How Long is the Saint Lawrence River?

Written by Colby Maxwell
Published: October 5, 2023
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The Saint Lawrence River (usually abbreviated St. Lawrence River) is an essential waterway for the Northeastern United States and Eastern Canada. The river has become one of the busiest shipping corridors in the world and an essential waterway for Canada. Let’s take a look at some of the features of the St. Lawrence River, specifically how long it is. Plus, we’ll look at its history and how it compares to other rivers in the U.S. and Canada.

How Long is the Saint Lawrence River?

Saint Lawrence river. Thousand Lakes National Park USA.

One of the most important rivers in North America, the St. Lawrence River connects to the Great Lakes to the Atlantic Ocean.

©Sergey Kotsegubov/

The St. Lawrence River is approximately 3,058 kilometers or 1,900 miles long.

The total length of the St. Lawrence River uses measurements from its start (known as headwaters) to its ending (known as the mouth). The river begins at the outflow of Lake Ontario, heading northeast until it empties into the Gulf of St. Lawrence. The St. Lawrence River is extremely pivotal as it connects each of the Great Lakes to the North Atlantic Ocean, forming the drainage system for the Great Lakes Basin. Essentially, this river helps drain the world’s largest region of freshwater.

In addition to connecting and draining the Great Lakes, it also forms part of the international boundary between Canada and the United States. Along the way, the St. Lawrence passes through the Thousand Islands archipelago, the St. Lawrence Seaway, and multiple dams and rapids. Its importance as a boundary and as a shipping lane for both the United States and Canada is hard to understate!

Comparing the St. Lawrence River’s Length

RiverLength (km)Length (mi)Mouth
Missouri3,7672,341Mississippi River
Mississippi3,7342,320Gulf of Mexico
Yukon3,1851,980Bering Sea
St. Lawrence3,0581,900Gulf of St. Lawrence
Rio Grande3,0571,900Gulf of Mexico
Mackenzie2,6351,638Beaufort Sea
Colorado2,3331,450Gulf of California
Columbia2,0001,243Pacific Ocean
Saskatchewan1,9391,205Lake Winnipeg

Let’s compare some rivers! The St. Lawrence River is the fourth longest river in North America, but it does have first place as the longest river that drains into the Atlantic Ocean. Still, it’s slightly shorter than the Yukon River, which drains into the Bering Sea, and slightly longer than the Rio Grande, which drains into the Gulf of Mexico.

Interestingly, the St. Lawrence River is also longer than any other river that originates or flows entirely within Canada or the United States, such as the Mackenzie, Colorado, Columbia, Saskatchewan, and Nelson Rivers. Essentially, it’s the largest river that also crosses the US-Canada border.

Looking at cargo, it carried roughly 149 million metric tons of cargo to domestic ports during 2021. It is similar to the Mississippi River System, which carried 500 million tonnes of cargo (2019), although there aren’t any cruise lines of the same scale that go up the river. Without the St. Lawrence, many of the goods produced near the Great Lakes region suffer from a lack of cheap transport to international markets via the ocean.

The History of the St. Lawrence River

Aerial view of Montreal showing the Biosphere Environment Museum and Saint Lawrence River during Fall season in Quebec, Canada.

From ancient times to the first human habitation, and even to the present, the St. Lawrence River has been a dominating natural feature of the northeast.

©R.M. Nunes/

The St. Lawrence River formed a very long time ago, during the end of the last Ice Age. The process, known as “glacial retreat,” happened over 10,000 years ago. In fact, the same process that helped to create the river is also what carved the Great Lakes into their shapes today. Since the glacial retreat, various Indigenous peoples for 8,000 years or so have lived around the St. Lawrence River.

Many thousands of years later, the river was explored by Jacques Cartier in 1534 and again traversed in 1535. Cartier named the river and claimed it for France, taking inspiration from a Christian martyr in his naming process. The river became the main route for the French colony in modern-day Canada until it was eventually given (ceded) to Britain in 1763 after the Seven Years’ War. Throughout most of the colonial period, the river was extremely important for things like trade and transportation.

Even today, the St. Lawrence River is essential for the region. In 1959, Canada and the United States completed the St. Lawrence Seaway, officially allowing large ocean-based ships to reach the Great Lakes. Transportation via water from the ocean to the interior of the United States and Canada provides huge economic opportunities to both countries, and many businesses and cities have rapidly grown as a result. Currently, the river supports around 45 million people.

The photo featured at the top of this post is © Kristi Blokhin/

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About the Author

Colby is a writer at A-Z Animals primarily covering outdoors, unique animal stories, and science news. Colby has been writing about science news and animals for five years and holds a bachelor's degree from SEU. A resident of NYC, you can find him camping, exploring, and telling everyone about what birds he saw at his local birdfeeder.

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