The southern channel is named by the Atlas of Canada as the Attawapiskat River, and is the source location listed in the Infobox at right.

The elevation of the river drops significantly along these two outflow channels, descending from the higher ground of the Canadian Shield to the flatter and more boggy Hudson Bay Lowlands.

The river continues east, and makes a bend to the north at Pym Island at 52°12′20″N 86°19′28″W / 52.20556°N 86.32444°W / 52.20556; -86.32444[8] at an elevation of 174 metres (571 ft).

The Streatfeild River joins from the right at an elevation of 148 metres (486 ft), and the outlet river from McFaulds Lake, centre of the Northern Ontario Ring of Fire geological area, joins from the left 17 kilometres (11 mi) further downstream at 52°48′10″N 85°54′45″W / 52.80278°N 85.91250°W / 52.80278; -85.91250[9] at an elevation of 139 metres (456 ft).

[11] Less than 100 kilometres (62 mi) from its mouth, the Attawapiskat has carved out several clusters of spectacular high limestone islands, nicknamed by canoeists the "Birthday Cakes".