This frightening map is part of a 1987 study released to us by Hydro-Quebec after an access-to-information request. It was one of the earliest emergency studies of what a dam breach in the La Grande complex would look like. It is based on a possible rupture of LG-4 taking down the other dams downriver.

The study offers no other emergency planning advice: evacuation plans, contact numbers, reasons LG-4 might breach, etc., that are normally found in an emergency plan.

Hydro-Quebec claims it has emergency plans for La Grande, but refused to
release or even identify them.

Potential flood zones
Breach in cascade of LG-4, LG-3, LG-2 and LG-1

The water would flood into James Bay 426 km downstream 18h26 after LG-4 busts. The tidal wave would be 27.3 metres high (90 feet).

Chisasibi is not identified on the map. It is 422 km downriver from LG-4. The water would arrive here 17h52 after the LG-4 rupture. Chisasibi would be flooded by 30 metres of water, and the water would arrive at Fort George Island at 18h13.

The only highway out of town is cut off 30 km upriver from Chisasibi at 17h05 after the initial LG-4 breach. Here the water leaves the Chisasibi River valley and floods the coastal plain.

20 km

Reference: La Grande Complex
Report 07442-RA-87/47
December 1987
Service Hydraulique, Direction équipments de production

The water coming down
the river from LG-4
would arrive at the main
LG-2 dam (322 km downstream) 16h07
after the initial dam breach.