A team of divers this week scoured the murky floor of southern Minnesota’s Cedar River to find out if some of the water’s most unique and important animals survived.
The Department of Natural Resources has released thousands of endangered black sandshell and mussel mussels into the river over the past three years in hopes of restoring what had been a vibrant and productive waterway.
The Cedar, like many of Minnesota’s rivers, has been stuck in a negative feedback loop for decades. Pollution and overfishing have killed most of its shelled critters. Without mussels to filter the water and anchor the soil, the quality of the river only deteriorated. That, in turn, killed fish and invertebrates, making it harder for mussels to bounce back.
The river was once so full of life that the mussels kept entire industries afloat. The city of Austin, near the upper reaches of the Cedar, was nicknamed “Pearl City” because of the pearls produced by a mussel species. Old jewelry advertisements boasted of pearls taken straight from the riverbed.
Tim Ruzek of the Mower County Soil and Water Conservation District is helping open a mussel exhibit at a nearby wildlife center and has found photos of proud harvesters standing next to piles of mussels taken and killed for their pearls and shells, which made into knots.
“It’s amazing how overfished it was at the time,” he said. “I don’t know how they never thought it would run out.”
Over the past decade, the DNR has aggressively attempted to reintroduce endangered mussels to waters where they once thrived. The project is based on an environmental trust fund that is supplemented by proceeds from the state lottery.
Mussels are more important to river ecosystems than perhaps any other species. They constantly filter the water from bacteria, such as E. coli. They sequester chemicals and poisons by depositing them in their shells. Since nothing eats the shell of a mussel, it is one of the few natural ways to remove a chemical from the food web. They also excrete small granules that are full of nutrients for smaller fish and minnows.
Each mussel has a muscular foot that it uses to burrow into a soft river bed, holding sediment in place and helping to reduce erosion.
The state has been successfully recovering mussels in the upper Mississippi and Cannon Rivers for years. Since 2019, the DNR has been working to restore black sandshell and mucket clams — both on the state’s endangered species list — in the Cedar River.
It is not clear exactly when the mussels disappeared from the river. Shells still commonly found show that muckets and black sandshells once thrived there, said Madeline Pletta, DNR biologist.
“That’s what’s so unique about mussels,” she said. “If they die out, their shells are still there, maybe for 100 years. So we know exactly where they were. If a fish dies, how would you ever know it was there?”
In addition to problems with overfishing, mussels were killed in much of the state as growing cities began dumping raw sewage into rivers. There were also dams that separated the population from each other. The cedar, which cuts through miles of farmland, has been contaminated by fertilizer runoff and erosion as rainwater flows from channeled tributaries.
But some of that is starting to change. The Soil and Water Conservation District has helped install several large earthen berms to protect shorelines and store rainwater. The City of Austin plans to start a $105 million project to replace the wastewater treatment plant, which will reduce phosphorus pollution. And the river now has thousands of mussels constantly working to filter the water and keep it in place.
During this week’s research visit, Pletta and a handful of divers collected bag after bag of healthy, live mussels. They quickly sorted them by type on the grass and measured those that were tagged. Pletta held up a black sandshell—marked B381—and could see the annual rings on the shell, marking the years like rings in a tree trunk.
“They love their habitat,” Pletta said. “They stay, they don’t die, they don’t get washed away. They’re happy.”
Pletta took a large black sandshell mussel from the bag, which seemed pregnant to her. She used a pair of tongs to carefully pry the clam open a few inches to look inside.
“Oh, she’s really pregnant,” Pletta said with a smile, knowing she could hold one of the first black sandshell clams to reproduce on the Minnesota side of the Cedar River in at least 40 years.