PORTLAND, Ore. -- A group of scientists and volunteers took to the waters of the Willamette River near Corvallis over the summer to get an up close look at one of the largest known mussel beds on the river.

Willamette Riverkeeper executive director Travis Williams lead the expedition. What he and his team of researchers discovered under the water was breathtaking: Tens of thousands of fresh water mussels.

The species was the Western Pearl Shell.

Many of the mussels were close to 100 years old.

But when the researchers went to count the juveniles in the area, it appeared the mussels had stopped reproducing.

"We did not find a juvenile among that entire bunch," Williams said.

"It's a red flag for sure," said Celeste Searles Mazzacano, an entomologist who has studied freshwater mussels for more than a decade.

She said freshwater mussels are critical to river health for several reasons. For one, they're natural filters and cleanse the river of particles and bacteria. They also provide habitat for aquatic insects that salmon depend on to survive.

"These guys are just really super-hard working components of a healthy stream," she said.

What could be causing the problem? The experts say it could stem from a change in river flow due to dams, warmer water temperatures, or too much pollution.

Williams said although the study focused on an area of the Willamette River near Norwood Island, it makes him wonder if the problem is occurring elsewhere.

He admitted more studies need to be done so we can all better understand these often hidden and overlooked native creatures.