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The St. Croix River is still in good health but is facing pollution threats, according to two new 

Popular for its scenic beauty and recreation opportunities for boaters and 

, the St. Croix remains one of the cleanest tributaries to the Mississippi River, thanks in part to a large forested watershed that helps filter out contaminants.

But the reports highlight some troubling trends, including increasing development that leads to more polluted runoff entering the river.

The St. Croix flows more than 160 miles before joining the Mississippi River at Prescott, Wis., forming a large part of the border between Minnesota and Wisconsin. It was one of the first rivers in the United States to be designated for protection under the federal Wild and Scenic Rivers Act in 1968.

The Minnesota Pollution Control Agency’s 

 finds much to be positive about, said Pam Anderson, who manages the MPCA’s surface water monitoring program.

"The St. Croix River is a beautiful river, (with) lots of intact natural shorelines,” Anderson said. “We have relatively good water quality. We see low levels of bacteria. We have excellent fish communities and good bug communities." 

Much of that cleanliness can be attributed to the scenic river protections and the heavily forested watershed, said Monica Zachay, program director for the St. Croix River Association. which released its first “State of the St. Croix River” report.

“Forests are really good for water quality,” Zachay said. “They’re our best defense against water quality degradation as when it rains. Those natural ecosystems soak in the water into the ground and they act as natural filters."

However, the reports highlight some threats to the river, including too much phosphorus from farm and urban runoff, which can cause algae growth. The stretch of the river from Taylors Falls to Lake St. Croix 

is considered impaired because of phosphorus levels

Cities and industries discharge stormwater and treated wastewater into the river, which can contain phosphorus and other pollutants. That includes chloride from road salt or water softeners. Levels of chloride in the St. Croix are relatively low but rising, according to the MPCA report.

“We are adding more chloride to the system than should be there, and that's something that can't really take out of the water,” Anderson said. “So it's imperative that we do what we can to minimize what goes into the system.”

Urban stormwater also can raise the temperature of streams, which can threaten fish and other aquatic life. 

The St. Croix is home to 41 different species of freshwater mussels, including five that are listed as federally endangered. But 26 of the 41 mussel species live in water that is close to the maximum temperature that they can tolerate, so preventing further warming of the water is crucial, the MPCA report states.

Other contaminants have been detected in the river, including pharmaceuticals, microplastics and synthetic chemicals known as PFAS, which are used in a variety of consumer products because of their durability and resistance to heat and water.

And the St. Croix also faces a potential threat from invasive species, including zebra mussels and 

Zachay said local governments, soil and water conservation districts and other groups have been making strides toward improving the St. Croix’s water quality, with efforts such as adopting conservation tillage practices to reduce runoff and adding retention basins and rain gardens to capture stormwater. 

Unlike some rivers that are “so far gone,” the St. Croix still stands a chance of seeing improvement and possibly being removed from the state’s list of impaired waters, Zachay said.

“We are right kind of at this tipping point where we could actually see significant results in our lifetime,” she said.

Minnesota News

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Kirsti Marohn

Author

Popular for its scenic beauty and recreation opportunities for boaters and anglers, the St. Croix remains one of the cleanest tributaries to the Mississippi River. But two new reports highlight some troubling trends.

Trees with yellow, green and orange leaves on the banks of a river. 

Christine T. Nguyen | MPR News 2019

Closeup of hands holding different mussel species. 

Two new reports find St. Croix River still clean, but at risk

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