2018 01 09 Lake pan ice small

By David J. Hawke – It may be cold outside, but the legendary "Lake Skidoo-be-gone" is still claiming its victims, swallowing up motorized snow sleds on a weekly basis. The lake may look flat and solid… but often it isn't, much to the surprise of unsuspecting sled operators.

That layer of frozen water that covers the big ponds and lakes is a marvel of nature, yet despite centuries of close scrutiny by ice-fishermen (fisher-people? fisher-folks? ice anglers?), ice harvesters, skaters and other 'hard water' users, a definitive criteria for "safe use" is still vague.

 Although guidelines of uses per certain thickness have been published, they are just that – guidelines. Each lake is different in how the water flows, freezes, and responds to temperature changes. Add to that the weight variables of one snowmachine or ten, or a single skater versus a full-blown hockey game, and a 'hard and fast' rule of ice safety is impossible to determine.

As with just about any function of nature, the phenomenon of ice formation is simple yet complex. When the temperature of the liquid water reaches zero degrees Celsius, it solidifies into ice. Simple. Now come the somewhat more complex parts.

Knowledge nugget #1. When the temperature of lake water drops to a chilly plus 4 degrees Celsius, it sinks. Huh? Well, temperature is related to density, and cold water is denser than warm water. Plus 4 degrees Celsius is the magical temperature of ice formation. Read on.

The water that cools first as that is the surface layer which is exposed to the minus freezing temperatures of the air. Thereby a thin layer of cool water (now at just plus 4 degrees) begins to sink into the warmer, less dense water below. And the next night a new layer of cool water is formed, which then sinks and mixes with the warmer water below.

Envision a large mixing bowl of dough into which you are adding food colouring. Add a bit, mix a bit, add a bit, mix a bit… and eventually the entire wad of dough is of equal colour. The same process is occurring within the lakes, but rather than food colouring it's a bit more cool water that is being created and added. Eventually the entire water body is at a constant temperature of plus 4 degrees Celsius. Now ice formation can start.

Knowledge nugget #2. The freezing cold air temperature now hits the surface of the lake and that magical process of transformation from liquid water to solid ice begins. As long as the water below remains at that magical plus 4 degrees, the now colder surface layer won't sink and ice begins to form.

Knowledge nugget #3. An interesting characteristic of ice is that it is buoyant, due to air bubbles that become trapped within the ice. Another fact is that water expands as it changes from liquid to solid. This expansion and buoyancy is why lake ice cracks and heaves… the shores don't give way so there is no where to go but up.

Sometimes, due to this heaving, an air pocket forms under the ice leaving a space where a bit of heat may collect. This heat (small amount that it is) may come from geo-thermal releases from the much warmer bedrock below, or be brought to the lake via a river's inflow. This causes the plus 4 degree water to interact with the zero degree ice, resulting in a re-changing of the solid ice to liquid water (a sneaky thaw).

Knowledge nugget #4. Snowmobiles seem to crash through the ice more often than skaters or 'hard water' anglers. Why is that?  The answer is because new ice tends to bend. If the layer of ice is only a few inches thick, the weight of the machine and rider causes it to bend downwards. However, as the machine zips across the ice, this buckling isn’t really noticed by the driver.

As the ice is pushed downwards, a certain displacement of water occurs, resulting in a small wave being created just under the ice. Similar to the bow wake from a moving boat, this little wave of water gains strength and density as it is constantly pushed forward by the snowmobile's travel. This wave travels just ahead of the machine.

If the ice is thin enough, the underwater wave can suddenly force its way upwards and split the ice apart. Several victims (who were ever so lucky to have survived the experience) state that "the ice just opened up in front of them". They aren't lying… they were there and they caused the event to take place.

Knowledge nugget #5. The thickness of the ice can change within the radius of a few feet. This thinning can be caused by water movement, such as the inflow of a steam into the lake. Ice has a difficult time forming on moving water so any flow, even minimal, can seriously impede ice formation.

I was once with a group of teachers who were visiting a trapline in winter. The trapper had us line up along the shore to face a beaver lodge but warned that no one was to go past this designated point. A late-comer hadn't heard this warning and walked around the group and towards the lodge. Even wearing snowshoes he broke through the ice in about 1.786 seconds. Watching him go down was like observing the Titanic slip away; it seemed to take forever but that was just our heightened senses trying to get our sluggish bodies in motion.

A Native teacher was in the group, visiting from the very northern regions of the province. She immediately flattened herself against the ice, slide otter-like to the man, grabbed his coat, and requested of us, in a rather urgent and demanding tone, to grab her and pull! And we did!

Warming up back at the tour bus, the trapper explained that this man had walked between the lodge and the beaver's winter food pile. Because the beavers travel underwater from lodge to food pile, their movement disturbs ice formation. Therefore the area between the two sites has thin ice, which he was about to explain to us just as the late-comer provided a very memorable demonstration.

Other options? Go snowshoeing in a forest, and buy your fish at the market. Works for me.

David Hawke is a naturalist who is well known for his outdoor writing and photography. David has worked for several agencies and organizations around Lake Simcoe. In his weekly blog, he shares his observations and insights related to our local natural environment.

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