The Lower Niben flows from the south end of Niben Bay, past and through the city of Leyawiin, and empties into Topal Bay. (map)
It is a very abundant source of Nirnroot. Leyawiin completely blocks the river, so it must be assumed that the water flows underground or through the sewers, because otherwise the city would act as a dam, and be completely flooded.
Water's Edge sits on its northwestern shore, with Rockmilk Cave a short distance to the north, and the Ayleid ruin Telepe and Undertow Cavern further south of Water's Edge. Fort Redman, Fisherman's Rock, and Amelion Tomb all sit on the eastern shores.