Wild celery, coongrass and half a dozen other underwater plants that have not been seen in the Potomac River around Washington for up to 50 years are beginning to return, according to federal hydrologists.

Last year, after a four-year study, scientists at the U.S. Geological Survey's headquarters in Reston reported that decades of pollution had killed virtually all plant life in the upper Potomac.

"But we said things weren't irreversible and that plants could come back" as the river got cleaner, hydrologist Virginia Carter said yesterday. This summer she and fellow Interior Department scientists were ecstatic to discover their predictions were coming true. "This is very exciting, well at least to me . . . and to those who enjoy fishing and using the river for recreation purposes," said Carter, because "it is a significant indicator of the health" of the river.

It's also good news for turtles, geese, ducks, muskrats, fish and other wildlife which feed on the underwater grasses.

Scientists, however are issuing mixed messages about the river this summer. Blue-green algae, which thrives in polluted waters, also has returned to the upper Potomac for the first time in almost a decade.

The unsightly, floating mats of algae can kill underwater plants by depriving them of sunlight, and last week forced the closing of boat rental facilities at Pohick Bay Regional Park south of Mount Vernon. The algae also has troubled members of the Virginia State Water Control Board, who say the state may have to reconsider its recent decision to relax standards for sewage treatment plants that discharge into the Potomac.

The return of both plants and algae has apparently been aided by heavy spring rains, which washed fertilizer from lawns and fields into streams and also may have washed small aquatic plants from the streams into the Potomac, said Carter.

The underwater plants, which include pondweed, elodea and two completely new species seen for the first time in the upper Potomac--hydrilla and water-stargrass--have managed to survive in small quantities in tributaries of the Potomac, she said. Most haven't been seen for decades in the river, Carter said, and some of the plants may have been transported to the area by wild duck, since their seeds are popular with many species of diving duck.

Carter said she had found a large, thriving bed of wild celery in the Washington Channel, along Fort McNair's seawall. Besides finding grasses growing the wild in various parts of the river from Piscataway Creek to the Woodrow Wilson Bridge, the scientists planted the underwater grasses along the Prince George's County shore, at Rozier Point just south of the Woodrow Wilson bridge, "and they have flourished," she said.

The big question, she said, is can they survive the winter and the blue-green algae, and become firmly established underwater residents.