The Hydrology of the St. Lawrence Basin

Introduction Hydrological Cycle The Great Lakes The St. Lawrence River Bibliography

There are approximately 1 billion km3 of water covering three quarters of the earth's surface mostly in the form of oceans, rivers, lakes, snow and ice (glaciers). Hence there is a great need to have an understanding of the principles behind the ways in which water flows around on the planet. Expansive amounts of water, water vapour, ice, and energy are in continuous movement in an endless cycle called the hydrological cycle.

The St. Lawrence has the tenth largest drainage basin in the world.

The hydrology of the region is relevant when discussing the Great Lakes, one of the largest supplies of available freshwater; and the St. Lawrence River, the tenth largest drainage basin on the planet, discharging water into the Atlantic Ocean at a rate of 10,100 m³/s. The numerous streams, rivers and lakes contribute to the hydrology of the region with a measurable amount of complexity that is not fully understood today. This section will familiarize you with some basic facts concerning the hydrology of the region. A description of the hydrological cycle will be provided to illustrate the central premise of the water systems. A description of the hydrology of the Great Lakes will then be given to explain where the water comes from. Lastly, some hydrological features of the St. Lawrence will be elaborated upon to show some more detailed phenomena.

The hydrological cycle is a term describing the interaction of water with the planet.

Only 1% of all the water on the planet is available to us.

Water moves continuously throughout the planet. This elaborate system of movement is referred to as the hydrological cycle or system. The system has been operating for billions of years between the subsurface to the lower portion of the atmosphere transforming and moving water depending on the environmental conditions. In the hydrological cycle, water precipitates out of the atmosphere and into the oceans or land, where it eventually flows overland or beneath the ground to go back to the oceans. This theory has been confirmed by the fact that the total quantity of water on the planet has remained fundamentally constant since the geological past.

Looking at the table below one can see that less than 3% of the water on the planet is fresh water, and of that amount, only 1% (139,000 km³) is available for meeting our freshwater needs. The usable fraction of the freshwater is located in rivers, lakes, underground and in the atmosphere. Therefore it is imperative that the interaction of water is well researched so that living organisms may enjoy a comfortable existence.

The Great Lakes are integral for providing water to the St. Lawrence.

The Great Lakes Have an Immense storage capacity.

All of the Great Lakes are among the top 20 largest lakes in the world.

The Great Lakes contains 22,700 km³ or 25% of the world's freshwater in lakes. These lakes have the ability to store water when excess is recorded and then release it slowly. Hence the Great Lakes are important in injecting a consistent flow of water into the St. Lawrence. For instance, the St. Lawrence River's maximum flow reaches 1.5 times its average and twice its minimum, illustrating the immense storage capability of the Great Lakes. But how does it work?

Following the principles behind the hydrological cycle, moisture gets carried into the Great Lakes basin by way of the movement of air masses in the form of rain, snow, hail or sleet. Consequently, some precipitates descend to the Great Lakes contributing to the amount of stored freshwater while other portions return to the atmosphere. The remaining precipitation falls to the ground and flows to the lakes as surface runoff or groundwater. Sandy soils, gravel, and certain types of rock encourage groundwater flows, while impermeable rock and clays facilitate surface runoff. However, as westerly winds carry moisture into the basin, it also loses moisture through evaporation and transpiration, and outflow into other rivers. The outlets for the Great Lakes are the St. Mary's River (Superior), Straits of Mackinac (Michigan), St. Clair River (Huron), Niagara River/Welland Canal (Erie), and the St. Lawrence River (Ontario). The St. Lawrence River is by far the largest outlet in terms of discharge.

The St. Lawrence has the 14^th largest drainage basin in the world.

Stretches of narrow rapids and riverine lakes are characteristic between Cornwall and Donnacona.

Salinity rises from 0% to 20% between d'Orleans and La Pocatière.

There are three layers of water in the Lower Estuary.

