every lands, preparing them to go into the sun-scorched desert for one last time.