On Saturday the National Park Service ran a free boat trip down the Potomac to show 30 citizens how alive Washington's River has become.

They took oxygen samples and tested for chlorine and orthophosphates and nitrates. They checked for siltation and acidity. Everything looked great.

But the crowning touch was to come when biologist Jim Rasin dragged a net behind the boat in an effort to pickup all kinds of aquatic life. He had told everyone aboard that there were 64 documented varieties of fish and marine life in the river, from bass and shad to eels and minnows.

He dragged the net at Fort Washington; he dragged it off Hains Point; he dragged it off the lightship Chesapeake in the Washington ship channel. And each time it came up barren.

Rasin loves the Potomac. He grew up fishing here and now he works for the Interstate Commission on the Potomac River Basin. He knows the river is getting cleaner. The empty net was breaking his heart.

"I hope they don't think the river's dead," he said after everyone had left. He was leaning against the railing at Hains Point, watching the river go by.

From the distance came a little aluminum skiff. It pulled up 50 feet away. Two guys aboard started pitching plastic worms against some abandoned pilings.

One anglers' rod bowed double. He heaved back on the tackle, started reeling and before Rasin had time to smile, a 2-pound, 12-ounce largemouth was flailing away in the boat.

"Hot dog," said the scientist. "I had a feeling that was going to happen."

Bass in the ship channel? By all means.

In fact, enough bass to lure two bass clubs to D.C. waters to stage tournaments this month.

The fish Rasin saw won the Ft. Meade Bass Club's tournament. Two weeks earlier the Laurel Bass Anglers has convened in the nation's capital and hauled in 55 pounds of largemouth in one day.

Said Sgt. Richard Tatum, who belongs to both the Laurel and Meade outfits, "The Potomac was the best tournament the Laurel club has had all year."

Largemouth bass are not the world's most demanding water quality sticklers, but they do not prosper in foul waters. Does this mean the Potomac is getting better?

Says John Brink, Chief of the District's bureau of air and water quality control. "The river is in better condition around D.C. today than anyone living here can remember it ever being.

The water has improved to the extent that if you go out and look for bass in the city proper you can find them. This was not true several years ago.

"In the Washington Channel alone," he added, "we've eliminated something like 13 overflows where raw sewage used to get in." Those improvements came during the Southwest redevelopment of the early '70s, when sewer and storm drains were separated so heavy rains no longer would send massive amounts of sewage into the river.

Brink said water quality in the ship channel nowadays sometimes matches that of waters around Chain Bridge, which are practically unaffected by pollution from the Anacostia River, Alexandria and the District's Blue Plains sewage treatment facility.

How clean is the Potomac? Tough question, because like all fast-flowing streams it changes, day by day. But Brink and his colleagues are optimistic. New standards they have drafted and sent to the city council include a goal of approving sections of the river above Key Bridge and parts of upper Rock Creek for swimming and other "primary contact" sports.

When? No one is saying. But it's a goal, and that's a start.

As it stands, D.C. waters of the Potomac both above and below Key Bridge are approved for "secondary water contact recreation," meaning boating, fishing and recreation along the shore. The same stipulations apply to Rock Creek and the Anacostia.

As for the fish, Brink said the government has no ban against eating fish taken from the river. It does, however, recommend that all fish taken for eating be cooked thoroughly.

None of which affects the bass club folks. They put their fish back alive, so they can catch them again when they come back next year.