Maryland's largest river is the Patuxent, which flows along the back borders of some of the state's busiest counties -- Montgomery, Prince George's and Anne Arundel -- as well as more rural Howard, Calvert and St. Mary's.

With all the growth the Patuxent has tolerated over the last 50 years, it might well be a dirty, decaying place to avoid, particularly in the sections closest to Washington. But Jerry Phoebus, who fishes it regularly near his home in Prince George's, will tell you he'd rather be on the winding, marshy backwaters of the Patuxent than just about anywhere else in the state.

There's wilderness here, 10 miles from the Beltway.

"Up ahead there's a sunken boat we call the barge," Phoebus said toward the close of a delightful, modestly successful bass fishing trip last week. "We should pick up some monster bass in there."

He eased his boat up the shoreline and killed the engine, but before he'd retrieved the first cast it was clear something else already had been fishing there -- something about five feet long and scaly, with a hinged jaw and a large meal to digest.

Phoebus drew closer with the electric trolling motor until the boat crunched to a stop. The object of his attention, perched in the sun on a timber of the wreck, was unperturbed.

A big snake it was, obviously fresh into a meal of a 10-inch-long catfish. Everything but the fish's head protruded from the snake's vast jaw, and by flipping its tail the victim made clear it had not yet expired.

A snake consuming a fish bigger around than the snake itself is quite a sight.

If you've ever pulled the drawstring out of a pair of sweatpants and had to rethread it, you can appreciate the eating technique. The snake held the catfish steady in its jaw, then drew its slinky body forward around more fish. When the snake eased its scrunched-up muscles and relaxed its jaw for a moment, the fish was drawn inexorably down the gullet.

Nor did it take long. It seemed every time you looked there was less fish, and in about three minutes the grisly drama was complete. A great lump lay just aft of the snake's jaw, and no fish at all was visible outside.

"That should hold him for a week or two," Dr. Vagn Flyger of the University of Maryland's Animal Sciences Department said later, after confirming that it was doubtless a water snake, harmless to humans. Flyger said cold-blooded snakes generally eat only one-tenth as much as warm-blooded mammals do, so "that was a good catch."

Certainly it was better than the snake's human competitors were doing. After four hours of fishing, they'd boated only four largemouth bass, and "now we've wrecked that spot," said Phoebus, eyeing the cloud of mud raised as he backed out with the trolling motor. "But it was worth it."

That it was, as were many other sights along the way as we explored waters Phoebus said haven't changed in more than a decade.

The Patuxent is under close scrutiny these days as a kind of talisman in Maryland's efforts to clean up the Chesapeake Bay. Scientists regard the river as a small-scale replica of the bay overall, with many of the same problems of overpopulation, but on a more workable level.

Maryland is planning to spend millions over the next several years to reduce nutrient enrichment of the river from the sewage treatment plants that pump effluent into it and to limit runoff from farms, urban storm sewers and land development. Scientists will monitor the effects.

"The theory," said Rich Dolesh, who runs the Maryland-National Capital Park and Planning Commission's Patuxent River Park, "seems to be, 'If you can't save the Patuxent, you'll never save the Chesapeake.' "

There are obvious potential problems. Reliable sources estimate that during periods of extreme low-flow in summer up to half the water flowing past Jug Bay, where Phoebus fishes, is treated sewage effluent.

Evidently the treatment plants are doing a good job, because the river keeps producing and the wildlife, with some exceptions, thrives. Phoebus, who owns a flower shop in College Park and has lived hereabouts all his life, had the best crabbing he can remember this year, and bass fishing remains as good as ever in his favorite haunts, he said.

Ducks also abound in the Patuxent estuary, feeding happily in the miles of marshland bordering the winding main stem. The largest concentration of Canada geese on the Western Shore of the Chesapeake overwinters here, feeding at the federal Patuxent Wildlife Research Center in Laurel and at Merkle Wildlife Refuge a few miles below Wayson's Corner.

But not everything is rosy. Dolesh worries about oysters, canvasback and black ducks, rockfish and bald eagles, all of which have had their troubles.

"The river is holding its own," said Dolesh, "but barely."

Which leaves him, Phoebus and a lot of other fans of the wild, not to mention the snakes, ducks, geese, fish, herons, ospreys, frogs and muskrats, hoping for more.