Rising at an altitude of over 800 metres in the Réserve faunique des Laurentides, the Rivière-à-Mars flows for 95 kilometers from south to northeast, in a tumultuous and rugged descent. It flows into Baie-des-Ha!-Ha! of the Saguenay Fjord after crossing the borough of La Baie, in Saguenay. The river offers a multitude of pools for salmon and trout fishing (anadromous Saguenay brook trout). Salmon fishing is practiced by wading, mainly on the 7 kilometers of river that are most easily accessible.
From 1894 to 1935, Price Brothers operated a private fishing club on the Rivière-à-Mars. In 1930, the construction of a dam limited access to salmon and allowed timber to be floated until 1952. Following major seeding and development work, including the creation of a salmon fishway, the Association des pêcheurs sportifs de la Rivière-à-Mars was formed in 1983, and fishing reopened in 1992. In 1996, the Saguenay flood destroyed all the river's facilities, prompting further work, including the creation of a new fish pass.
The river has recovered well from this catastrophe, as evidenced by annual runs of 200 to 500 salmon, some reaching over 13 kilograms. The waters of Rivière-à-Mars are cool and slightly dark, tinted towards copper. For salmon fishing, there are five sectors, two of which have restricted access. There's also a salmon and sea trout interpretation trail and an underwater observation window.