Rivers, watersheds and the water that flows north

Fort Providence, Northwest Territories, late April. The Deh Cho Bridge crosses the Mackenzie River here, and under it the ice is breaking up. Robert Sabourin is 63, Métis, a retired highways foreman. For thirty winters, before the bridge, his crew kept the ice road open across this river. He still comes down to watch the break-up, and he knows which way every drop is going. North.

So here's the question. Where does Canada's water go? Which ocean gets the rain that falls on your roof, and why does most of it run away from most of us?

Environment and Climate Change Canada says our rivers flow into five drainage basins: the Pacific, the Arctic, Hudson Bay, the Atlantic, and, for a small corner of Alberta and Saskatchewan, the Gulf of Mexico. Here is the odd part. About 60 per cent of Canada's fresh water drains north, while about 85 per cent of the population is found along the southern border. Water and people go opposite ways.

The Mackenzie is our longest river: 4,241 kilometres from the headwaters of the Finlay in the Rockies to the Beaufort Sea, tenth-longest in the world, by the Atlas of Canada's measure. Then the St. Lawrence at about 3,060 kilometres, and the Nelson, the Saskatchewan and the Fraser. And the Milk River, about 1,000 kilometres, leaves Alberta for the Missouri, the Mississippi and the Gulf of Mexico.

Lakes, too. Environment and Climate Change Canada counts 563 lakes bigger than 100 square kilometres, more lake area than any other country. Great Bear Lake, at about 31,000 square kilometres, is the biggest lake entirely inside Canada, and Great Slave Lake next door is the deepest in North America, at 614 metres. Robert's river drains both of them.

Find it on the map. This is the Atlas of Canada's map of drainage basins, from Natural Resources Canada, five basins in five colours. Put your finger on Calgary. The Bow River runs east into the Saskatchewan, the Nelson and Hudson Bay. Slide north past Edmonton to the Athabasca, and the colour changes: that water goes to the Arctic. The line between two colours is a divide, the high ground between two rivers. Find yours.

Let's change one fact. Tilt the Prairies so they drain south. The Saskatchewan rivers would join the Missouri and feed the Mississippi, the way the Milk River already does. Hudson Bay would lose the Nelson's fresh water, and Manitoba would lose the river that fills Lake Winnipeg. Same rain, same snow, a different ocean, and a very different map of who shares water with whom.

The mistake people make is calling the St. Lawrence our longest river, because it is the famous one. It is long, about 3,060 kilometres by Environment and Climate Change Canada's count, but the Mackenzie beats it by more than a thousand. The official study guide puts it another way: the Mackenzie system is the second-longest in North America, after the Mississippi.

Here's the rule, in one breath. Five basins, and the divides between them are high ground: water in Canada mostly goes north, and people mostly stay south.

This week, find your own watershed. Open Toporama, the Atlas of Canada's map viewer, find your street, and follow the nearest creek downstream with your finger until it reaches a big river. Then ask which of the five basins that river belongs to. Rain on your roof ends up in an ocean. Now you know which one.

Next time, three coasts, the islands of the Arctic, the highest tides in Canada, and the one sentence about our fresh water that is actually true.