The St. Lawrence River, the outlet from Lake Ontario, flows 530 miles in a northeasterly direction to the Gulf of St. Lawrence, with a fall of about 245 feet. The major portion of this fall, some 227 feet, occurs between Lake Ontario and Montreal Harbour, 183 miles from the lake.
The International Section of the river extends for a distance of 115 miles from Lake Ontario to St. Regis, New York, where it passes entirely into Canadian territory. After leaving Lake Ontario, for 63 miles the river is wide and deep, with little current. In the first 50 of these miles, the channel is filled with what is known as the "Thousand Islands." The fall from Tibbetts Point at Lake Ontario to Ogdensburg, New York, is only about 1 foot. Beginning about a mile upstream of Brockville, Ontario, and continuing to downstream of Ogdensburg, New York, a distance of about 13 miles, the channel is nearly straight, approximately one mile in width and some sixty feet in depth. About 6 miles below Ogdensburg is Galop Island. Most of the improvements and changes made in the river were made at and below this point.
The 55 mile stretch of river, from the head of Galop Island to the international boundary in Lake St. Francis, used to be a swift-flowing section with a drop of over 90 feet. The Galop Rapids, which flowed around Galop Island, were removed when the channels were widened and deepened, in the 1950s, for the St. Lawrence Seaway and Power Project. A dam below Galop Island, near Iroquois, Ontario, now controls the levels in this reach. The Iroquois Lock is used by ships to navigate past this dam.
The reach below Iroquois, which once consisted of many narrow channels filled with rapids, is now a reservoir impounded by the Long Sault Dam and the Moses-Saunders Powerhouses. This man-made lake, known as Lake St. Lawrence, covers some 100 square miles of area. The difference in elevation between this reach and the next is overcome by the U.S. Eisenhower and Snell Locks at Massena, New York.
Downstream of the Powerhouses, the river divides into two channels around Cornwall Island and then widens to form Lake St. Francis. With the exception of a small area at the upstream end of the lake, about 3 miles of United States shoreline, Lake St. Francis and the downstream St. Lawrence River lie entirely within Canada.
The natural regime of the outlet from Lake Ontario has undergone changes, at least since 1825. By 1850, work in the St. Lawrence River provided a minimum channel depth of 9 feet from the Atlantic Ocean to Lake Ontario. The natural control of Lake Ontario outflows was at the Galop Rapids, located on either side of Galop Island, approximately 70 miles downstream from Kingston, Ontario. Man-made changes to the natural control began in 1876, with dredging in the Canadian Galop Rapids channel (completed in 1888). Changes continued with the realignment of the Galop channel (from 1897 to 1901), improvements to the North Channel and construction of the Gut Dam (from 1903 to 1908). The Gut Dam was removed in January 1953. Dredging was begun in 1890 to remove a series of 12 shoals between Three Sisters Islands and Brockville, Ontario. This work was completed in 1901.
Between 1884 and 1905, a canal building program, undertaken by the Government of Canada, enabled ships with a 14-foot draft to navigate from the Atlantic to Lake Superior. In 1918, a submerged weir was built in the St. Lawrence River near Massena, New York, to facilitate the diversion of water for the generation of power. In 1934, the Cornwall Canal was started to allow navigation around the Long Sault Rapids.
Prior to construction for the St. Lawrence Seaway and Power Project, the character of the St. Lawrence River was significantly different than it is today. The Galop Rapids flowed on either side of Galop Island; the water broke over a rocky ledge and fell about 14 feet in 8 miles; the channel south of Galop Island, known as the American Galops, or Red Mills Rapids, entered a relatively quiet pool from which water rejoined the main river through several passages among a group of Islands.
Paralleling the Galop Rapids on the Canadian side was the Galop Canal, with two locks, Lock 27 at the upper end and Lock 25 at the lower end. The canal, at one time, followed the river closely and its lower portion formed the water front and harbor at Cardinal, Ontario. The canal was later moved to pass, in a deep cut, behind the village of Cardinal.
Downstream of Lock 25, for a distance of 4 miles, the river, although swift, was navigable. It then entered the Rapide Plat, with a fall of 12 feet in 4 miles. Originally, the river here flowed in two channels, one on either side of Ogden Island. At an early date a dam was constructed across the American channel at Waddington for power purposes. Above the dam, this channel was known as the Little River. There was no perceptible current, although there was a small leakage through the dam.
The Rapide Plat Canal carried navigation past these rapids. There were two locks in this canal, Guard Lock 24 at its head and Lock 23 at its foot at Morrisburg, Ontario. About ten miles below Lock 23, in the Canadian channel, at the head of Croil Island, were the Farran Point Rapids and the Farran Point Canal, with a single lock (No. 22) and a lift of about 4 feet. The canal was about 1-1/4 miles long. The American channel to the foot of Croil Island was wide and deep.
Between the foot of Croil Island and the head of Long Sault Island was a channel called the Sny. South of Long Sault Island were the South Sault Rapids, while north of the Island the channel was relatively deep and wide for nearly 2 miles to the head of the Long Sault Rapids. The St. Lawrence River Power Company (U.S.) contructed a dam across the South Sault and diverted water by a canal through its power plant to the Grass River.
The river, as it entered the Long Sault Rapids, was broken into several channels by islands; the hydraulic conditions were rather complex. Navigation past these rapids was by way of the Cornwall Canal, 11 miles in length and with a total lift of 48 feet by 6 locks. Lock 21 was at the head of the canal and Lock 15 at its foot at Cornwall, Ontario. Below Cornwall the river entered Lake St. Francis and no further critical sections occurred until after it ceased to be an international boundary water, at St. Regis, New York.
In 1952, the Governments of Canada and the United States applied to the International Joint Commission for approval to construct certain works for the development of power in the International Rapids Section of the St. Lawrence River. The construction, maintenance and operation of the proposed works were approved subject to a number of conditions established by the Commission in it's Orders of Approval of 1952, which was amended in 1956. The resulting project is known as the St. Lawrence Seaway and Power Project.
Construction of the Seaway began in 1954 and was completed: after five years and the dredging of over 360 million tons of rock; after the resettlement of thousands of people and entire towns; after changing river channels and the homes and habits of thousands of its inhabitants; after the construction of seven new locks (three in the international section) and after the construction of the world's largest joint power facility.
The Moses-Saunders Dam and Powerhouses, the Long Sault Dam, which is a spillway capable of passing the total flow of the St. Lawrence River, and the Iroquois Dam were completed in August 1958. The extensive channel enlargements, to widen and deepen the navigation channel to 25 feet for the entire length of the river, were completed and the Seaway was opened for navigation in 1959. The channel enlargements significantly increased the outflow capacity of Lake Ontario; control dams were designed to cope with the worst known (as of 1955) floods and droughts, as well as to compensate for the increased flow capacity.
Since 1960, the outflow from Lake Ontario has been completely controlled as directed in the International Joint Commission's Orders of Approval. The Commission established the International St. Lawrence River Board of Control to ensure compliance with the provisions of the Orders. Since October 1963, Plan 1958-D has been the operational regulation plan controlling Lake Ontario outflows.