Wearing hipboots, Dave Plott and Ken Shanks stepped over a pile of rusted beer cans into the muddy water of the Collington Branch of the Patuxent River, taking oxygen readings from the brown stream flowing beneath a swirl of autumn leaves.

Plott, who works for the Maryland Department of State Planning, and Shanks, of the state Department of Health and Mental Hygiene, were members of a 12-person team from various state and county agencies who spent the day in Prince George's County hiking the tributary of the ailing river system.

Yesterday's expedition is part of an attempt to assess the extent of pollution and other problems in the 110-mile river and its tributaries, determine what can be done to reverse the damage, and -- if possible -- prevent further harm. It was the second day experts had been out studying the 88-mile Collington Branch, and the effort is being repeated throughout the seven counties that make up the Patuxent River watershed.

The Patuxent, Maryland's largest intrastate river, was once a clear, fast-flowing waterway, serving as a major thoroughfare for tobacco traders, a rich harvesting ground for clams and oysters, and a popular spot for hunting and fishing.

Today, the Patuxent is "a river in distress," according to the Patuxent River Policy Plan, adopted earlier this year in an attempt to revitalize the river. Sewage treatment plants dump increased amounts of waste as the population grows. Increased development brings more "urban runoff," the residue of fertilizer, pesticide, oil and other pollutants that is eventually washed into the stream. Algae and sediment clog the water, causing a drop in the number of fish and blocking the growth of aquatic plants.

"People are used to it being a beautiful, clear river," said Michael Pawlukiewicz of the county Department of Environmental Resources, who is directing the Prince George's part of the project. "The Patuxent generally has seen some serious decline."

The evaluation is the first comprehensive study of its type in the state. Environmental planners hope it will serve as a model for future water studies, including the newly created Chesapeake Bay Critical Areas Commission, which is to undertake a similar analysis of the bay region.

Population in the 582,500-acre Patuxent River basin grew by 42 percent between 1970 and 1980 and is expected to increase 35.4 percent more to a total of 477,570 by 1990.

"The type of urban development we've had in the past has caused pollution in the river," said W.C. (Bud) Dutton Jr., the Prince George's County representative on the Patuxent River Commission. "But with the right type of environmental controls, that doesn't have to be."

Those who hiked the stream yesterday took readings of oxygen levels to determine whether fish and plants could survive, looked for signs of runoff from various pollutants and examined the banks for indications of erosion. They came from an array of state and county agencies, including the county Park and Planning Department, the Washington Suburban Sanitary Commission (WSSC) and the county Soil Conservation District.

"These are people who generally work from maps and abstraction," Pawlukiewicz said. "This is kind of a link to reality. Part of the idea is to get them out in the field to see what they're working on."

Pawlukewicz decided to start with the relatively undeveloped Collington Branch because a comprehensive rezoning and master plan revision for the Bowie-Collington area are both under way, along with a WSSC revision of its storm-water runoff management plan for the region.

Although the Collington Branch appears to be in fairly good shape, according to Pawlukewicz, at the start of the day yesterday he found garbage littering the stream off Peerless Avenue in Upper Marlboro. Empty oil cans and beer bottles were scattered along the bank, while a brown plastic garbage bag was snagged in a dead tree limb. Farther up the street stood a junked garbage truck, with bags of waste overflowing from it.

"I'm really excited," Pawlukewicz said. "This is the kind of stuff we can do something about."