Hydrography of the St. Lawrence River

The map below shows the major hydrographic divisions of the St. Lawrence River, which include the Fluvial Section, the Fluvial Estuary, the Upper Estuary and Saguenay River, and the Lower Estuary and Gulf.

Map of the St. Lawrence River showing the main hydrographic divisions, including the fluvial section, the fluvial estuary, the upper estuary dans Saguenay river, and the lower estuary and gulf.

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The Fluvial Section

Map: Fluvial Section of the St. Lawrence River

  • Covers a distance of over 240 km from Cornwall, Ontario, to the outlet of Lake Saint-Pierre.
  • Up to 10 km wide.
  • Characterized by the presence of rapids, lakes, islands and islets, and several water development projects.
  • Weak tidal influence.
  • Presence of fresh water.
  • Natural physiography has been altered by human intervention, including the construction of hydroelectric works, creation of the St. Lawrence Seaway, dredging of the ship channel, shore modification, creation of the Expo 67 islands, and fill work.
  • Water in the Montréal region is marked by the confluence of the Ottawa River and the St. Lawrence River, creating fluvial widenings like the Lake of Two Mountains, Lake Saint-Louis, the Mille Îles River and Rivière des Prairies.
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The Fluvial Estuary

Map: Fluvial Estuary section of the St. Lawrence River

  • Stretches more than 160 km from the eastern tip of Lake Saint-Pierre (Pointe-du-Lac) to the eastern end of Ile d’Orléans.
  • Width varies from 870 m near the Quebec City bridges to 15 km at the easternmost end of Ile d’Orléans.
  • Average water depth in the main channel varies from 13 to 40 m.
  • Presence of fresh and slightly brackish water, occasional strong tides and a relatively straight main channel.
  • The Portneuf dock and the harbours of Bécancour, Trois-Rivières and Quebec City are the sites most modified by human intervention. Thereare no major water development works in the fluvial estuary.
  • The shores in the Quebec City region have been almost wholly degraded by construction of Champlain Boulevard and the Dufferin-Montmorency highway in the early 1970s.
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The Upper Estuary and Saguenay River

Map: Upper Estuary and Saguenay River

  • Covers an area 150 km long from the eastern tip of Ile d’Orléans to the mouth of the Saguenay River.
  • Average width of this section of the river is 17 km.
  • Depth varies from 100 to 300 m.
  • Fifty-odd islands and islets are present in the area, including Ile aux Grues, Ile aux Coudres and Ile Verte, are found in this river stretch.
  • Characterized by the mixing of fresh and salt water, driven by a combination of high-intensity currents and the influence of the tide. The resuspended sediment makes for highly turbid water, which is especially pronounced in the area between Ile d’Orléans and Ile aux Coudres.
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The Lower Estuary and Gulf

Map: Lower Estuary and Gulf

  • Over 230 km long and an average 42 km wide.
  • Depth varies from 100 to 400 m in the Gulf, except in the Magdalen Shelf, where depths rarely exceed 70 m, and in the area around Cabot Strait, where depths are greater than 500 m.
  • Marine area of the St. Lawrence dotted with large islands, including Anticosti Island, the Magdalen Islands, Bonaventure Island, and the Mingan and Sept-Îles archipelagos.
  • Heavily influences by the tide, barometric pressure, air temperature, wind, local freshwater inputs, coastal relief and the rotation of the Earth.
  • Lower estuary begins upstream, in the deepwater upwelling zone located at the mouth of the Saguenay River, and at the head of the Laurentian Channel, an underwater valley of more than 350 m in depth.