| Rivière du Sud | |
| Subdivision Type1: | Country |
| Subdivision Name1: | Canada |
| Subdivision Type2: | Province |
| Subdivision Name2: | Quebec |
| Subdivision Type3: | Region |
| Subdivision Name3: | Chaudière-Appalaches |
| Subdivision Type4: | Regional County Municipality |
| Subdivision Name4: | Montmagny Regional County Municipality |
| Length: | 23km (14miles) |
| Source1: | Lake Saint-Mathieu |
| Source1 Location: | Saint-Mathieu-de-Rioux |
| Source1 Coordinates: | 48.1721°N 68.9944°W |
| Source1 Elevation: | NaNm (-2,147,483,648feet) |
| Mouth: | Saint Lawrence River |
| Mouth Location: | Rimouski |
| Mouth Coordinates: | 48.357°N -68.7564°W |
| Mouth Elevation: | 4m (13feet) |
| Tributaries Left: | (Upstream) Rossignol Creek, Lac des Coulombe outlet, Lac Fournier and Lac Caouette outlet, Ernest-Boucher stream. |
| Tributaries Right: | (Upstream) Morin stream, Beaver stream, Simon stream, Grand Lac Malobès outlet, Cold stream. |
The Rivière du Sud-Ouest is a tributary of the southeast coast of the St. Lawrence River. This river flows in the administrative region of Bas-Saint-Laurent, in Quebec, in Canada, in the regional county municipalities of:
The Southwest river rises in the outlet of Lake Saint-Mathieu (length: ; altitude:), at Saint-Mathieu-de-Rioux. It then follows the St. Lawrence River in a northeast-southwest direction to flow into the Havre du Bic. During its course, it crosses Petit Lac Saint-Mathieu and Lac de la Station. Its two main tributaries are the outlet of Grand Lac Malobès and the Neigette River. A fall from a height of is located near its mouth.
The flow of the south-west river is generally slow, its course being located on a plateau whose average altitude is . The last eight kilometers show more rugged terrain.
From the mouth of Lake Saint-Mathieu, the southwest river flows over divided into the following segments:
Upper course of the river (segment of)
Upper course of the river (segment of)
The watershed has an area of .
The toponym "south-west river" was formalized on December 5, 1968 at the Commission de toponymie du Québec.[2]
The river hosts a small population of Atlantic salmon of around 40 spawners.[3] The latter goes up the river on where the fall of constitutes an insurmountable obstacle. Salmon spend the first two years of their lives in the river. The second species in abundance is generally the American eel, which unlike salmon, goes up the river over its entire length to Lake Saint-Mathieu.