The Mongeau River (in French: rivière Morigeau) is a tributary of the south-eastern bank of the rivière du Sud (Montmagny), which flows north-east to the south shore of the St. Lawrence River.

The main neighboring watersheds of the Morigeau river are: The Morigeau River originates from the northeast side of route 283, in the municipality of Montmagny, only 320 metres (1,050 ft) of the municipal boundary of Notre-Dame-du-Rosaire.

[1] The Morigeau River flows northeast, more or less parallel (on the northeast side) to the rivière du Sud (Montmagny), to the south shore of St. Lawrence River and to the rivière des Poitras.

[1] From its source, the Morigeau river flows over 23.5 kilometres (14.6 mi), divided into the following segments: The Morigeau river empties on the south-eastern bank of the rivière du Sud (Montmagny).

[1] The toponym Morigeau river was formalized on December 5, 1968, at the Commission de toponymie du Québec.