Lake St. Lawrence


Lake St. Lawrence is the forebay of the Moses-Saunders Power Dam. It is located in the International Section of the St. Lawrence River shared by Canada and the United States. This river acts as the hydraulic outlet of Lake Ontario.

In conjunction with the construction of the Saint Lawrence Seaway Project that started in 1954, the Moses-Saunders Power Project was also built. A navigable channel through the Long Sault rapids needed to be created, along with dams, locks and other control structures. This included the Moses-Saunders Power Dam constructed near Cornwall, Ontario and Massena, New York. When construction of these structures was completed, inundation began on July 1, 1958 and resulted in the formation of an artificial widening and deepening in the river which was named Lake St. Lawrence. Inundation of the river caused a dozen villages in Ontario, now collectively known as "The Lost Villages", to be flooded. There was also inundation on the New York side, but no communities were as widely impacted[7].  

Lake St. Lawrence extends upstream from Moses-Saunders Power Dam southwest approximately 46 km to the village of Iroquois, Ontario. The lake’s width is up to 7 km with a surface area of 259 km2 [1].

map lake st lawrence
Figure 1: Map of Lake St. Lawrence. Source: Environment & Climate Change Canada

The most notable use of the lake is as the forebay to the Moses-Saunders Power Dam. Outflows through this power dam are regulated by the International Lake Ontario – St. Lawrence River Board  (ILO-SLRB or Board) and can vary widely depending on water supply conditions and the water levels of Lake Ontario and the St. Lawrence River. With the forebay located immediately upstream of the dam, Lake St. Lawrence can be subject to a wide range of water-level fluctuations.

moses saunders nypa
Figure 2: The Moses-Saunders hydropower dam. Source: NYPA

Recreational boating is popular on the lake among local residents and visitors to the area, and it is home to the Stormont Yacht Club and several marinas. There are also several notable campsites and beaches along its banks. Fishing is an especially popular activity. The lake is frequented by waterfowl and wading birds, especially during migration seasons.


Ice formation on Lake St. Lawrence and the potential for frazil ice generation can have a major effect on regulation of outflows in the St. Lawrence River. Flow reductions are often required to ensure the formation of a safe and stable ice cover, but modern ice management practices have significantly reduced the frequency and magnitude of ice jams and localized flooding that were a frequent occurrence along the river historically.

Water Levels

The International Lake Ontario-St. Lawrence River Board oversees and regulates the flow from the Moses-Saunders Dam, which affects Lake St. Lawrence water levels. The Board regulates outflows from Lake Ontario in accordance with Regulation Plan 2014, which was adopted in 2017 and developed in accordance with the International Joint Commission’s  (IJC’s) 2016 Order of Approval[2].

The Board also communicates with the public about water levels and flow regulation, and works with its Great Lakes-St. Lawrence River Adaptive Management (GLAM) Committee, which monitors the performance of the regulation plans used in the regulation of outflows in the Great Lakes-St. Lawrence River System[3].

Water levels in Lake St. Lawrence depend on several factors, such as the corresponding levels of Lake Ontario, wind, ice, and aquatic vegetation effects, hydropower “peaking” operations at Moses-Saunders Power Dam (i.e., changes in outflows within the day to best meet electricity generation demands), and the outflow of the river. Since outflows are regulated through the dams at the downstream end of the forebay, the last two factors above result in an interesting hydraulic effect. That is, as outflow is increased, Lake St. Lawrence levels fall, whereas as outflow is decreased, Lake St. Lawrence levels rise. This phenomenon and other processes (e.g., peaking, ice management, Iroquois Dam operations, wind effects, etc.) are featured in a series of online learning modules[4].

