The ice abides: If Thoreau had a Secchi disc

Your lake is probably clearest at the time you’re least likely to be looking down into it – which is now, in winter, the water encased in ice and covered with snow.

Lakes clear up in winter for a variety of reasons. Cold water slows down the growth of algae (phytoplankton). In addition, the snow cover shuts out sunlight so that the algae cannot photosynthesize (make food) and so die back. Also, in essentially a closed vessel, the water is very still – no wave action from wind or boaters means bottom sediments are not stirred up and particles suspended in the water can settle out. There’s no rainfall runoff to wash soil and debris into the lake. Water still enters from springs, but that’s groundwater, essentially clear.

If you’re lucky enough to experience an early winter when the lake freezes solid enough to walk on, and a week or so goes by with no snow, you can enjoy a real spectacle. Even in fairly deep water, you can see all the way to the bottom and make out every detail.

Scenes like this remind me of a favorite passage from Henry David Thoreau’s Walden, telling about the clarity of the author’s beloved pond:

“Once, in the winter, many years ago, when I had been cutting holes through the ice in order to catch pickerel, as I stepped ashore I tossed my axe back on to the ice, but, as if some evil genius had directed it, it slid four or five rods directly into one of the holes, where the water was twenty-five feet deep.

“Out of curiosity, I lay down on the ice and looked through the hole, until I saw the axe a little on one side, standing on its head, with its helve erect and gently swaying to and fro with the pulse of the pond; and there it might have stood erect and swaying till in the course of time the handle rotted off, if I had not disturbed it. Making another hole directly over it with an ice chisel which I had, and cutting down the longest birch which I could find in the neighborhood with my knife, I made a slip-noose, which I attached to its end, and, letting it down carefully, passed it over the knob of the handle, and drew it by a line along the birch, and so pulled the axe out again.”

I wonder what a Secchi disc clarity measurement would have yielded in water that clear. Of course, Walden pond was naturally clear – Thoreau reported seeing the bottom 25 to 30 feet down while canoeing even in mid-summer. Your lake most likely is not that clear; my lake (Birch, near Harshaw, Wis.) certainly isn’t. Still, I long to experience an early December of clear ice. It almost happened this year. A couple of cold, still nights created a smooth skin, but then came snow. Now about a foot of snow lies on the ice; there will be no looking down into clear, cold water this year.