Canada has a very long and irregular coastline; Hudson Bay and the Gulf of St. Lawrence indent the east coast and the Inside Passage extends along the west coast. The straits between the islands of N Canada form the Northwest Passage, but until the 21st century the passage was ice-clogged year-round. During the Ice Age all of Canada was covered by a continental ice sheet that scoured and depressed the land surface, leaving a covering of glacial drift, depositional landforms, and innumerable lakes and rivers. Aside from the Great Lakes, which are only partly in the country, the largest lakes of North America—Great Bear, Great Slave, and Winnipeg—are entirely in Canada. The St. Lawrence is the chief river of E Canada. The Saskatchewan, Nelson, Churchill, and Mackenzie river systems drain central Canada, and the Columbia, Fraser, and Yukon rivers drain the western part of the country.
Canada has a bowl-shaped geologic structure rimmed by highlands, with Hudson Bay at the lowest point. The country has eight major physiographic regions—the Canadian Shield, the Hudson Bay Lowlands, the Western Cordillera, the Interior Lowlands, the Great Lakes–St. Lawrence Lowlands, the Appalachians, the Arctic Lowlands, and the Innuitians.
The exposed portions of the Canadian Shield cover more than half of Canada. This once-mountainous region, which contains the continent's oldest rocks, has been worn low by erosion over the millennia. Its upturned eastern edge is indented by fjords. The Shield is rich in minerals, especially iron and nickel, and in potential sources of hydroelectric power. In the center of the Shield are the Hudson Bay Lowlands, encompassing Hudson Bay and the surrounding marshy land.
The Western Cordillera, a geologically young mountain system parallel to the Pacific coast, is composed of a series of north-south tending ranges and valleys that form the highest and most rugged section of the country; Mt. Logan (19,551 ft/5,959 m) is the highest point in Canada. Part of this region is made up of the Rocky Mts. and the Coast Mts., which are separated by plateaus and basins. The islands off W Canada are partially submerged portions of the Coast Mts. The Western Cordillera is also rich in minerals and timber and potential sources of hydroelectric power.
Between the Rocky Mts. and the Canadian Shield are the Interior Lowlands, a vast region filled with sediment from the flanking higher lands. The Lowlands are divided into the prairies, the plains, and the Mackenzie Lowlands. The prairies are Canada's granary, while grazing is important on the plains.
The smallest and southernmost region is the Great Lakes–St. Lawrence Lowlands, Canada's heartland. Dominated by the St. Lawrence River and the Great Lakes, the region provides a natural corridor into central Canada, and the St. Lawrence Seaway gives the interior cities access to the Atlantic. This section, which is composed of gently rolling surface on sedimentary rocks, is the location of extensive farmlands, large industrial centers, and most of Canada's population. In SE Canada and on Newfoundland is the northern end of the Appalachian Mt. system, an old and geologically complex region with a generally low and rounded relief.
The Arctic Lowlands and the Innuitians are the most isolated areas of Canada and are barren and snow-covered for most of the year. The Arctic Lowlands comprise much of the Arctic Archipelago and contain sedimentary rocks that may have oil-bearing strata. In the extreme north, mainly on Ellesmere Island, is the Innuitian Mt. system, which rises to c.10,000 ft (3,050 m).
Canada's climate is influenced by latitude and topography. The Interior Lowlands make it possible for polar air masses to move south and for subtropical air masses to move north into Canada. Hudson Bay and the Great Lakes act to modify the climate locally. The Western Cordillera serves as a climatic barrier that prevents polar air masses from reaching the Pacific coast and blocks the moist Pacific winds from reaching into the interior. The Cordillera has a typical highland climate that varies with altitude; the western slopes receive abundant rainfall, and the whole region is forested. The Interior Lowlands are in the rain shadow of the Cordillera; the southern portion has a steppe climate in which grasses predominate. S Canada has a temperate climate, with snow in the winter (especially in the east) and cool summers. Farther to the north, extending to the timberline, is the humid subarctic climate characterized by short summers and a snow cover for about half the year. The huge boreal forest, the largest surviving remnant of the extensive forests that once covered much of North America, predominates in this region. On the Arctic Archipelago and the northern mainland is the tundra, with its mosses and lichen, permafrost, near-year-round snow cover, and ice fields. A noted phenomenon off the coast of E Canada is the persistence of dense fog, which is formed when the warm air over the Gulf Stream passes over the cold Labrador Current as the two currents meet off Newfoundland.