The rivière Méchant Pouce (in English: Bad Tumb River) flows in the municipalities of Saint-Marcel (MRC of L'Islet Regional County Municipality) and Sainte-Apolline-de-Patton (MRC de Montmagny Regional County Municipality), in the administrative region of Chaudière-Appalaches, in Quebec, in Canada.

The main neighboring watersheds of the Méchant Pouce river are: The Méchant Pouce river takes its source from mountain streams, north-west of Violon Lake (altitude: 426 metres (1,398 ft)), in the Notre Dame Mountains.

Violon Lake straddles the townships of Arago Saint-Marcel and Patton Sainte-Apolline-de-Patton.

The confluence of the Méchant Pouce river is located in the hamlet of L'Espérance.

[2] The toponym Rivière Méchant Pouce was made official on December 5, 1968, at the Commission de toponymie du Québec.