The Rivière-à-Mars is a tributary of the Saguenay Fjord in the Saguenay–Lac-Saint-Jean administrative region in the east of the Canadian province of Quebec. This part is easy to hike and is very wild and really worth seeing.
The Rivière-à-Mars rises in the Laurentian Mountains in the Laurentides Wildlife Reserve. From there it flows in a predominantly northerly direction. It takes in the Rivière à Mars Nord-Ouest on the left. Shortly before reaching the town of La Baie, which is part of Saguenay, it flows through a 700-meter-long gorge. It then turns east-northeast, flows through the center of La Baie and finally flows into the Baie des Ha! Ha! on the south bank of the Saguenay Fjord. The Rivière-à-Mars is 129 kilometers long. It drains an area of 660 square kilometers. To the east, its catchment area borders that of the Rivière Ha! Ha!, which also flows into the Baie des Ha! Ha!. To the west is the catchment area of the Rivière du Moulin, which flows into the Rivière Saguenay in Chicoutimi. The average discharge of the Rivière-à-Mars is 14 cubic meters per second.