Wherever you go in Canada, you’ll hear the question, “Are you going to the lake this weekend?” That’s more than you can say for some other phrases, like, say, “Confederation” and “cultural identity.”
Lakes are, of course, depressions with water in them. Those depressions can be formed in a number of ways, from tectonic and volcanic action to the scouring of a basin at the foot of a waterfall, but prairie lakes were all made by glaciers (except for those made by the damming of a stream by beavers, muskrats or politicians).
At least five times in the last two million years, continental ice sheets have scoured this region. The last one retreated about 10,000 years ago. As it melted, the sediments and rocks frozen into it piled up, forming hills or rocky plains. This gravelly material is called “moraine.” Sometimes, blocks of ice fell off the retreating glacier and were buried in the moraine. When these blocks melted, the moraine collapsed, forming circular depressions. Some are large enough to retain water year-round; others fill only from rain or spring run-off. These “prairie potholes” cover a large part of the plains and are major breeding grounds for waterfowl.
As the rocks frozen into the glaciers moved over bedrock, they ground away at it. Softer rock would give way first. When the glaciers retreated, this left many depressions behind. Filled with water, they now dot northern Canada.
At the edge of the glacier, huge lakes formed when melting water was trapped by moraine. One such lake, Lake Agassiz, extended from almost as far northwest as Prince Albert to present-day Lake Superior.
A lake’s nature depends on its depth and size and on its biological component. Water is very slow to heat up and cool down. This is why most Canadian lakes are so cold during the swimming season: the short summer just isn’t long enough to heat up that much recently frozen water.
Surface water warms more rapidly than bottom water, which, being colder, is also more dense. As a result, lakes consist of a layer of warm water over a layer of cold water. Wind causes some mixing of the two layers. A persistent wind will literally pile the water up at one end of the lake; after the wind stops, the whole lake slowly rocks back and forth. As well, as winter approaches, the upper layer cools more quickly than the lower layer, eventually becoming heavier and sinking to the bottom, overturning the whole lake.
A high algae and plankton content makes the surface layer warmer, because brown water traps more energy than clear water. So a cloudy brown lake can be warmer to swim in than a crystal-clear lake (although in a clear lake the interface between the cold and warm layers is actually deeper than in a cloudy lake, because the algae and plankton also keep energy from penetrating).
Algae and plankton are worse in southern lakes because they’re warmer and they receive a greater influx of nutrients than rock-bound northern lakes. In the south, the lakes are surrounded by rich soil, whose nutrients are constantly deposited in the lakes by run-off.
Unfortunately, it’s possible to have too many nutrients. This can lead to “eutrophication.” It happens naturally to lakes as they age, but it can also happen when lakes receive massive quantities of nutrients due to increased erosion (from farming or shoreline development) or pollution (human or animal waste, phosphates from fertilizers or detergents). The water becomes so nutrient-rich that more life forms grow than there is oxygen to support. A lot of them die–and their decomposing bodies take even more oxygen from the water. The only thing that thrives is weeds; they choke the lake and eventually turn it into a marsh.
Some shallow prairie lakes are not very pleasant to swim in. One reason is blue-green algae. When it blooms in fresh water, which it does in response to phosphate pollution, it produces noxious odors and toxic compounds that can kill fish and even dogs.
Another nasty possibility is “swimmer’s itch.” It’s caused by the developmental stages of waterfowl flukes, tiny parasitic flatworms–microscopic versions of the leeches that are also occasionally encountered.
But poisonous algae blooms and bloodsucking worms aside, our lakes are pretty nice places to spend a hot summer’s day. If some politician really wants to find an issue every Canadian can agree on, he could do a lot worse than promoting this amendment to the Constitution: “All citizens shall have the right to spend the day at the lake whenever it’s just too darn nice to work.”
He’d get my vote!
