Since the days of the first settlers, the St. Lawrence wetlands in the Montreal area have shrunk by 80%.
The St. Lawrence is made up of five main water masses and nine secondary water masses associated with the main tributaries. Each of these water masses has its own distinct natural physical and chemical characteristics.
The Quebec portion of the St. Lawrence River widens in three places, forming stretches of open water large enough to be considered lakes, but with a typical river flow. These three fluvial lakes are Lake Saint-François, Lake Saint-Louis and Lake Saint-Pierre.
The St. Lawrence–Great Lakes hydrographic system is one of the largest in the world. It drains more than 25% of the Earth’s freshwater reserves and influences environmental processes across much of North America.
An estimated two thirds of the riverbanks between Montreal and the Lake Saint-Pierre archipelago are under erosion.
Covering some 1,610,000 km2, the drainage basin of the St. Lawrence–Great Lakes hydrographic system is the 13th largest in the world and the 3rd largest in North America, after those of the Mississippi and Mackenzie rivers.
The St. Lawrence–Great Lakes hydrographic system ranks 16th in the world in mean annual flow, measured at 12,600 m3/s just off Quebec City.
The St. Lawrence–Great Lakes hydrographic system ranks 17th in the world in length, spanning 3,260 km from Lake Superior to the Cabot Strait.
The Quebec portion of the St. Lawrence River is fed by 244 tributaries. The tributaries with the greatest flow are the Ottawa, Saguenay, Manicouagan and Saint-Maurice rivers and the Rivière aux Outardes.
The low water levels observed in 1999 and 2001 dried out the marshes of the St. Lawrence, causing the vegetation in the marshes of the fluvial section to lose up to 75% of its biomass.
To date, it is estimated that at least 163 non-indigenous aquatic species have been introduced into the Great Lakes over the past 200 years. Of these species, 85 have been sighted in the St. Lawrence River.
Since the 1980s, PBDE levels in the belugas of the St. Lawrence Estuary have shown a thousandfold increase.
The first mention of the presence of zebra mussels in the waters of the Great Lakes dates back to 1985. They were first observed in Quebec in 1989, and their distribution is currently limited to the St. Lawrence River, as far as the Montmagny Islands and the Richelieu River.
Biological invasion by exotic species is the second largest cause of biodiversity loss, after the destruction of natural habitat. There are over 5,000 naturalized exotic plant species in North American ecosystems--80 in the St. Lawrence alone.
Mollusc populations exposed to municipal wastewater effluents have a higher proportion of females, increasing from the normal 41% to close to 70%.
A specimen of the Chinese Mitten Crab was discovered in September 2004 in Lévis, on the south shore of the St. Lawrence River, opposite Quebec City. The Chinese Mitten Crab is an exotic species whose presence in the waters of the St. Lawrence signals that the river and estuary are at risk of establishment and invasion.