Tracking the health of lakes in Washington County

It’s a cool fall day and the trees surrounding Goose Lake in Scandia are dappled yellow, gold and green. Becca Oldenburg Giebel and Matt Downing, water monitoring experts from the Washington Conservation District, are out on a boat in the middle of the cold, grey lake collecting water samples and tracking the lake’s progress toward seasonal “turnover.” As the fall winds blow and air temps drop, the deep, cold water at the bottom of the lake will begin to mix with warmer surface water until eventually, the entire lake will be uniformly cold.

While on the lake, Oldenburg Giebel and Downing measure water temperature, dissolved oxygen, and water clarity and collect samples to send to the Metropolitan Council for further analysis. There, staff in the lab will measure total phosphorus (the nutrient that feeds algae), chlorophyll-α (the photosynthetic component found in algae and aquatic plants) and TKN (the sum of organic nitrogen and ammonia, which can contribute to eutrophication). The goal is to track overall lake health and determine whether water quality is improving or declining over time. Here’s what recent data tell us about five local lakes.

Long Lake, on the west side of Stillwater is a small, shallow lake that is only 22ft at its deepest. Ninety-five percent of the lake is littoral – a term used to describe areas less than 15ft deep that are dominated by aquatic plants. Officially, Long Lake is classified as impaired for excess nutrients but water monitoring shows a statistically significant improving trend for water clarity, chlorophyll-α, and total phosphorus. Long Lake is part of the Brown’s Creek Watershed District.

Grandfather and grandson explore Long Lake in Stillwater. The lake is shallow and dominated by aquatic plants.

Lake McKusick is another small, shallow lake in Stillwater. It is located within the Middle St. Croix Watershed Management Organization. In the past, McKusick was listed as impaired for excess nutrients, however, it was removed from the state’s impaired waters list in 2012 after water quality begin to improve. In 2020, three water samples collected from Lake McKusick exceeded the Minnesota Pollution Control Agency’s standard for shallow lakes but, overall, the lake has a statistically significant improving trend for water clarity and total phosphorus.

Swans on Lake McKusick, a shallow lake in Stillwater.

Further north, in the Carnelian-Marine-St. Croix Watershed District, water monitoring tells different stories for three popular recreation lakes. The first, Big Carnelian, is 67ft at its deepest and only 30% of its area is littoral. The lake has a public boat launch and is blessedly free of aquatic invasive species. Water monitoring data show a statistically significant improving trend for total phosphorus, chlorophyll-α, and water clarity and the lake scored a grade of A- on the most recent Met Council report card.

At 1756 acres, Big Marine is the second largest lake in the county. It has three public boat launches and is popular for boating, fishing and swimming. Unlike Big Carnelian, however, two-thirds of the lake is littoral. Shallow bays provide ideal habitat for turtles, bass and heron, but are also vulnerable to invasive species. In recent years, Big Marine Lake Association has worked to manage Eurasian watermilfoil and curly-leaf pondweed in the hopes of preventing these plants from taking over the lake. Big Marine has a statistically significant improving trend for total phosphorus, chlorophyll-α, and water clarity and consistently scores A grades on the Met Council report card.

Big Marine is another lake in Washington County known for consistently high water quality. The south bay of the lake is quite shallow, so there are many rooted aquatic plants like water lilies. Shallow lakes provide excellent habitat for wildlife, but aren’t good for boating and swimming. On Big Marine, boaters head for deeper water in other parts of the lake.

Lastly, we have Square Lake, a popular destination for swimming, scuba diving, and paddling. For decades, this deep, clear lake has been regarded as one of the best lakes in the Twin Cities metro. Unfortunately, water monitoring shows a statistically significant declining trend for water clarity in Square Lake. Though the lake is still very clear, it is not as clear as it used to be. One explanation comes from a multi-year research study conducted by Hamline University Biology Professor Leif Hembre, which found that trout stocking in Square Lake was affecting water clarity and quality. The problem was that trout had disrupted the natural food web by eating daphnia (a type of zooplankton) that feed on algae. Though the Minnesota Department of Natural Resources has stopped stocking trout, water clarity trends have not yet rebounded. Overall, however, Square Lake is still in very good health. It has no aquatic invasive species and continues to receive A grades for water quality.

Square Lake in northern Washington County is known for its clear water and is popular for swimming, scuba diving, and boating.

During 2020, Washington Conservation District monitored water quality on 71 lakes in Washington County, collecting nearly 600 samples over the course of the year. It’s an enviable job on warm summer days, but also includes many days with oppressive heat, frigid winds, or angry water, covered in whitecaps. Currently, water monitoring staff are back at their offices, crunching numbers and prepping for another round of data collection once the ice finally melts.