By Boak Wiesner
Thoreau himself, during winters, measured the depths of Walden and some surrounding lakes, using fishing line and a small stone, finding depths of over 100’. What’s impressive around here is that the depths of some of the lakes are deeper that most of the hills around. For instance, the difference in elevation of the ski area at Hyland Hills is hardly more than the depth of Crystal Bay of Lake Minnetonka.
These kettles left by the glaciers provide a lot of relief, at least on a small scale. Right here at the Arboretum, this is especially apparent. Wood Duck Pond, just west of the main buildings with the very steep hills next to it, is a prime example. You’ll have to be “on your toes,” literally, to snowshoe up the trail this winter. A kettle formed where a small chunk of ice got left behind in the upper parts of the ground moraine – when it melted, a little lake was born, the sides falling into the space left by the melted ice.
At this time of year, the lakes are generally frozen over. With the ice on top. How unique! Water molecules, because of their very polar chemical structure, bond quite strongly to other water molecules, so much so that as they slow down and freeze, the resulting solid ice is more spread out, and hence less dense, than it is as a liquid. This is very much an anomaly.
But what good thing that is for all the denizens of the wetlands around here, to be insulated from the atmosphere. While the terrestrial animals up here will have to contend with dramatic daily and weekly changes in temperature, sometimes more than 60°F. in a short time, frogs, turtles and other creatures dwelling in and on the bottom of the lake will enjoy the relatively balmy 39°F. all winter long. Right now when the ice is fairly thin and pretty clear, you can lay down on it and perhaps get a glimpse of them moving around, albeit slowly. The bright, sunny days we enjoyed recently were great conditions for doing this.
Drill some holes yourself to check the thickness before venturing out too far. And drop a line in. What impressed Henry David more than how deep Walden was, were the fabulous colors of the pickerel that ice fishermen were catching “Ah, the pickerel of Walden! … are themselves small Waldens in the animal kingdom.” Around here, of course, they’d be Minnewashtas, not Waldens!
Boak Wiesner is a Minnesota Naturalist volunteer.