We tend to think of the Great Lakes as five separate bodies of freshwater. But all five Great Lakes are very intimately connected to each other. The lakes are all part of one big watershed. The watershed is an area of land that drains surface water down to a single point. In this case, that point is — you guessed it — The Great Lakes.

A map of the great lakes watershed showing flowlines toward the Atlantic Ocean

A still of the Great Lakes- St. Lawrence watershed and subwatersheds from the interactive Biinaagami map. Click here to explore for yourself.

Have you ever wondered how big ships travel through the Great Lakes?

Their waters all flow together in one big system. The Great Lakes are connected by close to 5,000 tributaries: a series of smaller lakes, rivers, streams, and straits flowing into larger bodies of water.

Water in the Great Lakes comes from thousands of streams and rivers covering a watershed area of approximately 520,587 square kilometres (or 201,000 square miles). The flow of water in the Great Lakes system move from one lake to another eastward, ultimately flowing into the Atlantic Ocean.

Let’s track the journey of a water droplet from the furthest western point on Lake Superior

A single drop of water finds its way into Lake Superior either by rainfall or runoff. It takes more than two hundred years to make its way through the Great Lakes system and out to the Atlantic Ocean.

Here’s the flow-by-flow:

  • From Lake Superior, water drains into the St. Marys River and flows into Lake Huron

  • Lake Huron and Lake Michigan are connected directly by the Straits of Mackinac

  • Lake Huron waters flow into the St. Clair River, which drains into Lake St. Clair

  • Lake St. Clair, in turn, drains into the Detroit River, and empties into Lake Erie

  • At the end of Lake Erie, water flows into the Niagara River, dropping 52 metres (170 ft) as it flows over Niagara Falls and into Lake Ontario

  • From Lake Ontario, water flows into the St. Lawrence River and ultimately runs out the Atlantic Ocean

A droplet of water, at the end of its two century long journey, experiences a total elevation drop of approximately 182 meters (or 600 feet).

The Great Lakes’ connection to the ocean

We’ve now traced the path of a single drop of freshwater from Lake Superior all the way through the Great Lakes system and into the oceanic salt water of the Atlantic. The distance from the furthest port in Duluth, Minnesota to the Atlantic is 3,769 kilometres (2,342 miles).

The Petawawa River, a river in the St. Lawrence drainage basin. Photo by Scott Parent.

We’ve seen how water travels across the Great Lakes through an intricate, interconnecting series of tributaries small and large, modest and majestic.

But here’s a fact that may be surprising: the Great Lakes are an essentially closed system.

Perhaps what’s even more impressive than the flow of water through this complex interconnected system is how relatively little water actually leaves the Great Lakes Watershed each year. Great Lakes water is only replenished by 1% annually; the remaining 99% is a one-time gift from melting glaciers.

Compare this percentage to that of an average lake, such as Lake Simcoe — where the total annual outflow is 900% the volume of water in the lake!

This means the Great Lakes are especially sensitive and vulnerable to certain risk factors, like pollution. Pollutants can (and do) travel from one lake to the next. They also tend to persist for long periods of time because water exits the Great Lakes at such a slow pace.


So when you’re out there on the water, take a moment to drink in the sheer size, depth, and power of the Great Lakes. Soak it all in and, as they say, go with the flow.