Canada’s water drains through five main watersheds, each of which is supported by several sub-basins. Running in a north-south orientation, the Rocky Mountains create the Continental Divide. All drainage areas west of that divide flow to the Pacific Ocean watershed. The largest rivers in this region are the Fraser, the Yukon and the Columbia rivers.
Water that flows into the Arctic Ocean or into the channels of the Arctic Islands is considered to be a part of the Arctic Ocean watershed. The largest river flowing into the Arctic watershed is the Mackenzie. Flowing 4,241 kilometres from its origins on the Columbia Icefield in the Canadian Rockies, the Mackenzie overlies portions of five provinces and territories – British Columbia, Alberta, Saskatchewan, Yukon and Northwest Territories. The Mackenzie is Canada’s largest river basin, and the tenth largest river basin in the world.
Approximately 30 per cent of Canada’s water drains to the Hudson Bay watershed. While they are ultimately considered to be part of the Arctic Ocean, Hudson Bay, James Bay, Ungava Bay and Foxe Basin are referred to as part of the Hudson Bay drainage area. The basin is fed by the Nelson and Manitoba’s Churchill rivers, which flow eastward from the Continental Divide, as well as Quebec’s La Grand Rivière. The Hudson Bay drainage basin includes portions of Alberta, Saskatchewan, Manitoba, Ontario, Quebec, Northwest Territories and Nunavut, as well as four U.S. states – Montana, South Dakota, North Dakota and Minnesota.
Most drainage basins in eastern Canada and the Maritimes flow to the Atlantic Ocean watershed, particularly the Great Lakes-St. Lawrence system. Other major Canadian rivers contributing to the Atlantic Ocean watershed include Labrador’s Churchill River and New Brunswick’s Saint John River.
A small portion of Canada’s freshwater also drains to the Gulf of Mexico watershed. The Milk River, which is the northernmost tributary of the Missouri-Mississippi system begins in Montana then flows north into Alberta before looping back into Montana. In Saskatchewan, Big Muddy Creek and the Poplar River are also part of the Missouri-Mississippi drainage.
In some parts of Alberta and Saskatchewan there are closed watersheds or areas known as internal drainages, which do not drain into any ocean.
- Watersheds that are upper elevation, such as those in the mountains of B.C and Alberta, and the glaciers of Canada’s eastern Arctic region, produce stream flow from snow melt, rainfall and icemelt. Located at higher elevations where temperatures are consistently lower, glaciers act as giant water stores, which release water late in the summer when temperatures are high, and the previous winter’s snowmelt has been depleted.
- Canada’s eastern watersheds are supplied by rainfall and snowmelt, which also serves as a store of freshwater, filling rivers as it melts in the spring and early summer months.
- Located on the Columbia Icefield of Jasper National Park in the Canadian Rockies, 3456-metre Mount Snow Dome is considered a likely hydrological apex, from where meltwater ultimately flows to three oceans – the Pacific, via the Columbia River; the Arctic via the Athabasca; and the Atlantic via the North Saskatchewan River, which drains to Hudson Bay. According to some definitions, water flowing to Hudson Bay is considered to be part of the Atlantic Ocean. Another possible hydrological apex is Triple Peak in Montana’s Glacier National Park, while the third is in Siberia.
This interactive piece features a map and explanation of Canada’s Snow Dome. Users can also select to learn about its drainage into the Pacific and Arctic Oceans, as well as Hudson’s Bay.