The westerly limit of the river itself has been set at Everett Point, Lake Ontario. The section from KINGSTON to MONTRÉAL is called the International Rapids, as sudden drops in the riverbed create a series of rapids (this portion was flooded in the 1960s, forming Lake St Lawrence). The river begins as an extended arm of Lake Ontario, choked with numerous islands, beginning with Wolfe Island and including the THOUSAND ISLANDS near Gananoque, Ont. It trends NE past Brockville, Prescott and Morrisburg to Cornwall, where it broadens to form Lake St Francis.
The Beauharnois Canal now carries shipping safely past the former rapids of Lachine and Les Cèdres to another widening of the river, Lac St-Louis, SW of Montréal. The Ottawa joins the mainstream through channels to Lac St-Louis and over a NE route via Lac des Deux Montagnes, Rivière des Mille-Iles and Rivière des Prairies.
The land between the various channels at the confluence forms the archipelago on which the city of Montréal is built. The port of Montréal has been developed since the 19th century by dredging and canals.
From Montréal to TROIS-RIVIERES, the river is generally calm and unaffected by tides. A number of long, narrow islands continue to divide the mainstream and a large cluster, similar to the group at Lake Ontario, lies at the mouth of the Richelieu River at Sorel. The river broadens into Lac St-Pierre, some 15 km wide, and narrows again at Trois-Rivières, at the mouth of the St-Maurice River. From here to Québec, the freshwater flow becomes reversible with the tides.
The river constricts at Québec where a promontory commands the entire upper course. The military value of the site was appreciated long before European settlement began.
Past Québec the river divides to encircle Ile d' ORLÉANS and steadily widens to 15 km at Cap-Tourmente, almost 25 km at Ile-aux- COUDRES. The water becomes brackish and tides are high. Near the mouth of the Saguenay River, the riverbed drops dramatically from 25 m to 350 m, forming a drowned valley in the lower estuary. The freshwater flow mingles with cold arctic saltwater. The town of TADOUSSAC sits on a terrace of sand and clay at the confluence, but the rugged Precambrian N shore is sparsely settled.
The S shore of the estuary, which forms a great curve towards Gaspé, is more open towards its hinterland, and major roads, including the TRANS-CANADA HIGHWAY, head inland from Rivière-du-Loup, Trois-Pistoles, Rimouski and Matane.
At Pointe-des-Monts, about 70 km E of Baie-Comeau and the mouth of the Manicouagan River, the N shore turns dramatically NNE for about 100 km to SEPT-ILES, near the mouth of the MOISIE R. The river doubles in width to over 100 km, forming a deep, broad submarine valley, in which strong currents pour in from the gulf along the N shore and sweep counterclockwise back to the E. The saline water of the estuary discourages ice, and the port of Sept-Iles is open year-round, despite its northerly location.
According to the ROYAL PROCLAMATION OF 1763, a line from the mouth of Rivière St-Jean on the N shore past the W tip of Ile d' ANTICOSTI to Cap des Rosiers on Gaspé marks the end of the river and the beginning of the gulf.