The St. Lawrence River has the fourteenth largest drainage basin in the world encompassing an area of 1,344,000 km² with a discharge rate of 2,830 m³/s. The drainage basin is separated from the watersheds of adjoining rivers by divides which are simply lands at a higher elevation. Small watersheds join the St. Lawrence and form one of the three continental watersheds in North America, called the Atlantic watershed. Accordingly, certain parts of the river have different hydrological characteristics which can be broken up as following: Cornwall to Donnacona, Donnacona to the Upper Estuary, and the Lower Estuary to the Gulf.

From Cornwall to Donnacona, the St. Lawrence is freshwater and always flows in the direction of the Gulf or downstream. Narrow rapid expanses and wider riverine lakes such as Lakes Saint-Louis, Saint-Pierre and Saint-François are typical for this part of the river. With the exception to ship channels, these inlet lakes slow down the velocity of water and deposit suspended solids in favoured parts of the lakes. The tributaries going into the river flowing near the banks sometimes travel great distances before blending with the waters of the river. The main water masses include: the Great Lakes, Ottawa River, Chateauguay River, Richelieu River, Yamaska River, Saint-Maurice River, Nicolet River, L'Assomption River and Bécancour River. The Great Lakes and the Ottawa River are the main water masses. The impact of the tributaries on river is very small because of the river's tremendous velocity. As a result water from the tributaries is kept from entering the center of the river, thus flowing downstream along the banks . This pattern has caused two separate water masses to flow beside each other for great distances before mixing completely. Variability in the location of water masses is dependant on the season, discharge rate, and water level.

The river begins to widen soon after Donnacona from a width of 1 km at Quebec City to 15 km at the eastern end of Île d'Orleans, and 20 km a few km upstream of Île aux Coudres. The underwater topography or bathymetry also changes significantly. Islands are located on the south shore, downstream of Quebec City; while the river is deeper in the north shore. The brackish water zone, or the area where saltwater and freshwater meet, extends from Île d'Orleans in proximity of La Pocatière raising the salinity from 0% to 20%. Tidal influences begin near Lac St. Pierre, increasing steadily into the Upper Estuary. The tidal movement may at times alter the course of the currents as the tide ascends, such as near Portneuf. Furthermore these tides affect the biology of the region. For instance, strong tides in the Upper Estuary have developed intertidal zones called wetlands. Typically these wetlands have a notable concentration of salinity, variable water levels, and specific forms of vegetation. Between Portneuf and Quebec City, the high speed of the river, turbulence from the Richelieu Rapids, and the effects of the tides cause the waters of the tributaries to mix into the river. The water becomes almost entirely homogenous above the Jacques Cartier River. However, the process begins again between Île d'Orleans and Île aux Coudres. The tidal forces cause a great amount of sediment to stir up which in turn contributes to trapping suspended solids. Consequently, saline stratification starts appearing at different tidal phases.

The Lower Estuary encompasses the area between the Saguenay River to Pointe-des-Monts. The St. Lawrence becomes too wide after Pointe-des-Monts to be called a river and is appropriately named the Gulf of St. Lawrence. Water circulation in the Lower Estuary is characterized by eddies, currents, upwelling of cold water, and the stratification of water. There are three water masses layered on top of each other because of the difference in temperature and salinity. The salinity increases as one descends while the temperature decreases. Not much is known about the circulation of the water at lower depth but upwelling of deep cold waters have been noted off of Tadoussac, on the north shore of Chaleur Bay, south of Anticosti Island, and near Mingan Island due to westerly winds. These upwellings continually provide the surface with the nutrients necessary to maintain life. In addition, some other features have been seen closer to the surface such as gyres. Gyres are great migratory eddies habitually located between Pointe-des-Monts and the western side of Anticosti Island. Gyres can also be seen in the area of Île du Bic and Pointe-des-Monts.

• Christopher, R. (1994). Geosystems: An Introduction to Physical Geography. New York: Macmillan College Publishing Company.
• Perrault, H., and Gingras, D. (1995). The St. Lawrence: The Ever-Changing River. Québec Science 33(6): 1-8.

Related Web Sites

An Environmental Atlas and Resource Book concerning the Great Lakes The Groundwater Foundation Groundwater: Nature's Hidden Treasure Saint-Laurent Vision 2000