Hydraulic Features

communities control structures
Figure 3: Location of communities and control structures on and around Lake St. Lawrence. Source: IJC

The IJC granted its approval for a cross-border construction project in 1952. In 1953, the Federal Power Commission issued a license for the New York Power Authority (NYPA) to develop the U.S. portion of the Moses-Saunders Power Dam which crosses the Canada–US border. In 1954, President Dwight D. Eisenhower signed legislation clearing the way for construction of both the dam and the St. Lawrence Seaway. Power was first generated in July 1958, and on June 27, 1959, Queen Elizabeth II and Vice President Richard M. Nixon formally dedicated the St. Lawrence Seaway and Power Project as a symbol of international cooperation[5].

The Moses-Saunders Power Dam provides power to both the Province of Ontario and New York State. The Board directs Ontario Power Generation (OPG) and the New York Power Authority (NYPA) on how much water to pass through this and other control structures located on the lake.

OPG owns and operates the R.H. Saunders hydroelectric generating station on the Canadian side of the Moses-Saunders Power Dam. The R.H. Saunders facility has a capacity of 1,045 megawatts[5]. OPG also manages the Iroquois Dam at the upstream end of Lake St. Lawrence. This gated structure is not used for hydroelectric generation and is primarily used to limit high water levels on Lake St. Lawrence as well as for ice management practices. Iroquois Dam protects Lake St. Lawrence from serious flooding.

NYPA owns and operates the Robert Moses hydroelectric generating station on the US side of the Moses-Saunders Power Dam. In 1981, NYPA’s half of the dam was renamed the St. Lawrence-Franklin D. Roosevelt Power Project in honor NYPA’s founder. This plant has a capacity of 912 megawatts[5].

Long Sault Dam was completed in the late 1950s. It is also managed by NYPA and is a second dam through which flow can exit Lake St. Lawrence. It is a massive arch spillway and is not used for hydroelectric generation.

long sault dam nypa
Figure 4: Long Sault Dam. Source: NYPA

With the dams impounding the river, there are locks at either end of the lake to allow vessels to bypass the dams. The Iroquois Lock is adjacent to Iroquois Dam at the upstream end. The Eisenhower Lock and Snell Lock can be found on the downstream end.

Eisenhower Lock
Figure 5: Eisenhower Lock. Source: SLSDC

In 1968, the Raisin Region Conservation Authority received IJC approval to divert up to 0.7 m3/s from Lake St. Lawrence at Long Sault, Ontario into the 26-km long Raisin River Diversion to supplement low summer flows in this Raisin River tributary. The Raisin River subsequently discharges back into the St. Lawrence River downstream of Cornwall. The Conservation Authority compensates OPG for any water use bypassing the power dam beyond a set base amount that is rarely exceeded.

The village of Massena, New York’s water treatment plant draws less than 0.1 m3/s from Lake St. Lawrence through a structure named the Massena Intake via a 600-mm raw-water pipeline[5]. Just north of the power dam lies the Cornwall Canal inlet. This structure directs about 5 m3/s continually through a historic canal to prevent stagnation. The City of Cornwall, Ontario draws about 0.5 m3/s from Lake St. Lawrence through an intake located near the Cornwall Canal inlet.  

Local population

Several communities call the area surrounding Lake St. Lawrence home. The largest is Cornwall, Ontario’s easternmost city of about 46,900 people[6]. Cornwall's industrial base has shifted to a diverse mix of manufacturing, automotive, high tech, education, food processing, distribution centers and call centers. Other population hubs include South Stormont, a township in eastern Ontario, Canada, in the United Counties of Stormont, Dundas and Glengarry. Communities such as Long Sault, Ingleside, Morrisburg and Iroquois, Ontario, as well as Massena, Wilson Hill, Louisville, and Waddington, New York all border Lake St. Lawrence. The townships of South Dundas and South Stormont stretch along the Canadian shore and St. Lawrence County lies along the US shore. Several islands dotting the lake have significant populations, with many featuring active community associations.

