It lies mostly in south and eastern Ontario and Quebec in Canada, and Upstate New York and Vermont in the United States.

This ecoregion is a transition area between the taiga to the north and the temperate deciduous forest to the south and thus contains a variety of habitats including freshwater marshes, dunes, bogs, fens, and hardwood and conifer swamps.

Large mammals including Canada lynx (Lynx canadensis), cougar (Puma concolor), caribou (Rangifer tarandus), wolverine (Gulo gulo), elk (Cervus canadensis) and eastern wolf (Canis lycaon) have been completely extirpated from this ecoregion; remaining mammals include white-tailed deer (Odocoileus virginianus), American black bear (Ursus americanus), coyote (Canis latrans), moose (Alces alces), snowshoe hare (Lepus americanus), eastern chipmunk (Tamias striatus), bobcat (Lynx rufus), American red squirrel (Tamiasciurus hudsonicus) and eastern gray squirrel (Sciurus carolinensis).

The Saint Lawrence River is one of the most polluted in the world[citation needed] and these surrounding forests are vulnerable to clearance for agriculture and urban development including the cities of Montreal, Ottawa, Quebec City and suburbs of Toronto, Syracuse and Albany, New York.

The fragmented blocks of remaining habitat include: the eastern end of Lake Ontario; Missisquoi National Wildlife Refuge in Vermont; the Chaumont Barrens, Rome Sand Plains, the Albany Pine Bush and the proposed Split Rock Wildway in New York; Bruce Peninsula (the barrier between Georgian Bay and the main section of Lake Huron), Alfred Bog, Luther Marsh, the Ganaraska Forest and Carden Plain in Ontario; and Mont Saint-Hilaire, Lac Saint-François National Wildlife Area and Cap Tourmente National Wildlife Areas in Quebec.