This ecoregion occurs between Lac Mistassini and Manicouagan Reservoir and includes the Lac Saint-Jean valley and plains in the Laurentian Highlands. It is marked by predominantly cool summers and cold winters. The exception is in the Lac Saint-Jean plains and Saguenay River valley where summers are warm and moist. The mean annual temperature is approximately 0°C. The mean summer temperature is 12.5°C and the mean winter temperature is -12.5°C. The mean annual precipitation ranges north to south 800-1000 mm. Total precipitation is generally less than 900 mm in the Lac Saint-Jean-Saguenay river area. This ecoregion is classified as having a high to mid-boreal ecoclimate. It forms part of the larger boreal coniferous forest that occurs in the Laurentians of south-central Quebec. Closed stands of black spruce and balsam fir are dominant along lower slopes, whereas upper slopes are dominated by more open stands of black spruce with some white spruce and paper birch, usually associated with lichens and feathermosses. The warmer Lac Saint-Jean valley is dominated by mixed woods composed of tall to intermediate, closed stands of sugar maple, beech, and yellow birch on upland sites, whereas eastern hemlock, balsam fir, eastern white pine, and white spruce prevail in valleys. In the drier, northern parts of the region, white, red, and jack pine, along with spruce and balsam fir are more common. Eastern white cedar and black spruce are associated with wetlands. Much of the region has been deforested. The ecoregion, composed mainly of massive Precambrian granites and gneisses, takes the form of a rolling, glacial drift-covered, old erosion surface that lies at about 400-600 m asl. In the North Shore upper hills, paper birch and aspen occur sporadically on disturbed sites, whereas jack pine occurs on drier sites. The region rises abruptly above the St. Lawrence River. Its southern edge is deeply incised by several wide, large rivers that gather volume in the high, undulating interior and create a mountainous appearance where relief is commonly at 300-600 m. Humo-Ferric Podzols with Dystric Brunisolic and some Luvisolic and Organic soils are dominant in the ecoregion. Permafrost is found in isolated patches with low ice content, in the northern reaches of the ecoregion. Characteristic wildlife species include caribou, black bear, wolf, moose, lynx, and snowshoe hare. Bird species include Canada goose, ruffed grouse, and American black duck. Land uses are limited but include forestry, hunting, fishing, and recreation. Significant agricultural areas occur in the warmer climate of the Lac Saint-Jean valley and plains. The major communities include Chicoutimi, Sept-Îles, Labrador City, Baie-Comeau, and Jonquière. The population of the ecoregion is approximately 386 400.
This ecoregion is part of the Boreal Shield ecozone.