Phase I of the La Grande complex included construction of La Grande2 (now called Robert-Bourassa), La Grande3 and La Grande4 generating stations on the Grande Rivière, as well as impoundment of the reservoirs associated with each power plant. In addition, the Caniapiscau River was diverted into the Grande Rivière watershed in order to increase the generating capacity of the three stations, and the Eastmain and Opinaca rivers were both diverted into La Grande 2 (now called Robert-Bourassa) reservoir, thus increasing the output from La Grande-2 (Robert-Bourassa) generating station. The Transtaiga highway was also built during Phase I.

Phase I cost $10.2 billion to build: $7.4 billion for the complex structures and facilities, and $3.1 billion to build the transmission grid.