They made their way down the Potomac River in dozens of canoes, scanning the calm waters and wooded shorelines for anything that didn’t belong: floating tires, bottles, cans, pieces of furniture — the man-made refuse that constantly infiltrates one of the Chesapeake Bay’s main tributaries.

On Aug. 9, scores of people filled 35 canoes to join a watershed cleanup effort organized by Keep Loudoun Beautiful, a nonprofit organization dedicated to preserving Loudoun County’s scenic beauty and natural resources and educating the public about the area’s environmental challenges.

Over several hours on a sunny summer afternoon, the team of volunteers collected an alarmingly typical volume of debris from the river and its banks: more than 90 tires, 45 bags of recyclables and about 20 bags of trash, enough to fill an eight-cubic-yard dumpster with trash bags, said Blake Netherwood, vice president of Keep Loudoun Beautiful and chief organizer of the watershed cleanup.

“It could have been so much more than that,” he said of the tires and other refuse pulled from the river and its banks. “But you run out of space in the canoes.”

The organization typically conducts two watershed cleanups a year: one along the Potomac on Loudoun’s border and one focused on internal areas of the county, Netherwood said. In April, the group combed the shallows of Goose Creek and its tributaries. In addition to the watershed cleanups, Keep Loudoun Beautiful also organizes smaller-scale community cleanups across the county throughout the year, Netherwood said.

New volunteers are sometimes surprised by the things they find in the river, including the large number of tires, he said. Some people pitch their old tires into the water rather than pay a fee to have them disposed of properly, he said.

“Joe Homeowner changes his tires every few years, and if the garage charges him for getting rid of the tires, what does Joe Homeowner do? If he was a responsible type, he’d go to the landfill and spend two or three bucks a tire,” he said. “I’m not saying that people don’t do that, because they do. But it doesn’t take very many of them not to do that before suddenly you have a lot of tires appearing in the waterways.”

Netherwood, who started volunteering with Keep Loudoun Beautiful more than 10 years ago, said the organization has done its best to keep up with the rapid population growth in Loudoun and the effects of that influx of people on the county’s environment. Those effects often go unseen, Netherwood said.

“In general, the amount of trash in the Potomac doesn’t change that much. And why is that? Well, it isn’t because the population isn’t growing. It isn’t that we aren’t having more and more people leaving more trash,” he said. “The reality is that one or more times per year, typically several times a year, there are outrageous amounts of water that come down the Potomac and the creeks, and it flushes them out.”

He said that heavy rains don’t erase the trash problem, but just move it a few miles downstream or into a larger body of water, such as the sensitive and struggling environment of the Chesapeake Bay.

Keep Loudoun Beautiful was begun in 1972, when founder Agnes Harrison and several other county residents collected roadside trash and took it to the Loudoun County Government Center, where they presented it to county supervisors as evidence of a growing community problem. The nonprofit organization has generally received about $15,000 in funding from the county government — until last year, when mounting budget pressure led supervisors to cut funds to certain nonprofit groups, including Keep Loudoun Beautiful.

The organization is surviving on money that remained from the year before, Netherwood said, along with support from commercial and private donors. The group hopes that funding will be restored from the county or other sources in the next fiscal year.

“It’s a real challenge for us,” Netherwood said. “We’ve been entirely self-sustaining, and we don’t really have contributions from individuals or firms exceeding $1,000.” Running even a bare-bones watershed cleanup event costs upwards of $2,000, he added.

The organization’s work is ongoing by nature: There is no “mission accomplished,” Netherwood said, only the hope that the group’s efforts will help encourage people to recycle and reuse materials so that less litter will end up in the county’s roads, waterways and natural landscapes.

“Keep Loudoun Beautiful is all about recognizing that we’ll never be in a position to say — well, we’ve done our job. People are going to do things that we would rather they not do in terms of spoiling landscapes and waterscapes,” Netherwood said. “As citizens, we can look the other way and not do anything about it, or be a part of Keep Loudoun Beautiful and try to do something about it.”