The Great Lakes-St. Lawrence River System

The Great Lakes-St. Lawrence River System consists of a chain of lakes and outlet channels. The excess waters from one lake drain through its outlet channel into the next lower lake downstream in the system, or, in the case of Lake Ontario, through the St. Lawrence River to the Atlantic Ocean.

Water level elevations are based on International Great Lakes Datum 1985 (IGLD 1985), which is a vertical reference system adopted in January 1992 to replace the previous reference system which was IGLD 1955. The zero for IGLD 1985 is located at Rimouski, Quebec, at the mouth of the St. Lawrence River; water level elevations in the Great LakesSt. Lawrence River System are measured above mean water level at this site.

Lake Superior is the uppermost lake, with a chart datum elevation of 183.2 metres, 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 metre. Through the St. Marys Rapids, a distance of about 1 km, the river falls about 6.5 metres. The remaining fall of about 0.6 metre is on the lower river between the rapids and Lake Huron. A control dam, locks and hydro diversions have regulated the discharge from Lake Superior since 1921.

Lakes Michigan and Huron are connected by the broad and deep Straits of Mackinac and are treated as one lake for hydrologic and hydraulic considerations. Chart datum on both lakes is 176.0 metres. These lakes discharge through the St. Clair River, which falls 1.6 metres to Lake St. Clair (chart datum of 174.4 metres), and the Detroit River, which falls 0.9 metre to Lake Erie (chart datum of 173.5 metres). 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 metres lower than Lake Erie. About 95 metres of the elevation drop occurs between the head of the Cascades upstream of Niagara Falls to the Lower Rapids about 10 km downstream of the Falls. A control structure between the Canadian shore and Goat Island is used to maintain the level in the ChippawaGrass Island pool for power generation and to provide the required minimum flow over the Falls. This structure is not used to regulate the level of Lake Erie.

Lake Ontario, with a chart datum of 74.2 metres, is the lowest of the Great Lakes. The outflow from Lake Ontario has been regulated since 1960 with the completion of 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 metres to Lake St. Lawrence, a man-made lake formed behind hydro-electric and control dams upstream of Cornwall. The flows out of Lake Ontario and into Lake St. Lawrence are moderated by the control structure and lock at Iroquois.

The river drops about 26 metres at these dams and the Eisenhower and Snell Locks, then flows into Lake St. Francis which has a chart datum of about 46 metres. Through a series of lakes, navigation channels and locks, the river drops to Montreal, with a chart datum of 5.6 metres at Jetty Number 1. In the 272 km between Montreal and Quebec City, the river fall about 7.5 metres, to a chart datum in Quebec City of -2.0 metres IGLD 1985.