The Mattagami flows 440 km from its source southwest of the city of Timmins, to Portage Island, where the Mattagami's confluence with the Missinaibi River forms the Moose River, about 100 km from its outlet at James Bay.
The Mattagami flows 440 km from its source, on the Canadian Shield southwest of Timmins, to Portage Island in the boreal wilderness of northern Cochrane District. Here the Mattagami's confluence with the Missinaibi River forms the Moose River, about 100 km from its tidewater outlet at James Bay.