The water level of the river varies with the seasons and the precipitation; the spring flood generally occurs in April.

The Grosse Décharge Ouest rises at the mouth of Côté lake (length: 0.17 kilometres (0.11 mi); altitude: 620 metres (2,030 ft)).

The mouth of this lake is located at: From its source, the course of La Grosse Décharge Ouest descends from the mountain on 7.7 kilometres (4.8 mi) with a drop of 279 metres (915 ft), according to the following segments: The Grosse Décharge Ouest pours into a bend on the west bank of the Mares river in Baie-Saint-Paul.

This mouth is located at: From the mouth of La Grosse Décharge Ouest, the current descends on 11.8 kilometres (7.3 mi) the course of the Mares river; then on 7.8 kilometres (4.8 mi) with a drop of 15 metres (49 ft) following the course of the Rivière du Gouffre which flows into Baie-Saint-Paul in the Saint-Laurent river.

The toponym "La Grosse Décharge Ouest" was formalized on March 25, 1997 at the Place Names Bank of the Commission de toponymie du Québec.