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.