The Great Lakes and the St. Lawrence

The Welland Canal, Lock Three, Thorold, Ontario, a viewing platform on a warm October afternoon. Marisol Reyes is twenty-nine, a marine engineer whose parents came from the Philippines. She's on the deck of a laker full of iron ore, and over the next few hours her ship will be lifted, lock by lock, 99 m. That's the height of Lake Erie above Lake Ontario.

So here's the question. Five lakes, one river, and a staircase of water from Duluth to the Atlantic. How big are the Great Lakes, why does Niagara fall, and what does a laker have to climb?

The United States Environmental Protection Agency keeps the table, because the lakes are shared. Five Great Lakes, and not one of them is entirely ours: Superior, Huron, Erie and Ontario have the border running through them, and Lake Michigan is entirely American. Together they hold roughly one-fifth of all the surface fresh water on the planet, and a third of all Canadians make their homes around them.

Superior is the giant: 82,100 km², 406 m deep, and a drop of water stays in it about 191 years. Erie is the shallow one, 64 m at most, and flushes through in under three years. Manitoulin Island, in Lake Huron, is the world's largest island in a freshwater lake, Statistics Canada notes.

Now the staircase. The Atlas of Canada gives the surface heights: Superior 184 m above the sea, Huron 177, Erie 174, and Ontario just 75. Between Erie and Ontario the water drops 99 m. Most of that is Niagara, where the Horseshoe Falls carries about nine-tenths of the flow, New York State Parks says. Then the St. Lawrence runs about 3,000 km to the sea.

Find it on the map. The map now shows the Great Lakes in profile, redrawn from a United States Army Corps of Engineers diagram with the Atlas of Canada heights. It's a cross-section: distance left to right, height up. Superior sits high on the left. Follow the steps down: a small one to Huron, a tiny one to Erie, then the big cliff to Ontario. That cliff is Niagara. Find sea level on the right.

The St. Lawrence Seaway opened in 1959, the Seaway corporation says, and runs 3,700 km from the Atlantic to Duluth. Between Montreal and Lake Erie there are fifteen locks: seven on the river and eight on the Welland Canal, which is Marisol's staircase. More than two and a half billion tonnes of cargo have moved since. Today about 37 million tonnes a year moves through, a third of it grain.

Let's change one fact. Lower Lake Erie to Lake Ontario's level. The ledge stays, but nothing pours over it: no Niagara Falls, no hydro stations on either bank, and no need for the Welland Canal, because ships would sail straight from one lake into the other. Marisol's staircase, and the towns built along it, exist because of 99 m.

The mistake people make is arguing about the biggest lake in Canada. Both answers are right with a qualifier. Lake Superior is the largest freshwater lake by area in the world, but we share it with the United States. Great Bear Lake, at about 31,000 km², is the biggest lake that is entirely inside Canada.

Here's the rule, in one breath. Five lakes, one wholly American, a ninety-nine metre step at Niagara, and a seaway of fifteen locks to the sea.

This week, find your nearest Great Lake or river on Toporama and follow it downstream with your finger until it reaches the sea. Notice where it drops.

Next time, rivers and watersheds: five ocean basins, the Mackenzie at more than four thousand kilometres, and the water that flows north.