The river's source is east of Little Bear Lake and north-east of Narrow Hills Provincial Park[2] in a hilly plateau called Cub Hills.

Over 10,000 years ago at the end of the last ice age, the retreating glaciers shaped the landforms of the Cub Hills forming the valleys, lakes, and streams.

The Mossy River flows east out of the hills through a glacier formed valley and into flatter land characterised by muskeg and boreal forest.

From the Cub Hills, the river travels in a south-easterly direction, being met by several other tributaries, including McDougal Creek, South Mossy River, and Brougham Creek, on its way to one of North America's largest inland fresh water deltas, the Saskatchewan River Delta.

[5] Brook trout were successfully introduced to Mossy River in 1979 and became naturalised.