I’m training for an Ironman triathlon and, despite my best intentions to prevent triathlon from taking over my life, triathlon has taken over my life. On a typical day I get up at 4:45am to fit in my first workout before starting work at 7am. I do my second workout over lunch, leaving the evening free for gardening, momming, and everything else. In between swimming, biking and running around the county, I take breaks to write newspaper articles, attend community events, plan workshops for local elected officials, and answer a never-ending stream of emails and phone calls.
This is a long way of explaining why I was swimming across Square Lake at 6:30am on the morning of July 4th. I woke up determined to fit in a “quick” swim, bike, and run before the Marine on St. Croix parade at noon, a BBQ with friends, and fireworks at night. The funny thing is that I was just one of many people who had also woken up early in the morning to swim in the crystal clear waters of Square Lake that day. In fact, there were at least a dozen other swimmers there, some of whom had arrived even earlier than I. The sky was clear, the water temperature was absolutely perfect, and halfway across the lake I popped my head out of the water and discovered a family of loons swimming casually beside me. Yes, I thought, Square Lake is good.
Square Lake is one of about 15 lakes in Washington County that consistently score “A” grades in the Metropolitan Council’s annual lake water quality report card. In fact, two-thirds of the best quality lakes in the Twin Cities seven-county metro area are located in Washington County. Square, in particular, is known for excellent water clarity, which makes it a favorite destination for swimmers, scuba divers, and beach-loving families from miles around. Why is water quality and clarity so much better in some lakes than others?
Lake science is actually a lot more complicated than you might think. To begin with, some lakes are shallow, while others are deep. Their bottoms can be rocky, sandy or silty and the different types of rocks and minerals all affect the water differently. Some lakes are groundwater-fed, while others collect rainwater and surface runoff. Lakes can be in a chain, connected by streams and wetlands, or isolated in glacial pockets in the landscape. The number and kinds of fish in a lake affect water quality and clarity, as do the plants and trees that grow around and in the water. Last, but not least, human activities and changes to the landscape impact lakes as well.
Square Lake is ranked in the top 1% for water clarity in the North Central Hardwood Forest ecoregion of Minnesota. It has a small watershed in comparison with the lake’s size, and receives 70% of its inflow from groundwater, 25% from precipitation, and only 5% from surface runoff. In addition, the lake is relatively deep – 68ft – which helps to keep the water clear since there is less chance of sediment on the lake bottom being stirred up by boats, wind, and wildlife. In other words, Square Lake’s water quality is partly due to “good genes.”
Though Square Lake is one of the cleanest and clearest in the metro area, human activities have still impacted the lake over time. Approximately 25% of the land draining to the lake is developed and another 22% is farmland. However, most of the lake is still buffered by woods and natural vegetation, thanks to careful development along the shoreline. For years, the Minnesota Department of Natural Resources stocked Square Lake with rainbow trout, until two decades worth of studies by the Carnelian-Marine-St. Croix Watershed District and Square Lake Lake Association showed that the fish were impacting the lake’s food web and causing poorer water clarity. There is curly-leaf pondweed in the lake, but so far, it has not been infested by Eurasian watermilfoil, zebra mussels, or other aquatic invasives. Through a lot of hard work and careful planning, we’re maintaining the delicate balance that allows us to love Square Lake without loving it to death.
Most of us don’t think about geology, ecoregions, watershed management, or fisheries science when we grab a towel and head to the lake. We paddle an oar, cast a line, dive in, drop anchor, wade around, speed along, skip stones, sit back, and enjoy the view. And every once in a while, we take a break in the middle of the lake to say hello to the loons.