How clear is your lake?

Is you lake clearer than the next one over? Clearer than the one where your best friends live? Maybe the questions don’t matter to you, but water clarity is one attribute people think of when judging the quality of a lake. So how do you find out whose lake is clearer? You could get a qualitative judgment by slipping on fins, a mask and a snorkel and kicking your way around both for a while, eyes down. Or you could get hold of an electronic turbidity meter and scan a test-tube sample taken from each lake.

Of course, there’s a middle ground, and that’s to make yourself a Secchi disk. It’s a very simple tool for measuring lake water clarity with reasonable accuracy, and it’s really quite a bit of fun to use. Secchi disks (pronounced SEKK-ee) are accurate enough so that freshwater biologists often use them for their water clarity measurements. Look at a Department of Natural Resources report on fish stocking for a lake and you’re likely to find a Secchi disk reading.

A Secchi disk is simply a circular, plate-like object on a rope that’s painted with quadrants, like a pie cut in four pieces. Two quadrants are white, the other two black. To use the disk, you go out in a boat to where the water is deep, then lower the disk slowly, slowly, until you can no longer see it. Mark the rope at the waterline. Then slowly, slowly, pull the disk up until you can see it again. Once more, mark the rope at the waterline. Now retrieve the disk and measure the distance from the disk to both marks. Take the average, and that’s your Secchi disk reading.

What the Secchi disk actually measures is how deep light penetrates into the water. Suppose your Secchi disk disappears at 10 feet below the surface. How deep does the light penetrate? That's easy, right? Ten feet. Ah, but no! The light you see bouncing back from the disk actually has to make a round trip, from the surface and back up to your eyes. So a Secchi reading of 10 feet means light penetrates to a depth of about 20 feet. 

A Secchi disk is quite easy to make. On the Internet, you can easily search up instructions on how to make a one good enough that a biologist would be happy to use it. Since you don’t need one good enough to sell in a scientific supply catalog, all you really need is a round object, a weight to make it sink, a drill to bore a hole in it, a length of rope, and a couple of cans of paint (ideally waterproof).

As long as you can get the disk to stay face up as you lower it, it’s good enough for your purposes. Try making one and checking your lake water clarity – not just once but at different times of year, including winter, through the ice. Keep a record of Secchi disk readings in your home or cabin log. You’ll enjoy it, and chances are the kids will love it.