Rivière Saint-Maurice, 563 km long, rises upstream from Réservoir Gouin, 200 km west of Lac Saint-Jean, Québec. It drains a basin of 43 300 km2. After its confluence with Rivière Manouane, it feeds Réservoir Blanc and then takes in the Vermillon, Trenche, Croche, Mattawin and Mékinac rivers.
Attawapiskat River, 748 km long, formed by the confluence of the Pineimuta, Trading and Otoskwin rivers at Attawapiskat Lake, in northeastern Ontario, flows east, jogs north and runs east to the flatland by James Bay. Its drainage area is 50 200 km2 and its mean discharge 626 m3/s.