The Robert-Bourassa Dam, known until its renaming as LG-2, is a 162-metre (531 ft) earth-fill dam on the La Grande River in northern Quebec, Canada, 117.5 km upstream of the river's mouth on James Bay. Its underground generating station, 137 m below the surface, holds 16 Francis turbines with a combined 5,616 MW. They were commissioned between 1979 and 1981, and the station produces roughly 26,500 GWh in a typical year. It is Canada's largest hydroelectric station and the largest underground power station in the world. Together with the neighbouring 2,106 MW La Grande-2-A station, which shares the 2,835 km² Robert-Bourassa Reservoir, it accounted for more than 20% of Hydro-Québec's installed capacity in 2009. It is the central plant of the James Bay Project, which supplies almost half of Hydro-Québec's output, and building it reshaped the lives of the region's Cree.
Where the Water Comes From: La Grande and Two Diverted Rivers
The plant works because engineers collected far more water than the La Grande River carries on its own. The river drains part of the Canadian Shield between the Laurentian Plateau and the James Bay coast. Its catchment above the dam covers 32,480 km² of granite, lakes and spruce taiga, shaped by glaciers that left moraine, clay and sand across the land. Hydro-Québec's engineers and the firm Rousseau Sauvé Warren chose this river over the southern Nottaway, Broadback and Rupert rivers in 1972. The La Grande option was cheaper, needed less flooding and had less impact on Cree hunting and fishing, and the silty soils of the southern rivers would have made damming harder.
Then the supply was boosted. The James Bay Project turned the Eastmain River and the Caniapiscau River into the La Grande basin, and each diversion adds about 800 m³/s. The Eastmain's flow at its mouth fell by 90% and the Caniapiscau's by 45%. As a result the La Grande now carries about 3,400 m³/s on average instead of 1,700. Releases follow Quebec's demand for electricity, which peaks in winter, so the seasonal pattern has been turned upside down: the winter flow that once fell to about 500 m³/s now reaches about 5,000 m³/s.
Before any of this could be built, the site had to be reached. In June 1971 crews set out from the mining town of Matagami, 630 km to the south, with a deadline of 450 miles in 450 days. Surveyors and lumberjacks went in by seaplane and helicopter. A 555-metre ice bridge over the Rupert River opened on 11 February 1972, and the temporary road reached the La Grande in December 1972. A permanent gravel road was finished on 20 October 1974 and paved over the next two years.
The 162-Metre Moraine Dam and Its 29 Dykes
The river was moved first. Two diversion tunnels, each 14.8 m wide and 18 m high, were drilled through the left bank and opened on 27 April 1975. They were sized for the La Grande's 65-year flood of 7,500 m³/s and carried the whole river for three and a half years. Today they serve as flood outlets with a design flow of 3,100 m³/s.
The dam itself is built mostly from local glacial deposits. Its watertight core is glacial moraine, which was available in large quantities within 10 km of the site. The core leans slightly upstream to resist the weight of the reservoir. Around it are fill zones of increasingly coarse material, and the whole slope is faced with riprap against wave erosion. The dam is 2,836 m long at the crest and about 550 m wide at the base, and it contains some 23 million m³ of fill. A concrete-faced rockfill design was rejected, partly on cost and partly because the face would need regular inspection, which would mean draining a reservoir far too large to empty.
The dam is only one part of the barrier, because the land around the reservoir is low. To hold the water at its operating level of 175.3 m above sea level, 29 dykes close the gaps in the surrounding ridges. Dykes D-1 to D-4 sit just north of the dam, the "Forebay dikes" D-5 to D-14 to the south, and the "Duncan dikes" D-17 to D-27 about 30 km further south. They range from D-26B, 82 m long, to D-20, 6,052 m long. Where the Duncan dykes blocked the Desaulniers River, Hydro-Québec built a pumping station in 1977 instead of flooding another 30 km². Its four pumps, each moving 1.7 m³/s, lift the water 54.3 m into the reservoir and keep Desaulniers Lake at its natural level.
The reservoir began to fill when the tunnels were closed on 27 November 1978. The water rose 15 m on the first day and 43 m in the first week. It reached the minimum operating level of 167.6 m on 2 September 1979 and the normal maximum of 175.3 m that December. The Robert-Bourassa Reservoir now covers 2,835 km², an area larger than Luxembourg, and holds about 61.4 km³ of water, of which about 19.4 km³ can be drawn down for power.
