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.
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 .
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.
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.