The "rivière à Mars" is a tributary of the Saguenay River and has its source in the Laurentides Wildlife Reserve.

The surface of the Mars river is usually frozen from the beginning of December to the end of March, however the safe circulation on the ice is generally done from mid-December to mid-March.

Taking its source at the mouth of a small lake at 869 m (2,851 ft) above sea level in the Laurentides Wildlife Reserve, the "rivière à Mars" flows from south to northeast in a descent comprising several tumultuous and rugged segments.

Following major seeding and development work, including the creation of a fishway for salmon, the Association of sport fishermen of the Mars river was formed in 1983, and fishing reopened in 1992.

The toponym "Rivière à Mars" was formalized on December 5, 1968, at the Place Names Bank of the Commission de toponymie du Québec.