Great Lakes-St. Lawrence River system
Learn about the lakes and outlet channels that make up the Great Lakes-St. Lawrence River system.
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About the system
The Great Lakes-St. Lawrence River system consists of a chain of lakes and outlet channels. The excess water from most lakes drain through their outlet channel into the next lower lake downstream in the system. However, Lake Ontario drains through the St. Lawrence River to the Atlantic Ocean.
Water level elevation measurements are based on International Great Lakes Datum 1985 (IGLD 1985). This is a vertical reference system adopted in January 1992 to replace the previous reference system (IGLD 1955). The 0 for IGLD 1985 is located at Rimouski, Quebec, at the mouth of the St. Lawrence River. Water level elevations in the Great Lakes-St. Lawrence River system are measured above mean water level at this site.
Lake Superior is the uppermost lake in the system. It has a chart datum elevation of 183.2 m, and discharges water through the St. Marys River into Lake Huron.
In the upper portion of the river, for the first 22 km, the level of the river falls about 0.1 m. Through the St. Marys Rapids, a distance of about 1 km, the river falls about 6.5 m. The remaining fall of about 0.6 m is on the lower river between the rapids and Lake Huron.
Since 1921, the discharge from Lake Superior has been regulated with:
- a control dam
- hydro diversions
Lakes Michigan and Huron
Lakes Michigan and Huron are connected by the broad and deep Straits of Mackinac. They’re treated as a single lake for hydrologic and hydraulic considerations.
Chart datum on both lakes is 176.0 m. These lakes discharge through the:
- Detroit River, which falls 0.9 m to Lake Erie (chart datum of 173.5 m)
- St. Clair River, which falls 1.6 m to Lake St. Clair (chart datum of 174.4 m)
The flows on the St. Clair-Detroit River system are dependent on the levels of both the upstream and downstream lakes.
The natural outlet from Lake Erie is through the Niagara River to Lake Ontario, which is about 99 m lower than Lake Erie.
About 95 m of the elevation drop occurs at Niagara Falls between the:
- head of the Cascades (upstream)
- Lower Rapids (around 10 km downstream)
A control structure between the Canadian shore and Goat Island is used to maintain the level in the Chippawa-Grass Island pool for:
- power generation
- providing the required minimum flow over the falls
It isn’t used to regulate the level of Lake Erie.
Lake Ontario is the lowest of the Great Lakes. It has a chart datum of 74.2 m.
The outflow from Lake Ontario has been regulated since 1960. This is done with the control works on the St. Lawrence River for the St. Lawrence Seaway and Power Project.
From Lake Ontario, the river drops about 1.7 m to Lake St. Lawrence. This man-made lake was formed behind hydro-electric and control dams upstream of Cornwall. The flows out of Lake Ontario and into the St. Lawrence are moderated by the control structure and lock at Iroquois.
The river drops about 26 m at these dams and the Eisenhower and Snell Locks. Then it flows into Lake St. Francis, which has a chart datum of about 46 m.
It drops to Montréal with a chart datum of 5.6 m at jetty number 1 through a series of:
- navigation channels
In the 272 km between Montréal and Québec City, the river falls about 7.5 m. The chart datum in Québec City is -2 m IGLD 1985.