Opinaca Reservoir is a lake in the northwest of the province of Quebec in Canada.

It is located 140 km east of James Bay, south of the Robert-Bourassa Reservoir and south of Lac Sakami.

It is connected by a 41 km narrows to the Eastmain Reservoir located east.

Opinaca Reservoir is a reservoir of the James Bay Project with an elevation of 216 m (709 ft) and an area of 1,040 km2 (400 sq mi).

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