Eastmain is a compounding of the river's former name East Main,[2] which was taken from the former Hudson's Bay Company outpost at its mouth.

This post controlled company trading operations in the East Main District on the eastern side of James Bay.

Since the late 1980s, most of the waters of the Eastmain River have been diverted and flow northwards through the Opinaca Reservoir, with a surface area of about 950 square kilometres (370 sq mi), and into the Robert-Bourassa Reservoir of Hydro-Québec's La Grande Complex.

The remainder of the Eastmain River contains only about 10 percent of the volume of its former flow, and is now subject to freeze-up in winter (see photo).

The Eastmain Reservoir will eventually have a surface area of about 600 square kilometres (230 sq mi), and the Eastmain-1 power plant will generate a maximum of 900 MW.