Counting Mussels in the Cold Reach of the Millrace

The question

In spring 2026, environmental science senior Delia Marchetti won the Chancellor’s Undergraduate Research Prize for two years of Millrace water temperatures. Her 14 loggers showed a 400-yard reach below the old Porter Woolen Mill site where groundwater seeps keep summer water four to six degrees cooler than the rest of the channel. She argued that this was why native freshwater mussels still turn up there and almost nowhere else.

Dr. Grace Beaumont, professor of biology and a freshwater ecologist, wanted to test that argument properly. Is the cold reach really where the mussels are? Are they reproducing, or are they old survivors? And what happens to them if the city’s planned stormwater changes near the channel alter the seeps?

The approach

The project combines three kinds of data:

• Mussel surveys. Timed snorkel and wading searches in 25-meter blocks along the whole Millrace each July and September. Every mussel is identified, measured, marked with a small numbered tag and returned to the spot it came from.
• Temperature. Readings from Marchetti’s four loggers that the city’s parks staff kept in place, eight new loggers in the cold reach, and the two Millrace stations of the Riverwatch network, which report every 15 minutes.
• Seep mapping. Students walk the channel on cold winter mornings with a handheld thermal camera to find open-water seeps, then measure their temperature and flow.

The work is coordinated with Dr. Everett Yoon, whose Riverwatch network recently won a $1.84 million award to extend monitoring onto the upstream tributaries, and with Dr. Ruth Okonkwo-Barrie, who supervised Marchetti’s original study. It is based in the Freshwater & Watershed Laboratory.

Students on the team

• Ingrid Solvang, senior, biology — field lead; holds the mussel-handling permit training and runs the survey blocks.
• Marcus Delacourt, junior, environmental science — temperature loggers and the Riverwatch data pulls.
• Pa Houa Xiong, junior, biology — species identification and the tag database.
• Caleb Strand, sophomore, environmental science — thermal-camera seep mapping.

Every field day follows the university’s field research safety rules: two-person teams, waders and a float plan, and no work in the channel after a storm while flows are up. Handling is kept to a minimum. Each mussel is out of the water for less than two minutes, measured on a wet tray, and pressed back into the gravel in the same spot, siphons up. The crew also logs what it finds that is not a mussel, including shopping carts, which the alumni of the 1970s will be pleased to hear are now rare, and one intact 1950s bicycle that is now displayed in the lab.

What they found so far

After two survey seasons (summer 2025 and summer 2026):

• The team found 214 live mussels of four native species. 187 of them, 87 percent, were inside the cold reach that Marchetti identified.
• Daily maximum temperatures in the cold reach stayed below 24 °C through both summers; the rest of the channel exceeded 27 °C on at least 18 days each year.
• Thermal mapping located 11 seeps along the reach. The three largest sit within 30 meters of each other below the old mill foundation.
• The team found 23 mussels shorter than 30 millimeters, which suggests at least some recent reproduction. That was the result Beaumont considered least likely when the project began.

“Delia’s map told us where to look. We expected to find old animals hanging on. Finding small ones means this reach is not just a refuge; it may be a nursery.”

— Dr. Grace Beaumont, professor of biology

What’s next and who’s involved

The team will share its seep map with the City of Porter Falls before the stormwater project is designed, with a recommendation that runoff from the parking lot above the reach be infiltrated rather than piped into the channel. Next summer it will add environmental DNA sampling to look for mussels too small to find by hand, and look for the host fish that mussel larvae need, building on the brook trout already using the restored Millrace fish passage.

Funding: $96,000 over three years from the Wesconsin Aquatic Heritage Fund and $12,000 from the Porter River Riverkeepers, with temperature data shared from the Riverwatch network. Students typically start after BIOL 2500 Ecology or ENVS 3500 Environmental Sampling and Analysis.