Physical Geography

Ontario lies within three distinct regions: the Canadian Shield (or Laurentian Plateau), the Great Lakes–St. Lawrence Lowland, and the Hudson Bay Lowland.

The Canadian Shield spans all the province except the north and the extreme southeast. It is an area of geologically ancient rock, dating from Precambrian times. Surface features stem mainly from the last Ice Age, when advancing and retreating glaciers scoured the land. On their retreat, they left a poorly drained land studded with thousands of lakes and laced by rivers. Forests and lakes cover most of this region; in some sections muskegs (bogs) are widespread. Settlements are found mainly along Lakes Huron and Superior and in scattered mining and lumbering areas in the interior.

Although the Canadian Shield appears to be flat, it rises gently southward. The highest point in the province is north of Sudbury, where the Shield reaches an elevation of 2,275 feet (693 m).

The Great Lakes–St. Lawrence Lowland, in the southeast, is made up of two parts—the valley of the St. Lawrence River and a peninsular area wedged between Lakes Ontario, Erie, and Huron. Separating the two sections is a low, narrow extension of the Canadian Shield that connects with the Adirondack Mountains in New York. The Thousand Islands, in the St. Lawrence River, are outcroppings of the Shield. Level to rolling land prevails throughout the lowland, although there are hilly uplands and escarpments. At one, the Niagara Escarpment, is the mighty Niagara Falls. The Great Lakes–St. Lawrence Lowland region is Canada's leading center of population, agriculture, industry, and commerce.

The Hudson Bay Lowland is a low-lying plain fringing Hudson and James bays in the north. Its surface is exceptionally flat and often marshy, for it consists of marine deposits that were raised above sea level in very recent geologic time. Scrubby forests cover much of the region; tundra vegetation of mosses, lichens, and other extremely hardy small plants occurs in the far north.

About 17 per cent of Ontario is covered by inland water, nearly all of which is accounted for by the Great Lakes and the lakes of the Canadian Shield. On the Shield the largest lakes are Lake Nipigon; Lake of the Woods, which Ontario shares with Manitoba and Minnesota; Lac Seul; and Lake Nipissing.

Slightly more than three-fifths of the land is drained by rivers flowing northward to Hudson and James bays; the rest by rivers flowing southward to the Great Lakes–St. Lawrence system. Except for the Ottawa River, a major tributary of the St. Lawrence, the northern rivers are longest. These include the Severn, Albany, Attawapiskat, and Moose-Mattagami rivers. Southern rivers, particularly the Niagara, the Ottawa, and the St. Lawrence, are chief sources of hydroelectric power.

Ontario has a continental type of climate, the result mainly of the province's northerly location in the North American land mass. Only in the south, especially the southeast, is the tempering effect of the Great Lakes felt. Winters are long and often severely cold. Average January temperatures range from about –20° F. (–29° C.) in the extreme north to around 0° F. (–18° C.) in the central part and 25° F. (–4° C.) along the lake shores in the southeast. Summers are short and vary from warm in the south to cool in the north. Average July temperatures range from about 55° F. (13° C.) in the far north to 70° F. (21° C.) in the extreme south.

Precipitation increases southward, varying from 15 inches (380 mm) annually near Hudson Bay to almost 40 inches (1,010 mm) in the southeast. Snowfall is heavy throughout most of the settled area, averaging some 70 to 120 inches (1,780 to 3,050 mm) a year. The far north receives less.

About two-fifths of Ontario is covered by forests, almost all of which are owned by the provincial government. Though most have potential use, only forests in the more accessible areas have been logged. Forests on the Canadian Shield consist primarily of virgin stands of coniferous trees, mainly spruces, larches, firs, and pines They extend northward hundreds of miles and become increasingly thinner and poorer in quality in the northern parts of the Shield. There are no trees in the far north, only tundra vegetation. In the south near the Great Lake, the forests contain increasing numbers of broad-leaved deciduous trees.