The Staircase of the Giants Spillway
Surplus water leaves by a different route from the water used for power. The spillway, part of dyke D-4 near the north end of the main dam, has a concrete control structure with eight gates, each 12.2 m wide and separated by 4.3 m columns. Cable winches raise and lower the gates, and all eight are opened at the same time only about once every fifteen years. The spillway's capacity is about 17,600 m³/s.
Below the gates, the water first loses energy in a 135-by-122-metre stilling basin. It then runs down a 1,500-metre channel cut into the rock, which drops 110 m to the La Grande in ten giant steps, each 9.1 to 12.2 m high and 127 to 200 m long. This is the feature that gives the spillway its nickname, the "Staircase of the Giants". The water speeds up from 11 m/s at the top to 22 m/s at the bottom.
137 Metres Down: The Underground Powerhouse and LG-2-A
The generating water takes a separate path, to a machine hall cut into the granite 137.2 m below the surface and about 6 km downstream of the main dam. The service tunnel, machine hall and surge chamber together required excavating 2.35 million m³ of rock. Unusually for the James Bay Project, the station was designed to imperial rather than metric measurements, even though Canada had gone metric in the 1970s. Workers reach it either by lift or through an access tunnel.
The 16 Francis units sit in two rows of eight, separated by the assembly area, workshops, control room, lift shaft and ventilation plant. Two suppliers built them. Marine Industries Limited made units 1, 3, 5, 7, 10, 12, 14 and 16, each 96 tonnes with 15 blades. Canadian General Electric and Dominion Engineering Works made the rest, at 111 tonnes with 11 blades. The turbines turn at 133.33 rpm under about 137 m of head, and the generators are rated at 370 MVA, rising to 390 MVA in winter. Quebec Premier René Lévesque inaugurated the station before 3,000 people on 27 October 1979. In 1991–1992 Hydro-Québec added the 2,106 MW La Grande-2-A station, which draws on the same reservoir.
Radisson, Chisasibi and the 1975 James Bay Agreement
The station is named after Robert Bourassa, Premier of Quebec in 1970–1976 and 1985–1994, who announced the project on 30 April 1971; in a 1985 essay he argued that Quebec's economic development relied on developing its natural resources. Building it was not peaceful. On 21 March 1974, amid rivalry between construction unions, workers at the LG-2 site used bulldozers to wreck generators and fuel tanks and set buildings on fire. The project was delayed by about a year, and the Cliche Commission, whose members included the future Prime Minister Brian Mulroney, went on to expose corruption in Quebec's construction unions.
The town of Radisson was founded in 1974 to house workers. It had about 2,500 people at the 1977 peak and about 300 today, and it remains one of only two non-Indigenous towns north of the 53rd parallel in Quebec. About 100 km west, the Cree village of Fort George stood on an island at the La Grande's mouth. The heavier regulated flow eroded the island and stopped solid winter ice from forming, so in 1981 the Quebec government moved the village to the mainland south shore, taking about 200 houses with it, and the new settlement was named Chisasibi, "Great River".
About 5,000 Cree and 4,000 Inuit lived in the territory, and a 1973 court ruling against the Quebec government forced the Bourassa government to negotiate with them. In November 1975 the James Bay and Northern Quebec Agreement gave them exclusive hunting and fishing rights over about 170,000 km² and about $250 million in compensation, in return for the right to develop the region's rivers. The reservoirs still flooded about 4% of the Cree's traditional hunting grounds, including about 10% of Chisasibi's. Flooded vegetation released mercury, and by 1981–82 levels in lake whitefish had risen up to four times above pre-flood values and in northern pike up to seven times. This led to the 1986 James Bay Mercury Agreement. In 2002 Quebec and the Grand Council of the Crees signed the Paix des Braves, which set new terms for later stations on the system.
Quebec's other giant dams make a useful comparison: the multiple-arch Daniel-Johnson Dam on the Manicouagan River is a very different design. The La Grande itself ends in the shallow southern arm of the inland sea described in our Hudson Bay guide, and the wider setting of the Shield and the north is covered in our overview of Canada's geography.