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Barely 15 000 years old, the St. Lawrence River is the world's youngest seaway!
When out in the middle of the river, you feel you are
at sea, and when close to shore, the physical traces
of this waterway's birth and evolution can be read on its rocky shoreline. The
St. Lawrence originates in the Great Lakes, but the waters from several other
rivers that flow into the seaway contribute to its powerful current. Learn about
the fascinating natural phenomena that create
the magic of the St. Lawrence River.
Things are splishing and splashing in the St. Lawrence River, whose waters are
fresh until a little past Île d’Orléans, located near Quebec
City. But the ocean's tides and currents have their grain of salt to add. The
river gradually transforms into a sea the more you travel east. The beautiful
islands in the St. Lawrence, buffeted by wind and storm, weave an enchanting spell
that beckons visitors to explore them in order to unravel their many mysteries.
Currents and tides
The seaway and the ocean
Did you know that a seaway is a type of river that connects with the
open sea and is deep enough to permit ocean shipping? Other seaways
around the world include the Seine in France and the Yangtze in China.
An estuary is the place where a seaway meets an ocean and where their
respective fresh and salt waters mingle. An ocean's tide affects a
seaway's currents at the estuary.
The man on the moon controls the tide
You might know that the pull of the moon's gravity, the force that
keeps everything on a planet from floating off into space, is responsible
for the ebb and flow of the tide on Earth. But the moon also has an
effect on our planet's land-based geographic features. Despite its
small size in comparison to the Earth, the moon even exerts its gravitational
pull on mountains. It is impossible to detect the minuscule movements
of earth formations with the naked eye. As water is more mobile, the
effects of the moon's gravity can be readily seen and measured.
A clock to tell what tide it is
Approximately every 6 hours, the tides change at certain points in
the ocean, and the effect is felt all the way to the shore, though
in an irregular manner. This is why the exact time of high and low
tide varies from one area to the next, even if the two areas are only
a few kilometres apart. Did you know that many fishermen and pleasure
boaters have a clock to indicate the number of hours remaining before
the next changing of the tide? Take note that the exact time on any
given "tide clock" only applies to a specific area.
Where does the water go during low tide?
It just goes somewhere else! If you tilt a bowl of water, you will
see that the level of water rises on one side while it drops on the
other, but the amount of water does not change. The same principle
applies to the tide in large bodies of water. Obviously, the water
in the sea does not drain away as if someone had pulled the plug out
of the drain of a giant bathtub! The amount of water in the ocean
does not change, it simply shifts elsewhere! During low tide, sailing
becomes more hazardous because a ship runs the risk of striking underwater
obstacles that now lie just beneath the surface.
Tides in the St. Lawrence
The tides are felt along the river all the way to Saint-Pierre Lake,
a shallow body of water attached to the St. Lawrence near Trois-Rivières,
about 1500 km (900 miles) from the Atlantic Ocean! And why would the
changing tide in the distant Atlantic manage to be felt so far inland?
Because of the sheer size of the St. Lawrence Estuary. In fact, the
tides in the St. Lawrence are stronger than in some seas.
Strong equinox tides
The equinox tides in the fall and spring are greater than at other
times of the year due to the stronger pull exerted by the moon and
sun. The St. Lawrence frequently jumps its banks during these times
of the year in Quebec City, near Place-Royale. The tides are very
strong in Quebec City because this is the location where the St. Lawrence
suddenly becomes much narrower. Did you know that high tides in the
St. Lawrence are felt in the Saguenay Fjord, where the water level
rises more than 6 m (18 feet)?
Underwater currents: it is very "windy"
under the surface
Tadoussac marks the beginning of the St. Lawrence Estuary, the place
where the fresh water of the river meets the salt water of the Atlantic
Ocean. If you could see a cutaway of this part of the river, you would
notice there are three distinct layers of water, each with its own
current. The current of the top layer heads towards the Atlantic,
carrying fresh water from the Great Lakes, which have an elevation
180 m (540 feet) higher than the St. Lawrence! The second layer comprises
extremely cold water, and its current heads in the opposite direction
of the top layer. This current, called the Labrador Current, follows
a deep underwater valley and runs along the North Shore all the way
to Tadoussac. The bottom layer is not as cold as the middle layer,
but has a higher salt content, making it heavier. This salt water
current comes from the Atlantic Ocean.
Whales fed by the Labrador Current
The St. Lawrence benefits from its powerful underwater currents, the
most fruitful being the Labrador Current that follows an underwater
valley all the way to Tadoussac. Why does it not go any farther inland?
Simply because a very high underwater cliff blocks the current at
the Saguenay Fjord. At this point, the depth of the river goes from
30 to 300 m (99 to 990 feet)! The cold water of the Labrador Current
mingles with the warmer surface water, creating dangerous turbulence.
But the icy cold water of the Labrador Current is rich in organisms
near the bottom of the food chain, including krill, a species of tiny
shrimp that is one of many whales' favourite food! The presence of
large quantities of krill is the reason whales will travel from the
St. Lawrence Gulf, along the North Shore and all the way to Tadoussac.
The whales do not come to visit us to chat, but to enjoy a marine
buffet provided by the Labrador Current!
The Gulf is the nursery of the St. Lawrence
The krill and small fish that escaped the whales' voracious appetites
begin swimming towards the St. Lawrence Gulf to the north, carried
along by the Gaspé Current. They are accompanied by clouds
of microscopic fish, mollusk and crustacean spawn that will reach
maturity in the waters near Anticosti Island.
Some of these small organisms will once again make the journey to
Tadoussac, where they will meet their fate in the stomach of a whale!
Talk about coming full circle!
As you can see, underwater currents contribute to the biodiversity
of the St. Lawrence, as well as the Earth's seas and oceans.
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