Learn Your Lakes. What’s a Watershed?

Last week was the first time in a while that hunkering down at home for days on end was a response to weather and not a decision driven by spiking COVID numbers. Here in south/central Wisconsin, it was day after day of gray skies and persistent rain and the perfect conditions to ponder profound questions. Or, well, at least pertinent ones. And the one question we thought people might be asking is where (other than my basement, of course) does all that rain go? And that means that today, class, we’re going to be talking about watersheds.

What Is a Watershed?

If you Google “What is a watershed,” you’ll land on a handful of different pages, like this one from the National Ocean Service that says “It’s a land area that channels rainfall and snowmelt to creeks, streams, and rivers, and eventually to outflow points such as reservoirs, bays, and the ocean.”

Which is…sorta helpful? But it’s hard to picture what that all means. So let’s use Madison’s chain of lakes as an example. Here is a map, courtesy of the Clean Lakes Alliance, that shows the watershed for the Yahara Lakes laid on top of a map of Dane County.

What you’re looking at is an outline of “the land area that channels rainfall and snowmelt” into Lakes Mendota, Monona, Waubesa and Kegonsa (and, yes, Wingra too!). That means that most drops of rain or flakes of snow that fall onto any area shaded blue on this map are eventually going to end up in one (or all) of these lakes*.

(*Not all rain or snowmelt makes it that far, of course. Some evaporates or gets absorbed into the soil or is taken up by the roots of plants or deposited into a groundwater aquifer.)

BUT, most of that rain and snow does end up moving across the land and into creeks and streams like Six Mile or Pheasant Branch or Door Creek and then flowing into one of the Yahara Lakes.

This is an important concept, because the things we do on land in the Yahara Lakes watershed end up in our lakes. Agricultural fertilizers like phosphorus in the topsoil of corn and soybean fields are carried into our warerways and fuel algae blooms in summer. Dangerous chemicals like mercury and PFAS fall out from smokestack emissions or run off of airport tarmacs and into our streams and lakes where they eventually work their way up the food chain into our fish.

On a positive note, wetlands that we have preserved in the watershed slow down this flow and filter many of these water quality threats out.

In short, you can tell a lot about a lake from its watershed.

Can Watersheds Have Watersheds?

Yes! And this is where the fate of a drop of water gets really cool. Obviously, water doesn’t just stop in Lake Mendota and call it good. The Yahara Lakes Watershed is connected to bigger and bigger watersheds downstream. The rain on your rooftop is just water at the beginning of a very epic journey.

Here’s one version of that story:


Now, if you’ll excuse me, I need to go figure out what the watershed for my basement is. Because some of the water landing on my roof or yard are taking a very annoying detour and I’d much rather have them heading toward Lake Monona!