Fresno State students and Kiwanis key club members joined other participants in the Great Sierra River Clean-Up this weekend.
Some scoured the water by canoe, and others swam with snorkels.
The volunteers pulled about 300 tires out of the river, including one they believe is a model-A tire from the 1920s or 30s.
Organizers say they've removed more than 7-thousand tires from the river over the years, and most of them are from tractor trailers.
"I believe the dumping was done on a commercial scale, and then flood waters have just carried the tractor trailer tires down the river. We've pulled tractor trailer tires from 15 feet up in trees," Rivertree Volunteers President Richard Sloan said.
Sloan says the tires and other trash are a hazard for many reasons, including the chance of chemicals ending up in in our drinking water.