When Fairfax County Supervisor Lilla Richards was a high school student in the 1950s, she and her friends often rented canoes in Georgetown and paddled over to the Watergate band shell concerts on the Potomac.

"As you passed the Georgetown storm drainage outlet, raw sewage was floating in the water," she said. "It made it a little less romantic."

Richards traveled Thursday on a much cleaner Potomac. The afternoon cruise aboard the Cherry Blossom concluded a two-day conference on the state of the Potomac, exactly 20 years after the river was declared all but dead.

A billion dollars invested in advanced treatment of sewage, principally at Blue Plains, has yielded a healthy river.

The fish are jumping, and the Potomac is clean enough for swimming.

Indeed, the river is one of the few environmental bright spots in the Chesapeake Bay watershed.

But while the conference, sponsored by the Metropolitan Washington Council of Governments, was a celebration of sorts, it also underscored that the region's growth threatens the river's recovery.

"We're all patting ourselves on the back for a success story," said Lee Zeni, executive director of the Interstate Commission on the Potomac River Basin.

However, he added, "you can't rest on your laurels with the growth projections. If we don't do better, we fall back."

James Collier, the District's water hygiene chief, said, "We've got nitrogen and toxics {removal} to deal with. And they're desperately intertwined with growth and land use."

What happens on the land affects the water.

Runoff from farms and housing subdivisions into streams and, ultimately, into rivers such as the Potomac results in nutrients entering the bay and robbing the water of oxygen needed to sustain aquatic life.

However, these "non-point" sources of nutrients are difficult to identify, quantify and control.

The region's population is growing faster than "moderate" projections had estimated. And most of the growth in the region is in largely undeveloped areas well beyond the Beltway.

Loudoun, Prince William, Charles and Frederick counties are expected to account for 40 percent of the population growth but only 13 percent of the employment growth in the next 20 years, according to COG demographer Robert E. Griffiths.

By addressing the identifiable industrial and sewage "point" sources of pollution first, said Ann Swanson, director of the governmental Chesapeake Bay Commission, "we took the easiest {problem} first. Now, we have to address the most politically charged, difficult question: growth. What is the state role in growth management programs? Are there times when we simply say 'no'?"

But John H. Foote, a land-use lawyer with the firm of Fairfax developer John "Til" Hazel, suggested that environmental concerns could translate into an economic elitism that severely limits housing choices for most people.

People have to have a place to live, he said, and closer-in locations carry a premium fewer and fewer people can afford.

"We don't know what we want to achieve in land-use planning," he said. "I fear that environmentalism can be seized by no-growth forces, by people increasingly isolated from the realities of the marketplace."