Lake Looks Like a River: Making Sense of the Lower St. Croix

How water moves through the broad, 25-mile stretch has big implications for cleaning up its algae problem.




3 minute read

Cross-posted from Field Notes, the St. Croix Watershed Research Station blog.

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Some people look at the lower St. Croix River and see a lake. Others see a river. Scientists see both – a perspective which is proving to be a key factor in the extensive efforts to reduce noxious algae in its waters.

“It’s basically four lakes with a warm river sliding over the top,” says Sue Magdalene.

The St. Croix famously slows down at the town of Stillwater, Minnesota, about 25 miles above where the river joins the Mississippi, a stretch known as Lake St. Croix. It hosts big boats, big fish, beaches, and broad waters where millions soak in the summer sun and set up fishing shacks on the ice in the winter, and admire the fall colors, the broad views, and blue sky reflected on red-tinted water.

The color of the water is a key concern, as more and more often it is painted blue-green by potentially toxic types of algae. These aquatic plants often bloom on the same hot summer days people love to swim in the St. Croix. They can be poisonous to humans and animals. Dogs die every year after swimming in the stuff.

Preventing these harmful algal blooms is a priority for Minnesota, Wisconsin, and the federal government. Reducing the risk of such algae means reducing the nutrients – principally phosphorus – that feed it. The question of whether Lake St. Croix is a river or a lake is key to shaping and sharpening the strategy to reduce the toxic algae.

To improve understanding of this complex behavior, the St. Croix Watershed Research Station is studying it in multiple ways, thanks to federal funding to balance the impacts of development in western Wisconsin after the new Stillwater bridge is completed. One of the scientists spends a lot of time on the river, collecting samples, working with volunteers and other agencies, and analyzing data. The other spends most of his time staring at a screen, immersed in code.

Jim Almendinger is the digital data water scientist, programming software to simulate the St. Croix watershed and to predict the impact of different methods to reduce pollution. Sue Magdalene is analyzing what’s happening on the river right now to measure current conditions, assess progress toward cleaner water, and provide “on-the-water” data that can help better calibrate Jim’s software, and guide conservation efforts.

What they are finding is a complicated system that defies neat definitions.

“The upper end, below Stillwater, is much like a turbid river, while the lower end is like a clear lake,” says Magdalene. It’s not only a lot like a lake, with water that stays put for long periods, it’s four lakes separated by shallow and narrow channels. The uppermost lake is from Stillwater to Hudson, where the Willow River enters and its sand has built up islands and barriers. The second lake is from Hudson to Afton, where Catfish Bar and Valley Creek form a bottleneck. The next is from Afton to the Kinnickinnic River, where the Kinni’s delta squeezes the river to a quarter of its width. The final pool is from there to Prescott and Point Douglas, which separates the St. Croix from the Mississippi.

Where the water enters the first pool, at Stillwater, it is still carrying a lot of sediment. While that sediment carries phosphorus, it also blocks sunlight from penetrating far into the water, and the solar energy isn’t enough to grow a lot of algae. Downstream, somewhere south of the I-94 bridge, most of the fine sediment has dropped out so the water is clearer and algae blooms more easily.

The four pools act a lot like lakes: during the summer they stratify into warmer, oxygen-rich upper parts, and colder, lower-oxygen water on the bottom. The waters on the bottom can very easily strip phosphorus out of the floating organic material and muck on the bed of the river, recycling the nutrients again and again, growing thicker and thicker blooms of algae. On the upper end of Lake St. Croix near Stillwater, it is probably buried by all the incoming sediment fast enough to prevent much “internal loading.”

“Recycling nutrients can cause a time lag between fixing runoff in the watershed and seeing improvements in the water. The lake still needs time to heal itself,” Almendinger says.

With this knowledge and much more, Almendinger is making the computer model match the real-world conditions. The simulation of the watershed will help identify where conservation could get the most bang for the buck.

The model shows what would happen with water quality when we “tweak a few knobs,” Almendinger says. The knobs might be runoff reduction practices like more wetlands, or vegetated buffer strips between fields and creeks, or ponds to settle sediment and nutrients out of rain runoff, or more rain gardens built in neighborhoods, or more farmers leaving corn stalks on the fields all winter.

Because of phosphorus recycling, planned efforts to reduce nutrients flowing into the St. Croix River may not be enough to meet the goal of a 20 percent reduction by 2020. There is already a big supply built up that will takes years to deplete. It may be that more runoff reduction and more time will be needed than anticipated to compensate for past pollution.


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Lake Looks Like a River: Making Sense of the Lower St. Croix