Both rivers take their common source from a marsh area located south of the village of Sainte-Barbe.

From the head swamp, the Saint-Louis River flows 12.8 km northeast to route 201, more or less parallel to the south shore of lake Saint Francis.

From this village, the serpentine river flowing 10.0 km north-east, crossing Saint-Étienne-de-Beauharnois; then it flows for 9.1 km passing on the west side of the city of Beauharnois and on the east side of the Beauharnois Power Station to its mouth.

Pointe Thibaudeau borders on the west side the small bay located at the mouth of the Saint-Louis river.

The toponym "Rivière Saint-Louis" was formalized on December 5, 1968, at the Commission de toponymie du Québec.