Where does the water in the Great Lakes come from?

We tend to think of the Great Lakes as five separate bodies of freshwater. But all 5 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.

Table of Contents Show

The Great Lakes Watershed
Journey of a water droplet in the Great Lakes system
The Great Lakes’ connection to the ocean
A closed system
How did the Great Lakes become fresh water?
How were the Great Lakes filled with water?
What rivers feed the Great Lakes?
Are Great Lakes drying up?

Graphic by Flavia Lopez

The Great Lakes Watershed

Have you ever wondered how big ships travel through the Great Lakes? Although the Great Lakes don’t physically touch one another, 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.

Chase (Unsplash: Link)

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.

Journey of a water droplet in the Great Lakes system

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.

diego_torres (Pixabay: Link)

Here’s a 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 meters (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

Graphic by Flavia Lopez

Did You Know?

Lakes Huron and Michigan can be considered one big lake! Lake Huron and Lake Michigan are connected by the Straits of Mackinac, which is at the same elevation as the two lakes.

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).

Image Ontario

A closed system

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.

Outflows from the Great Lakes are very small in comparison to their total volume: each year, less than 1% of the volume of the water in the Great Lakes flows out the St. Lawrence River.

Jp Valery (Unsplash: Link)

Compare this stat 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.

Flockine (Pixabay: Link)

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.

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.

How did the Great Lakes become fresh water?

Thousands of years ago, the melting mile-thick glaciers of the Wisconsin Ice Age left the North American continent a magnificent gift: five fantastic freshwater seas collectively known today as the Great Lakes — Lake Superior, Lake Huron, Lake Michigan, Lake Erie and Lake Ontario.

How were the Great Lakes filled with water?

The massive weight and movement of this glacier gouged out the earth to form the lake basins. About 20,000 years ago, the climate warmed and the ice sheet retreated. Water from the melting glacier filled the basins , forming the Great Lakes.

What rivers feed the Great Lakes?

Among the waterways linking the lakes are the St. Marys River, which flows from Lake Superior to Lake Huron; the Niagara River, connecting Lake Erie to Lake Ontario; and the narrow Straits of Mackinac, joining Lake Michigan and Lake Huron (hydrologically, the two lakes are considered a single body of water).

Are Great Lakes drying up?

Water levels in the Great Lakes have fluctuated since 1860. Over the last few decades, water levels have declined slightly for most of the Great Lakes (see Figure 1). The past few years, however, have shown notable increases toward the top of the historical range.