So much trash. So little time.

The 15th annual Potomac River cleanup scheduled Saturday will last only three hours, from 9 a.m. to noon. But within that short span, organizers hope to have more than 4,000 volunteers scouring 133 sites in the watershed, which encompasses 21 counties in four states and the District.

Last year, the target was tires, and 2,406 were collected, along with 1,867 balls, refrigerators, stoves, auto parts, mattresses, furniture, bicycles, even a 1957 Chevy.

This year, it's plastics.

The bane of environmentalists, the synthetic material has also been good for laughs (as in the one word of career advice imparted in the 1967 movie "The Graduate"), as well as jobs and the economy. But cleanup organizers say it's no laughing matter.

"We're not saying people should give up using plastic bottles," said Michelle Radez, cleanup coordinator with the Alice Ferguson Foundation, the Accokeek nonprofit group that began the cleanup tradition at two sites in 1989. "The focus is on trash that is very prevalent in people's lives. We decided on plastic bottles, because they're everywhere."

While in past years the organizers tracked the number of tires, they plan this year to get a good count on plastic bottles. But beyond that they are offering no solutions.

"We don't have the answer," said Tracy Bowen, the foundation's executive director. "We know what the problem is."

Radez said that "if we can provide the data on how many are out there, others can figure out how to make it better."

Among this year's cleanup sponsors is the Arlington-based American Plastics Council. "Obviously, we think plastics are a major benefit to society," said Judith Dunbar, the council's technical assistance manager. "They bring a lot of good to people's lives."

Because plastics are lightweight, she said, products that are shipped in them cost less, thus reducing fuel and energy costs.

"We certainly don't want to see plastic bottles litter the Potomac or anyplace else," Dunbar said. "We want them recycled or disposed of properly. Eighty-one percent of the population has access to plastic recycling, so there is no reason for people not to do it. They definitely shouldn't litter."

Beyond plastics, the Potomac cleanup's long-term goal is to make the river trash-free in 10 years.

"We want to put ourselves out of business," Radez said. "We could keep doing this forever and ever. It's a great community event. It brings a lot of people out to the river. But there has to be a point where we're changing our behavior, so we're not polluting the river in this particular way.

"There are some things citizens have no control over, like toxic chemicals and sediment, but trash they do have control over."

During the cleanup's 14 previous years, 24,000 volunteers removed 744 tons of trash from the Potomac and its tributaries. Organizers said the volume of tires has dropped during that time, even as other categories of trash have increased.

The past couple of years, Radez said, volunteers have removed more electronic items, such as computers and videocassette recorders, from the stream banks and river shores. Last year, volunteers hauled 122 tons of trash of all kinds from the watershed, which takes in 14,670 square miles.

The Potomac proper runs 383 miles from the Fairfax Stone, just inside West Virginia, to Point Lookout, where the river empties into the Chesapeake Bay. About 4.6 million people live in the watershed, including 3.7 million in the Washington area.

This year's cleanup sites are in nine counties in Virginia, seven in Maryland, three in Pennsylvania and two in West Virginia. Fairfax County has the most, with 34, followed by Prince George's, with 25; Charles, with 22; Montgomery, with 13; and the District, with eight. To locate a site near you, visit www.PotomacCleanup.org or call 301-292-6665.

The District sites are Fletcher's Boathouse; Kingman Lake; Kenilworth Aquatic Gardens; the Mall, West Potomac Park; Picnic Grove #1, Rock Creek Park;. Watts Branch, Anacostia River; Anacostia Park; and the Washington Canoe Club. All still had spaces for additional volunteers on Monday, except for the Washington Canoe Club.