Description
The St. Lawrence River begins as the outflow of the Great Lakes and widens into a large estuary near Ile d’Orléans, where the river’s fresh water first encounters oceanic
salt water and where the typical two-layer estuarine circulation begins. Continuing downstream, the surface water of the St. Lawrence becomes more and more salty, finally having a true
oceanic character at the head of the Laurentian Channel 
(off-shore
of Tadoussac), where strong upwellings bring deep waters to the surface. Among the deepest and largest estuaries in the world, the St. Lawrence maritime estuary extends nearly 250 km
before it widens at Point-des-Monts into the Gulf of St. Lawrence. This enclosed sea is connected to the Atlantic Ocean by Cabot Strait and the Strait of Belle-Isle.
As it makes its way toward the ocean, the fresh water of the St. Lawrence estuary remains on the surface, above the saltier oceanic waters, much like a layer of oil floats above water.
It takes a parcel of water about three months to travel down the estuary and reach Cabot Strait and the Atlantic Ocean beyond. The surface outflow of water from the St. Lawrence entrains
the deep entry of much saltier water from the Atlantic via Cabot Strait and the Strait of Belle-Isle. This deep water gradually moves into the St. Lawrence Gulf and up the estuary toward
the head of the Laurentian Channel, near Tadoussac, a voyage that can take several years. These large-scale, counter-current water movements generate the estuarine circulation that gives
the St. Lawrence its distinctive character.
As part of the Atlantic Zone Monitoring Program (AZMP), which includes the Newfoundland, Maritimes, Québec, and Gulf regions of the Canadian Department of Fisheries and Oceans
(DFO), 
scientists from Institut Maurice-Lamontagne (IML)
began oceanographic monitoring of the St. Lawrence ecosystem in 1996. This program provides DFO with the information necessary to detect, track, and predict changes in the productivity
and state of the marine environment. This information is essential to the understanding of large issues, such as the impacts of climate change. The data gathered from the monitoring program
also support the ecosystem approach for sound management of the St. Lawrence ecosystem, the aim of which is to conserve and protect the marine environment and resources for future generations.
Another important aspect of the program is that the data collected as part of the AZMP are used to build time series and add to historical data sets that will be required to respond to
future problems.
