JOHN YANG: Wisconsin is coming back from its worst drought in decades.

Along with unusually high temperatures, it's affected wildlife in and along the state's rivers.

While spring rains under the drought recovery in some places has been slow.

PBS Wisconsin's Nathan Denzin tells us about one species that's been hit particularly hard.

WOMAN: We reached peak drought severity in September of last year.

NATHAN DENZIN (voice-over): It's been a very dry few years for Wisconsin.

WOMAN: I started getting lots of phone calls from people saying, there's all these mussels out there and they're stranded, they're dying.

What can we do?

NATHAN DENZIN (voice-over): It's had a significant impact on a species we hardly think about.

WOMAN: I was no longer picking up a muscle and placing it gently in the water.

I was throwing them as fast as I could.

NATHAN DENZIN (voice-over): Much of southern Wisconsin was in a perpetual drought starting in the winter of 2021 through the spring of 2024.

ELLEN VOSS, Wisconsin River Alliance: Droughts are very stressful to aquatic organisms as well.

NATHAN DENZIN (voice-over): Ellen Voss is the climate resilience director with the Wisconsin River Alliance.

ELLEN VOSS: There's just less space for the things that animals and plants and insects and everything else need to survive.

NATHAN DENZIN (voice-over): One animal that can be squeezed out by low water is mussels.

LISIE KITCHEL, Wisconsin Department of Natural Resources: They're basically just, you know, a mollusk with two shells, and they live in the rivers and lakes, rivers and streams of Wisconsin.

NATHAN DENZIN (voice-over): Lisie Kitchel is a mussel expert at the Wisconsin Department of Natural Resources.

LISIE KITCHEL: They don't have a brain and they don't have eyes, so they, you know, they can't just figure out where the deeper water is.

NATHAN DENZIN (voice-over): She says there are tens of thousands of mussels on the bed of the Wisconsin river, including 40 different species.

LISIE KITCHEL: They filter as much as ten gallons of water a day per mussel.

And when there are hundreds and thousands of them, they really help purify the water.

NATHAN DENZIN (voice-over): But when the drought reached its peak in September, mussels started to wash up on shore banks.

JEAN ANMUTH, Retired Scientist: What I was really looking for was actually historic artifacts because the water was so low.

I'd never seen it this low.

NATHAN DENZIN (voice-over): Jean Anmuth is a retired scientist for the Department of Natural Resources.

She lives near the Wisconsin River in Prairie du Sac, northwest of Madison.

She was also the first person to contact experts when she saw stranded mussels.

JEAN UNMUTH: Mussels are a huge part of the ecology of this river.

So I thought, hey, I better ring the alarm.

NATHAN DENZIN (voice-over): A natural question is to ask about how dams on the Wisconsin River affect water levels.

LISIE KITCHEL: Because we had such a drought year, the dam did not have water to release.

NATHAN DENZIN (voice-over): The dams along this river are not holding back water and cannot release anything more than they receive, meaning they can't add more water to help mussels at the lower end of the river.

Alliant Energy, which owns the Prairie De Sac dam, in a statement said, our license requires us to maintain the water level.

Therefore, we cannot release more water during drought conditions.

ELLEN VOSS: In this scenario, on paper, nobody did anything wrong.

Everybody was in compliance with their license obligations the entire time.

And yet the outcome was thousands or tens of thousands, we don't know, dead mussels on the river.

NATHAN DENZIN (voice-over): That meant the only solution to save the mussels was to physically walk the shore banks and throw them back into the river.

LISIE KITCHEL: There were thousands of mussels saved.

I mean, individuals were picking up 100 mussels a day, 300 mussels a day.

It was pretty amazing what people were willing to do.

JEAN UNMUTH: It was getting to where I was spending three to 4 hours.

Forget the artifacts.

I was no longer looking at that.

NATHAN DENZIN (voice-over): Anmuth saved more than 1,200 herself.

JEAN UNMUTH: Multiply that times 365 days a year.

And just those 1,200 muscles, you're filtering over 3 million gallons of water.

NATHAN DENZIN (voice-over): 2024 has been called a weather whiplash in the state.

In May, some parts of Wisconsin were still in a drought, but overall, the spring and summer are experiencing an unusually wet season.

Still, should it dry up again, they'll be ready.

ELLEN VOSS: There is a live phone tree muscle rescue alert system in place.

LISIE KITCHEL: They can also contact me anytime, and I can give them advice as to what to do.

NATHAN DENZIN (voice-over): Until the time comes to walk the shore banks again, experts eyes will be glued to the weather.

For PBS News Weekend, I'm Nathan Denzin in Prairie De Sac.