Two raindrops land a few kilometres apart in the hills north of Thunder Bay. One trickles south into Lake Superior and, months or years later, flows past Montréal into the Atlantic Ocean. The other trickles north and ends up in Hudson Bay. The line between them is invisible on the ground, but it decides where the water, and anything dissolved in it, ends up. Canada's water is organized by these invisible lines into drainage basins, and the oceans around Canada move heat in great currents that shape our climate.
Drainage basins and divides
A drainage basin (or watershed) is all the land drained by one river and its tributaries (smaller rivers that flow into it). A river begins at its source, often a lake, spring or wetland, and ends at its mouth, where it empties into a lake or ocean. The high ground that separates one drainage basin from the next is a divide. A continental divide separates basins that drain to different oceans, as near Thunder Bay, where the divide separates water bound for the Atlantic from water bound for Hudson Bay.
| Ocean drainage area | Approximate area (km²) | Major rivers | Notes |
|---|---|---|---|
| Hudson Bay (with James and Ungava bays) | 3 860 000 | Nelson, Saskatchewan, Churchill, Albany, Moose, La Grande | Largest drainage area; much of it in Ontario, Quebec and the Prairies |
| Arctic Ocean | 3 580 000 | Mackenzie, Back, Coppermine | The Mackenzie system (about 4240 km) is Canada's longest river |
| Atlantic Ocean | 1 520 000 | St. Lawrence, Saint John, Ottawa | Includes the Great Lakes; the St. Lawrence is Canada's main shipping route |
| Pacific Ocean | 1 010 000 | Fraser, Columbia, Yukon, Skeena | Steep mountain rivers; the Fraser has major salmon runs |
| Gulf of Mexico | 21 600 | Milk River | A small area in southern Alberta and Saskatchewan that drains to the Mississippi |
Why watersheds matter
Everything that happens upstream reaches people downstream: fertilizer from farm fields, salt from winter roads, sewage overflows, eroded soil and floodwater. That is why Ontario has managed land and water by watershed since its Conservation Authorities Act of 1946. Conservation authorities map floodplains, run flood warnings, regulate building near rivers and protect wetlands and forests within each watershed. In 2025 the province passed a law creating a new Ontario Provincial Conservation Agency to oversee them, and it plans to merge its 36 conservation authorities into nine larger regional ones in February 2027. Supporters say this will make permits and rules more consistent; critics worry that local knowledge and local control will be lost. Many First Nations also describe their responsibility for water in watershed terms; Anishinaabe Water Walkers, for example, have walked the shorelines of the Great Lakes to honour and protect the water.
Ocean currents
Ocean currents are huge, steady flows of seawater driven mainly by the winds and by differences in water temperature and saltiness. Warm currents carry tropical heat toward the poles and warm the air over nearby coasts; cold currents carry polar water toward the equator and cool the coasts they pass. Canada, with the longest coastline in the world, feels both.
- North Pacific Current: flows east across the Pacific and splits near the BC coast, with the Alaska Current turning north. It keeps coastal BC mild in winter; Prince Rupert and Victoria rarely stay below freezing for long.
- Labrador Current: flows south from the Arctic along Labrador and Newfoundland, chilling the coast and carrying icebergs from Greenland down "Iceberg Alley."
- Gulf Stream: a warm current from the Gulf of Mexico that flows up the east coast of North America and then across the Atlantic toward Europe.
- Grand Banks: where the cold Labrador Current meets the warm Gulf Stream off Newfoundland, warm moist air passes over cold water and condenses into thick fog. The mixing also brings up nutrients, which once made the Grand Banks one of the richest cod fisheries on Earth, until overfishing led to the 1992 cod moratorium.
El Niño and La Niña
Every few years (roughly every 2 to 7), the surface of the tropical Pacific Ocean swings between warmer and cooler than normal. During El Niño, the central and eastern tropical Pacific is unusually warm; during La Niña, it is unusually cool. Because the ocean and atmosphere are linked, these swings shift the jet stream and change weather thousands of kilometres away, including in Canada. They tilt the odds rather than decide every season: other patterns and long-term warming also play a part, so say that El Niño made a mild winter more likely, not that it caused every warm day.
| El Niño | La Niña | |
|---|---|---|
| Tropical Pacific | Warmer than normal in the central and east | Cooler than normal in the central and east |
| Typical Canadian winter | Often milder than normal, especially in the West and on the Prairies, with less snow | Often colder than normal in western Canada and the Prairies, with more snow in the mountains |
| Atlantic hurricanes | Usually fewer | Usually more |