Inundation of the river caused a dozen Ontario communities, now collectively known as the Lost Villages, to be flooded. There was also inundation on the New York side, but no communities were as widely impacted. The Lost Villages are a prominent attraction for visitors and locals. These communities were inundated following unprecedented land expropriation for construction of the St. Lawrence Seaway and Power Project in the 1950s. Their inundation is a point of pride and historical culture for many local residents – especially those affected personally. Many communities were completely rebuilt on higher ground, with several present-day communities such as Long Sault, Morrisburg and Iroquois, Ontario containing houses and other structures moved from areas that were flooded[7]. Upper Canada Village, a park that opened in 1961 depicting life in a rural English-Canadian setting during the year 1866, is a part of the Project’s heritage preservation plan and contains many such structures[8]

The Mohawk Nation of Akwesasne straddles the intersection of an international border (United States/Canada) and provincial borders (Ontario and Quebec) on both sides of the river, primarily downstream of Lake St. Lawrence. Residents remain closely tied to Lake St. Lawrence and other reaches of the St. Lawrence River nearby.

Local Ecology

There are several native – some endangered – as well as invasive species present in Lake St Lawrence. Organizations such as the St. Lawrence River Institute of Environmental Sciences (SLRIES) and the Canadian Wildlife Federation help conserve the local ecology of the area. The lake is home to several species of concern, including threatened and endangered species such as Blanding's turtle, bald eagles, osprey, black tern, and the Indiana bat.

Numerous north-woods mammal species find their home on the shores and islands of the lake, including muskrat, beaver, flying squirrels, mink, deer, porcupine, etc. Winter ice cover on the lake provides important passage for animals between the mainland and islands.

Known as one of the great freshwater sport-fishing grounds in northeastern North America, anglers travel from around the continent to fish for species such as pike, bass (particularly smallmouth bass), and muskellunge.

There are several invasive species in the lake too. Zebra and quagga mussels, round gobies and sea lamprey are the most prominent.

The St. Lawrence River Valley is a key part of the Atlantic Migratory Fly-Way - a main pathway for seasonal migration of many bird species. The region has also been listed as an Important Bird Area by Audubon, New York. Bald eagles, which had not been seen on the lake for many years, are making a comeback and can be seen frequently – especially in the winter.

One principal endangered species is the American Eel, which has experienced a dramatic decline. Lake St. Lawrence provides an important link in their migratory path. SLRIES community outreach helps raise awareness of their plight, and allows citizen scientists to help document sightings and facilitate habitat association[9].

eel dfo
Figure 6: American Eel. Source: DFO

See Also

St. Lawrence River

St. Lawrence Seaway

Moses-Saunders Power Dam

Iroquois Dam

Eisenhower Lock

Ontario Power Generation

New York Power Authority

The Lost Villages

Upper Canada Village

Cornwall, Ontario

Massena, New York

External Links

International Lake Ontario-St. Lawrence River Board

Great Lakes-St. Lawrence River Adaptive Management Committee

International Joint Commission

St. Lawrence River Institute of Environmental Sciences

Raisin Region Conservation Authority


1.      International Lake Ontario-St. Lawrence River Board. Observed Conditions and Regulated Outflows in 2017. Report to the International Joint Commission. 25 May 2018. 51 pages. Retrieved 4 April 2019.

2. Website of the International Lake Ontario-St. Lawrence River Board. Retrieved 4 April 2019.

3. Website of the Great Lakes-St. Lawrence River Adaptive Management Committee. Retrieved 4 April 2019.

4. Website of the International Lake Ontario-St. Lawrence River Board. Learning Modules. Retrieved 4 April 2019.

5. Wikipedia page for the Moses-Saunders Power Dam. Retrieved 4 April 2019.

6.,_Ontario. Wikipedia page for City of Cornwall, Ontario. Retrieved 4 April 2019.

7. Wikipedia page for the Lost Villages. Retrieved 4 April 2019.

8. Wikipedia page for Upper Canada Village. Retrieved 4 April 2019.

9. Website of the St. Lawrence River Institute of Environmental Sciences. Retrieved 4 April 2019.