Abstract The St. Lawrence Lowlands region in the southern part of Canada encompasses an area that extends from Windsor at the southern tip of the province of Ontario to the St. Lawrence River at Quebec City (Fig. 1). This region comprises only 100,000 km 2 , or 2 p.rcent of the land surface of Canada, but contains morethan half the population of the country. Also it accounts for 75 p.rcent of the industrial production and 40 p.rcent of the agricultural income. Many of Canada’s major cities, such as Montreal, Ottawa, Toronto, and Hamilton are in this region (Fig. 1). The climate varies significantly across the 1,000 km from southwest to northeast but is a humid continental type with warm summers and cold winters. Annual precipitation varies from about 800 to 1,100 mm, and mean annual temperature varies from about 9°C in the southwest to 5°C in the northeast. The St. Lawrence Lowlands region is bounded on the northwest by the uplands of the Canadian Shield and on the southeast by faults and structures of the Appalachian Folded Belt (Fig. 3; Table 2, Heath, this volume). Relatively undeformed Paleozoic sedimentary rocks from Cambrian to Mississippian age overlie the Precambrian basement, and the sedimentary rocks are more than 1,000 m thick at the southwest end of the region. The region is an extension of the Glaciated Central Region, and the local topography is dominated by the effects of Pleistocene Wisconsin glaciation. Ice flow was strongly controlled by the preglacial drainage system now occupied in part by the Great Lakes; this control has had a strong influence on the glacial deposition, particularly the major moraines (Fig. 1) and outwash deposits, and consequently, the hydrogeology.