The shorts- and sneaker-clad band of men, women and children hunched intently over the shallow stream, occasionally scooping up rocks. They dropped the rocks into a bucket of water, gently scraped them with their hands, then poured the water through an embroidery hoop covered with sheer fabric.

A lost tribe of suburban hunter-gatherers? An Introduction to Gold-Panning class? Neither. The group of about 40 local residents was monitoring water pollution, aided by several handfuls of slithery, squirmy fly larvae.

Participants in the recent "How to Adopt a Stream" workshop learned to monitor stream pollution by collecting and examining fly larvae that dwell beneath rocks in fast-flowing portions of a stream. Different species of the larvae have varying levels of tolerance for pollution, making the tiny creatures living indicators of a stream's health, said Linda Fields, Howard County's recycling program manager and one of the workshop's organizers.

After listening to a slide program and lectures at Columbia Mall, participants set out for the middle branch of the Patuxent River. Their mission: collect, identify and number the stream-dwelling larvae of the stonefly, the mayfly and the caddis fly. The stonefly, according to Fields, lives only in pristine streams. Mayflies can tolerate a little pollution, while the caddis fly "is the least picky. It will live almost anywhere."

The group tested the water at several streams and locations, including spots downstream and upstream from a construction site. Participants saw the effects that erosion and sedimentation have on a stream, said Deborah Ward, deputy director of Save Our Streams, the statewide conservation group that sponsored the workshop.

"People didn't even get out and test the water at the {downstream} site," Ward said. "It was muddy. People just looked at it."

When the fly larvae gathered by participants were categorized and counted, the results were fairly good, Ward said. Most of the larvae collected were mayfly specimens, indicating that the Patuxent is relatively clean. The county's use of separate pipes for storm water and sewage contributed to the good finding, according to Fields.

Kathie Duck, an Ellicott City science teacher, says her 8- and 10-year-old sons relished their stints as naturalists/pollution fighters.

"We were really impressed that within a couple of minutes so many of us could come up with the creatures we were looking for," she said. "They felt it was worth their time."

The workshop included crash lessons on the ecology of a healthy stream and on how the watershed and surrounding land areas affect a stream's health. Residents learned how to spot illegal sediment runoff at construction sites.

Perhaps the workshop's chief benefit was raising awareness, Fields and Ward said. Most of the participants said they plan to "adopt" a local stream, using insect kits supplied by Save Our Streams to monitor the stream's health.

While environmental awareness is increasing in the county, residents remain unaware of simple things that directly affect the health of local streams, Fields said.

"Many people . . . don't realize a storm sewer is not a sewer at all . . . our storm drains go directly to streams. Also, when people fertilize their lawns, they don't realize it ends up in {Chesapeake} Bay.

Duck says she and her sons plan to adopt a stream.