After four years of dramatic improvement in water quality and contaminant level, the Charles River received a "B" from the U.S. Environmental Protection Agency (EPA) in its annual study of water quality.

Officials from federal, state and local government as well as community environmentalists were not discouraged by the grade, which means the river meets standards for safe boating. Officials at a press conference on Friday said they aim to make the Charles safe for swimming by Earth Day 2005.

Supporters of the river's cleanup effort acknowledge that the most difficult steps will come in the final stages of the cleanup.

"Last year, we reached a plateau because it was straightforward, just find the point source [of contamination] and the sediments," Robert Zimmerman, executive director of the Charles River Watershed Association said.

Though most of the sewage pipes flowing into the river have been identified and stricter regulations for storm drains have been set in place, there are still unidentified sources of pollutants, according to the officials. Sediment on the river bottom also presents a challenge to meeting the 2005 goal, for it can take years for another layer to form on top of the river bottom.

Nonetheless, those close to the clean-up efforts remain hopeful that the river will receive an A grade by 2005.

"We are fully intent on swimming in the Charles in 2005," said acting EPA New England regional administrator Ira Leighton at Friday's conference.

Wildlife along the river has improved since the clean up efforts began. This past year, porpoises and dolphins were seen in the Boston harbor and herons and other waterfowl are increasingly seen along the river's banks. The EPA predicts a record herring run up the river in the next few weeks.

"There used to be an odor, you used to see things floating in the river, but now you're going to see bubbles while the herrings make their migration," Leighton said.

The river's improvement is so dramatic that it looks nothing like it did thirty years ago, river advocates said.

"The only thing you could see at the MIT beach back then was carp,

One of the sturdiest fish," said Roger Frymire, a volunteer for the Charles River Association. "Up in Nashua, there was a paper company literally turning the river a different color every day."

The effort to save the river came from a combination of community action and government support.

"This really began back in the 1970s with the Clean Water Act, but the efforts of the Charles River Watershed Association show what a good nonprofit can do with some good science," said Roger Abele, a local EPA official who has been paddling on the Charles for the past 30 years.

For the past 7 years, the Charles River Watershed Association has taken samples for research into the river's condition. This year, volunteers took over 300 samples from 10 different sites to find sources of contamination in the river. The association then uses this information to inform public officials about problem areas.

"The Charles River Watershed Association shows what good non-profits can do with good science," Abele said.

To meet the goal of a swimmable, fishable Charles River, activists will now focus their efforts on educating the community about the importance of the health of the river and will also continue their campaign to encourage individual responsibility.

"The last part of this all will come from homeowners, individuals, dog owners who will change their behavior to reduce waste," according to Ira Leighton, the acting regional administrator of EPA New England.

Community education is another facet of the clean up efforts. The Watershed Institute, run by Max Kennedy, takes inner-city youth to the river. Through observation, the students learn the river's environmental value and will encourage people around them to respect the river, Kennedy said.

Environmentalists will continue to focus on reducing storm water runoff into the river, which carries with it pollutants from gasoline to laundry detergent.

"One of the reasons we've had no change in the river's grade was because this was a very wet year," Zimmerman said.

M.I.T plans to help the effort by sponsoring a contest for the best storm-water drain.

Activists are encouraged by the efforts targeted to make the Charles safe for swimming.

"Ten years ago, people had given up on the river. Now people want to spend their time recreating on it," Stephen Greene, a member of the Clean Charles Coalition said.

--Staff Writer Nicole B. Usher can be reached at usher@fas.harvard.edu