In 1832, the American adventurer Henry Schoolcraft led an expedition up the St. Croix River, before significant European immigration to the region. He was on his way back to Lake Superior after locating the headwaters of the Mississippi River.
The group’s first day on the St. Croix was spent ascending Lake St. Croix, the 25 miles of flat water above the river’s confluence with the Mississippi.
Schoolcraft described it as clean and clear: “The waters are beautifully transparent, and the margin exhibits a pebbly beach, so cleanly washed, that it would scarcely afford earth enough to stain the fairest shoe.”
Almost 200 years later, a lot has changed.
The river is still pretty clean, but not like Schoolcraft saw it. Its changes are linked to another one of his observations: he recognized the region's prairies as “valuable agricultural country” that would “sustain a dense population.”
Seeking the St. Croix’s story
Since Schoolcraft’s trip, the farms and families have come as predicted, with profound effects on the river. Research Station scientists have seen the evidence of these changes in the mud samples they’ve extracted from the bottom.
By taking six-foot tubes of mud from the bed of Lake St. Croix, scientists can look back at material that settled out of the water even before Schoolcraft passed through.
Putting the sediment under the microscope and through a battery of chemical tests has let them analyze how fast the mud was accumulating two hundred years ago, what species of algae were most prevalent, and how clear the water was.
There aren’t a lot of rivers where you can do that, because sediment doesn’t consistently accumulate in fast-flowing waters. But slow-moving Lake St. Croix keeps its own log book of the watershed’s history.
The story it tells will be the first part of a new series called “Research Revelations;” highlighting past and present findings. Watch for more in the months ahead, please subscribe if you are not already.
The St. Croix watershed has been occupied by indigenous people for perhaps 12,000 years, including the Dakota and Ojibwe to this day. Native people manipulated the landscape by setting fires, farming small plots, living in villages or seasonal camps, and just passing through.
The grasslands stayed intact, and the soil stayed in place. The river ran “beautifully transparent,” as Schoolcraft found it.
Five years after his expedition, the U.S. government paid the Dakota and Ojibwe about $40,000 total in cash and goods, with promises of future payments, and paid well-connected fur traders $160,000 to settle Indian debts (often fraudulent).
In exchange, much of what is now Minnesota, including the St. Croix, was opened to business.
The lumbermen were among the first to arrive, with the first sawmill opening up in 1839 at Marine on St. Croix (two miles upriver of the Research Station’s location today).
Logging the watershed’s old-growth white pine forests peaked in the 1880s and ended by 1914. Removing the forests altered water flows and erosion, and massive floods of logs floated downstream, ravaging the channel.
The lumber era coincided with the beginning of major changes in the St. Croix watershed, but did not appear to have a big impact on the water. At least, not compared to the agriculture and development that followed.
Breaking the sod
By 1850, hopeful farmers, many from Sweden and other Scandinavian countries, were riding steamboats upriver and embarking on new lives on land provided by the government. By 1880, farmers occupied almost all of the tillable acreage in the area.
When plows cut through the thick carpet of roots covering rich soil and drained wetlands, the way water moved across the land and into the St. Croix changed forever. Rainfall and snowmelt started to run off faster, with little to hold it back.
The river sediment shows us what happened next: Increased erosion of soil and stream-banks sent more and more material downhill and downstream.
In the century between 1850 and 1950, sediment accumulation in Lake St. Croix doubled, tripled, and then peaked in the 1950s at 130,000 tons per year — eight times the pre-settlement rate.
After that, it slowed considerably to its current rate of about 60,000 tons per year, four times the pre-settlement rate.
Post-War watershed moment
About the same time the sediment started slowing down, the St. Croix’s other primary problem was growing. Increasing phosphorus was feeding much more algae.
The arrival of modern farming in the 1940s and 1950s brought tools and techniques like tractors and fertilizers. Those changes also showed up in the bottom sediment.
Historic water quality can be determined by identifying the variety of tiny glass fossils of algae called diatoms buried in the mud. Diatoms live in the water, but when they die their fossils become buried in the mud as it accumulates. Different species thrive in different conditions, including more and less phosphorus.
Based on the different species of the diatoms in the layers of mud that accumulated over time, phosphorus started climbing in about 1920. Beginning in about 1950, it increased dramatically and has stayed high ever since. By the 1990s, the total phosphorus load was 460 tons per year, three times the pre-settlement load and the most during the 200 years that were studied.
The phosphorus increase could not solely have been caused by larger amounts of eroded sediment carrying more nutrients into the river, because it began well after the process of converting the land from prairies to farm fields was complete.
That points to the most likely causes: inorganic fertilizers and livestock feed supplements. Using these materials leads to more nutrients in the soil, which is easily washed away by rain.
Most of the extra phosphorus actually flows right out of the St. Croix into the Mississippi River at Prescott, Wis. The amount more than doubled between 1850 and the 1990s, increasing from 127 to 285 tons per year. Yet the St. Croix is still so clean, it actually dilutes the nutrient-rich river it joins.
Turning back time
Reading the history of the St. Croix watershed in its mud was a first step toward restoration. Research including what is described in this article was essential to a 2009 state and federal plan to reduce phosphorus flowing into Lake St. Croix.
The target is about 360 tons per year, roughly the same amount as during the 1940s. The primary strategy is slowing water on its journey from raindrop to river, decreasing erosion and runoff, making the watershed work more like it did historically.
Here are a few ways that's being done:
Repairing eroded gullies and redirecting stormwater can prevent tons of sediment and phosphorus from entering the river each year. Read about one example about how getting rid of a growing gully stopped significant erosion into the St. Croix River.
A rain garden like this one in Lake Elmo, Minn., can capture urban storm runoff and allow it to infiltrate the soil before rushing into the St. Croix, keeping lots of nutrients out of the water. Read more about how St. Croix community rain gardens.
Planting vegetation on farm fields that stays on the land all fall, winter, and spring (unlike most crops) can improve soil health, provide animal feed and other products, and reduce impairment of the St. Croix River. Read about how farmers in western Wisconsin are working together to expand conservation practices.
All this work and more means nutrients and soil will stay on the land where they can help grow food — rather than in the river, where they only feed algae.
Eventually, the waters should have less sediment and algae in it, once again not even "enough to stain the fairest shoe.”
Edlund, Mark & R. Engstrom, Daniel & Triplett, Laura & Lafrancois, Brenda & Leavitt, Peter. (2009). Twentieth century eutrophication of the St. Croix River (Minnesota-Wisconsin, USA) reconstructed from the sediments of its natural impoundment. Journal of Paleolimnology. 41. 641-657. 10.1007/s10933-008-9296-1.
Triplett, Laura & R. Engstrom, Daniel & Edlund, Mark. (2009). A whole-basin stratigraphic record of sediment and phosphorus loading to the St. Croix River, USA. Journal of Paleolimnology. 41. 659-677. 10.1007/s10933-008-9290-7.
This article was made possible with support from the McKnight Foundation's Mississippi River program.