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If the St. Lawrence River connects the Great Lakes to the Atlantic Ocean, why aren't the Great Lakes saltwater instead of freshwater?
from Billie Rae in Summerville, South Carolina, Age 12

Great question!

The St. Lawrence River can be divided into three broad sections: the freshwater river, which extends from Lake Ontario to near the city of Quebec; the St. Lawrence estuary, which extends from Quebec to Anticosti Island; and, the Gulf of St. Lawrence, which leads into the Atlantic Ocean.

View a map of the St. Lawrence river region
View a map of the entire Great Lakes-St. Lawrence River system

Originating from Lake Ontario near the Canadian town of Kingston, the St. Lawrence River is freshwater until near the Canadian town of Donnacona in Quebec. In this section, the river is always freshwater and flowing in the direction of the Gulf, or downstream. There are many tributaries that flow into the river, such as the Ottawa and Chateauguay Rivers. Because the water flow is quite fast in this part of the river, the water from the tributaries is kept from entering the center of the St. Lawrence River; this phenomenon creates two separate water masses that flow beside one another for a long distance before mixing completely.

After Donnacona, the river widens considerably and enters the brackish water zone, the area where freshwater and saltwater meet. In this section, the salinity of the water rises from zero to twenty percent! Tidal influences from the Atlantic start to affect the river, and the river becomes an estuary, with one of its main tributaries being Saguenay River. An estuary is where a freshwater river current meets an ocean tide, and is often abundant in wildlife. The St. Lawrence estuary is over 300 miles long, and is one of the most productive marine ecosystems along the Canadian coast. Over 718,000 seabirds of 19 species, such as the Atlantic puffin, the red-throated loon, and the Artic tern, and several different species of whales, such as finback, minke, Beluga, sperm and blue whales, are found in the St. Lawrence estuary.

The estuary deepens considerably as it heads toward the ocean, with the depth increasing from around 80 feet (25 meters) to 1,145 feet (358 meters). Once the waterway passes by Anticosti Island, the estuary becomes known as the Gulf of St. Lawrence. The Gulf extends 250 miles from the St. Lawrence River estuary to Newfoundland, where it becomes the Atlantic Ocean, and at its greatest width the Gulf is 500 miles (800 km) wide.

St. Lawrence River and Seaway, Encyclopedia Britannica
The Hydrology of the St. Lawrence Basin, Canada's Digital Collections
The Seabirds of the St. Lawrence, Environment Canada

St. Lawrence Region, College of Communication, University of Illinois
St. Lawrence Hydrographic System, Encyclopedia Britannica

Thank you for your question!

Answered on September 1, 2000

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