The St. Lawrence

The hydrographic system of the St. Lawrence, which includes the Great Lakes, is one of the largest in the world. With a total area of 1.6 million square kilometres, it ranks third-largest in North America after those of the Mississippi and Mackenzie rivers. It drains more than 25% of the world's freshwater reserves and shapes the environmental processes of the North American continent. From the Great Lakes to the Atlantic Ocean, the St. Lawrence extends over some 1,600 kilometres, and approximately 60% of Quebec's population lives along its shores. Crossing Quebec from west to east, it collects water from 244 tributaries.

Quebec City overlooking the St. Lawrence

The St. Lawrence is also a unique ecosystem constituting a treasure of biological wealth, providing a home to numerous bird, fish and plant species. This ecosystem is recognized worldwide, as reflected in the designation of its four Wetlands of International Importance (RAMSAR), the Lac Saint-Pierre Biosphere Reserve (UNESCO) and its UNESCO World Heritage Site, Miguasha National Park. Along with its coastline and 600 islands, the St. Lawrence has more than 500 protected areas, accounting for 20% of all protected areas in Quebec.

Fluvial Section, Estuary and Gulf

The St. Lawrence is much more than just a river; in fact, it has three distinct sections:

  • A fluvial section, which flows from Kingston, Ontario, to Lac Saint-Pierre near Trois-Rivières, Quebec. This section is entirely freshwater and not subjected to tides.

    The fluvial section widens in three locations referred to as "fluvial lakes" due to their ecological processes and water flow. These lakes are Saint-François, Saint-Louis and Saint-Pierre.

  • An estuary, or point of contact between the river and gulf, which is further subdivided into three zones: 
    • a fluvial estuary or freshwater estuary flowing from Lac Saint-Pierre, near Trois-Rivières, as far as the eastern tip of Île d'Orléans in the Québec City area. This zone is subjected to tides. The freshwater flowing into it originates in the Great Lakes and the river's many tributaries.
    • an upper estuary, also referred to as a brackish estuary, which runs from the eastern tip of Île d'Orléans to the mouth of the Saguenay River on the North Shore and to the western tip of Île Verte to the south. This is where the freshwater from the river meets the salt water of the gulf.
    • a lower estuary running between Tadoussac and Pointe-des-monts, Quebec. This zone is also the starting point of the Laurentian Channel, a very deep natural channel through which dense, nutrient-laden salt water from the Atlantic Ocean flows.

  • A gulf, or veritable inland sea, connected to the Atlantic Ocean and bordered by the North Shore (Havre-Saint-Pierre, Sept-Îles, Baie Comeau) and, to the south, the Gaspé Peninsula and the Magdalen Islands with Anticosti Island roughly in the centre. Emptying into the Atlantic Ocean, the gulf consists of salt water.