[2] The river is 320 kilometres (199 mi) in length and its name means "long water" in the Cree language.

A portion of the river's headwaters have been diverted into the Lake Superior drainage basin.

[2] It passes under Ontario Highway 11 and the Canadian National Railway (CNR) transcontinental main line (used at this point by Via Rail transcontinental Canadian trains) into geographic Bain Township in Unorganized Thunder Bay District, takes in the left tributaries Kenogamisis River and Burrows River, and reaches the Kenogami Lake Dam.

It continues northeast, takes in the left tributary Kawakanika River, reaches Chipman Lake, passes into Cochrane District at geographic Chipman Township, takes in the right tributaries Fernow River and Wabigano River, heads under a CNR line abandoned in 1986 (originally the National Transcontinental Railway main line) between the railway points Ogahalla to the west and Blanche to the east, and reaches the confluence with the left tributary Atikasibi River.

[10] A further stage was completed in 1948 when the Hayes Lake Dam (for water control) and Aguasabon Generating Station (for hydroelectricity), the former just upstream of the latter and the latter just upstream of the mouth of the Aguasabon River at Lake Superior, to support a pulp and paper mill at the adjacent town of Terrace Bay.