John Page Williams has been preaching the same sermon for 25 years. It's about underwater grasses, clear water and biodiversity. He's deeply enamored of all three, and as a senior naturalist for Chesapeake Bay Foundation he's had a chance to wander the bay for decades seeking them out.

He used to travel long distances to rural outposts to find what he was looking for, but not anymore. Lately, water quality has come to him.

"Isn't this amazing?" he asked last week as three of us solo-paddled CBF canoes on the Severn River just upstream of Annapolis, where he lives. A glance over the gunwale produced a sight you'd expect to see in some pristine backwater, not in sight of the Chesapeake Bay Bridge and the U.S. Naval Academy.

In the shallows 100 yards offshore, strands of wigeon grass, redhead and milfoil waved in the current. Minnows scurried nervously through clear water amid flashes of silver as small rockfish and perch slashed in and out, grabbing snacks.

Six years ago, according to state surveys, not a blade of submerged aquatic vegetation (SAV) existed in the Severn. The bottom was a virtual desert. Today, 400 acres of SAVs there harbor crabs, minnows, perch, pickerel, baby rockfish, menhaden and the like.

Underwater grasses also are blooming in the neighboring Magothy and South rivers, both of which were denuded after the floods of Hurricane Agnes in 1972 delivered what many thought was a final knockout blow to SAVs in much of the upper Chesapeake.

The turnabout couldn't have happened to a more appreciative fellow than Williams, who watched in wonder as Ray's Pond, a little Severn River feeder creek near his house, blossomed into a vegetation-choked classroom for his lectures on the environmental value of rooted aquatic vegetation. These days he spends as much time there as on the road, taking folks like me by canoe to Ray's Pond to show off the restorative powers of underwater weeds.

His message isn't all positive, though. The rebirth of SAVs in some stretches of the upper bay has been offset by significant declines in other areas, particularly the traditionally rich beds of Tangier Sound, where grasses declined 63 percent between 1992 and 1998.

Overall, bay grasses are up in the Chesapeake from an all-time low of 38,000 acres in 1984 to 63,500 acres last year, but that pales by comparison to the historical estimate of 600,000 acres before colonial man came along to mess things up with sewage, fertilizers and endless clearing of land.

When SAVs fail, the quality of a waterway nosedives. Grasses serve as food for waterfowl, nurseries for juvenile fish and crustaceans, and habitat for a broad range of tiny critters that larger species feed on, said Bill Street, restoration manager at CBF.

"It's a keystone species," Street said. "Without SAVs, the rest of the community won't come."

The weeds also help keep the water clear. "They stabilize bottom sediments, consume nutrients, dampen wave energy and reduce flow after storms so sediments can settle out rather than stay suspended in the water," Street said. "They're our best indicator of water quality and the health of the bay."

What makes them grow in some places and not others? "That's the million-dollar question," Street said. "If we knew, we'd be doing a better job restoring them where they're scarce."

Williams guesses the Severn River grasses came back after 20-odd years for a variety of reasons. He said intensive land development in the watershed came in the 1960s and '70s and the land has since had a chance to stabilize. Subsequent development was subject to critical areas regulations that minimize runoff and pollution problems.

As we poked our canoes into Ray's Pond, a short, shallow tidal creek ringed by trees, Williams led us to the headwaters for a first-hand look at how protecting land alongside a waterway fosters life in the water.

"See that little inlet?" he said. "Poke your canoe up in the marsh grass there and stick your hand in the water." I did, but even before I dipped a hand in the water I could feel cooler water on my bare feet through the aluminum of the canoe.

"Now look around at the variety of plant life in the marsh," said Williams, pointing to strands of wild rice, marsh grasses, shrubs and bushes.

Williams said houses that sat discreetly behind trees at the top of the hill above had been built recently along the ridges, while the thickly vegetated ravine that carried off storm water from them was left untouched. The result was a vegetated buffer with cool, clear water trickling down it all summer, which in turn fed a cornucopia of plant life above and below the water.

Across the creek, at the foot of an older community built in the 1970s, a road ran right down the heart of the ravine and warm, dirty water flushed through every time it rained. There, the creek bottom was sandy and bare but for a few struggling strands of vegetation and the water was warm and cloudy.

Obviously, Chesapeake Bay drainage is far more complex than the forces at play on opposite sides of one small creek off the Severn River, but Ray's Pond offered a clear example of how a little care in land use can directly affect the quality of life in the water downstream.

Washingtonians know all about the value of SAVs, having witnessed an explosion of aquatic life in the Potomac after hydrilla, milfoil and other grasses blossomed there in the last 20 years, a phenomenon largely credited to improvements at Blue Plains Sewage Treatment Plant.

There was a time when few folks appreciated the value and significance of submerged aquatic plants, and even sought to poison or mow them down when they clogged boating channels or interfered with swimming. These days, the blossoming of milfoil, redhead, celery and wigeon grass is cause for celebration in the urban and suburban settings where it's staging a comeback. It remains a mystery why aquatic grasses continue to decline in some rural areas where they used to thrive. Are city folks now doing a better job protecting the land than their country kin? It's a good question.

CAPTION: John Page Williams, left, and Shep McKenney inspect life drawn from 400 acres of aquatic grass that has reemerged.