The course of the river flows entirely in O'Sullivan Township and in Albanel, Mistassini and Waconichi Lakes Wildlife Sanctuary.

The surface of the "Waconichi River" is usually frozen from early November to mid-May; however, safe ice circulation is generally from mid-November to mid-April.

[2] The Waconichi River flows to the bottom of Poste Bay, which is an extension to the south of [Mistassini Lake].

The current first crosses the bay of the Post Office and passes the village of Mistissini (Cree village municipality) before passing through a strait to join Abatagouche Bay formed by the Abagouche Peninsula advancing on 32.8 kilometres (20.4 mi) to the North.

After circling this peninsula, the current runs westward across the Mistassini Lake across its width, then crosses an archipelago on the west bank, before reaching its mouth.