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Connecting the Great Lakes

            with the Atlantic Ocean

The St.Lawrence River is the second longest river in Canada, and runs 3059 km (1900 miles) from its source at Lake Ontario, through the provinces of Ontario and Québec, past the estuarine island of Anticosti into the Gulf of St.Lawrence. It connects the Great Lakes with the Atlantic Ocean through the St.Lawrence Seaway. The St.Lawrence is part of North America's most important hydrological system; one that stretches from Minnesota to the open Atlantic. The St. Lawrence is divided into three sections, the River, the Estuary and the Gulf.

St. Lawrence River, Estuary and Gulf

The River starts at Lake Ontario and runs eastwards past Quebec City. Just east of l'Île d'Orléans the fresh water meets oceanic salt water creating the typical two-layer estuarine circulation. By definition this marks the beginning of the Estuary. Only 2 km wide at the beginning the river widens continuously over the next 400m reaching 50 km at the estuary's end point at Pointe-des-Monts. From there the St. Lawrence widens into the Gulf which is  connected with the open Atlantic ocean by the narrow Strait of Belle Islle, north of Newfoundland and the Cabot Strait between Newfoundland and Nova Scotia.

A Question of Salinity

1. The Upper Estuary

From Île d'Orléans 150 km eastwards to Tadoussac, where the Saguenay Fjord meets the St. Lawrence. The average width is 17 km and depth ranges from 100 - 300 m. The fresh water from Île d'Orléans continuously gains in salinity from 2% to 10% to 22%.  At 22%  the water carries 22 grams of salt per kilogram of water. At Tadoussac, in the centre of our study area, the water has reached true oceanic character with a surface and deep water salinity of 26% and 34% respectively.

2. The Lower Estuary

The Lower Estuary starts at the head of the Laurentian Channel, a deep underwater trench of more than 350m in depth stretching over 230 km downriver from Tadoussac to Pointe-des-Monts. The water becomes slightly saltier with 28% at the surface and 34% at depth. On average the lower estuary is 42 km wide and depth varies from 100 - 400 m.

In the St. Lawrence Gulf salinity eventually reaches 30% and 34%.

Giants migrate far inland into the Estuary

During summer months, rorqual whales of the Northern hemisphere migrate into the cold,  nutrient rich feeding grounds of the St. Lawrence Gulf and Estuary. This huge ecosystem is highly complex and defined by different physical factors and constantly changing processes. Throughout the St. Lawrence, a deep water channel called the Laurentian Channel stretches over 1400 km connecting the St. Lawrence River, Estuary and Gulf with the open Atlantic.

From the Atlantic ocean nutrient rich cold water continuously flows into the St. Lawrence Gulf and Estuary at depth. Over hundreds of kilometres, water depth decreases slowly from 450 m at the Cabot Strait between Newfoundland and Nova Scotia to about 300 m near the channel head. There however, the channel shoals rapidly from within 20 km to less than 40 m in depth. The barriers created by banks, islands, shallow sills and the steep slopes along the North shore and along the southern edge of the channel (about mid-river) create strong upwellings where nutrients are concentrated and primary production is increased.