Below the giant boats that ply the St. Lawrence River live rich and diverse ecosystems. When the river waters rise or fall, naturally or as a result of human activities, all these ecosystems are affected. (Nathalie Letendre)

The St. Lawrence River is the dominant feature of the Quebec landscape. It flows a total distance of 1500 km, and over 70% of Quebec's population lives along its 4200 km of shoreline. The St. Lawrence is not a homogeneous unit. It is divided into three sections, (1) the freshwater reach, (2) the estuary, and (3) the gulf, on the basis of its natural characteristics: tides, salinity, vegetation, and sources of water.

The freshwater reach is fed by the Great Lakes basin and two large tributaries - the Ottawa River and the Saint Maurice River, and extends from Cornwall to Lake Saint-Pierre. This freshwater reach has no tides. The impact of climatic changes on this ecosystem may be felt on several levels, including a drop in water flow, which would affect access to quality water, and modification of wetland areas.

The very low water levels observed in 1998-1999 in the freshwater reach caused, among other things, a change in available habitat area, leading to an invasion of opportunistic land plants on bare and dry ground. The ecosystem was impoverished, notably because these species are of little interest to the fauna of the St. Lawrence - fish, ducks, amphibians, and reptiles.

Invasion of opportunistic land plants

Alain Armellin, St. Lawrence Centre

Examples of potential impacts in the freshwater reach:

• deterioration in water quality
• modification of habitat distribution
• loss of wetland habitats because of drying out
• reduction in biodiversity
• increase in dredging to maintain navigability
• greater risk to navigation and increase in the number of assistance and rescue operations

Summer 1994 - long-term mean level (Christiane Hudon, St. Lawrence Centre)

Summer 1999 - 1 m below the long-term mean level (Christiane Hudon, St. Lawrence Centre)

Did you know?
The freshwater levels of the St. Lawrence River have been monitored and recorded since the mid-1800s. The river's lowest levels were recorded in 1930 and in 1960. Now the trend is once again at the bottom of the curve.

Rising sea level

Coastal erosion, Rivière à Claude, Gaspé Peninsula. (François Morneau, Transports Québec)

Effects of high sea level, Pointe-aux-Loups, Îles de la Madeleine. (François Morneau, Transports Québec)

Between Lake Saint-Pierre and the Atlantic Ocean, the estuary and the gulf are sectors with tides in which the water ranges from fresh (upstream of Île d'Orléans) to brackish and saline. The saltwater portion of these reaches is being affected by increased erosion that climate warming could worsen.

General circulation models (GCM) are very imprecise at regional scales, although they all predict to varying degrees atmospheric warming above the Gulf of St. Lawrence. The main impacts likely to be felt in the estuary and gulf are a reduced flow of fresh water from the freshwater reach, an inflow of colder fresh water from Labrador, and an increase in ocean levels.

Examples of potential impacts in the estuary and gulf:

Hooded seal (Véronique Lesage)

• increased erosion
• less riparian habitat
• more vulnerable waterfront development (roads, property, buildings)
• reduced ice cover owing to warming of the water
• modified distribution of marine species
• possibility of accommodating larger ships

The main impact of climate warming on the St. Lawrence River would be the fluctuation in water levels. Whether they are fresh, brackish, or saline, the waters of the St. Lawrence River will either rise or fall. These fluctuations will affect various riparian and aquatic habitats, as well as biophysical and socio-economic aspects.

Hudon, C., 1997: Impact of water level fluctuations on St. Lawrence River aquatic vegetation; Canadian Journal of Aquatic Science, v. 54, p. 2853-2865.