From the roadside, the thick tangle of branches and wild plants bordering Quantico Creek seems untouched by human pollution.

But when the mayor of Dumfries and several young volunteers plunged into the thick undergrowth on Saturday, they found something unexpected: a huge pile of construction materials.

Mayor Gerald Foreman brushed off the brambles, climbed up from the creek bed and knocked on the door of the nearest building, a combined dental office and church. The man who answered said that the building’s roof and siding had recently been replaced. He had no idea that the discarded materials ended up in the creek area, and said he would take care of it within days.

The mayor promised to check back soon.

“This makes you feel good afterward. Stuff like this, you never would have known it was here,” Foreman said. Then he waded back into the brambles.

Sarah Sunder, 16, picks up some trash during the Potomac River Watershed cleanup. (Julie Zauzmer/The Washington Post)

The group was among the thousands of volunteers throughout the region who picked up trash along roadways and inspected streams as part of the annual Potomac River Watershed Cleanup. Foreman was accompanied by a team of four members of the Junior Reserve Officers’ Training Corps from Potomac High School and one energetic 6-year-old, about a third of the volunteer force that turned up Saturday morning to clean up the watershed in Dumfries.

The high school students showed off their knowledge of the outdoors as they dodged prickling branches and clambered for steady footholds in the untamed environment.

Clay Cornwell, 17, pointed out a beaver dam and a raccoon print — he knew it must be a raccoon because it had five toes. Between his shouts of excitement over a foxhole and a hollow tree that he was able to push over, he picked up a cigarette lighter, a Frisbee and a Nissan hubcap for the trash bags the volunteers carried.

All are tough kids — they described waking up at 3:30 a.m., a time when some of their fellow high school students have barely gotten to sleep, to complete their military drill practices. But when Zach Jamison, 16, found a cicada shell, Sarah Sunder, 16, cringed. “Ah — blech,” she said.

Sunder provided grossed-out sound effects for many of the items, natural and unnatural, that the group found along the stream. As the teenagers filled their trash bags with beer bottles, aluminum foil, a volleyball, two welcome mats, a piece of pipe and many unidentifiable pieces of paper, plastic and Styrofoam, Sunder narrated. “Oh, that’s disgusting! Oh my God, there’s so much more over there. You’re going to get us all killed. Ow!”

But she gamely picked up one piece of trash after another. And she didn’t hesitate to trek deeper into the forbidding area. “It feels like you’re going through an obstacle course,” she said.

Then she leaped across a narrow tributary, about three feet wide. Picking herself up on the other side of the bubbling water, she announced to no one in particular, “I’m actually really proud of myself because I jumped that. I didn’t think I could.”