You don't have to be Paul Bunyan to take a leisurely half-hour stroll around the entire perimeter of the Chesapeake Bay. Even a short-legged kid can do it - at the Chesapeake Bay Hydraulic Model Built by the U.S. Army Corps of Engineers on Kent Island.

In the shadow of the Bay Bridge - the real Bay Bridge - stands a 14-acre corrgated metal shed with a surprised inside: a mini-Bay with all the feeder rivers and islands. It holds 450,000 gallons of water - some of it salty and some of it fresh. And the tide?

"It operates just like the Bay," explains guide Dennis Mtchell: "Fresh watercomes in from the tributary rivers, and salt wate comes in from the ocean." We start our Bay walk at the James River, whose fresh water is seeping into the Bay at a rate determined by a computer hooked to the pipes. As we cross the river on a catwalk leading to Norfolk my daughter drops her brochure into the James, but Mitchell is unruffled.

"As you can see from the brochure floating on the water, the tide is going out right now," he continues. Metal strips in the water are not simulated pop tops but, Mitchell explains, "roughness strips," placed there to put the proper currents in the water.

The guide's voice is almost drowned out by the roar of the ersatz Altantic, a huge tank where brine is mixed with fresh water until the proper salinity is reached. We crossed the Maryland line, glancing over at Smith and Tangier Island, crossing the Pocmoke River, and approaching Salisbury.

"The two black lines are Route 50," says Mitchell.

As my kids ignore the bridge over the Choptank and wade through its in sneakers, Mitchell explains that the Bay was formed 10,000 to 15,000 years ago and the model completed in 1974 at a cost of $10 million. The purpose of the model, he says,is to enable scientists to study the effects planned actions may have on the Bay. On project that was studied was the proposed enlargement of the Baltimore Harbour channel. $"Here were in Baltimore," says Mitchell, adding that the harbor projects has been approved but that some boaters and enviromentalists are upset about where the city plans to dump the dredge material. $"They want to pile it on two unspoiled islands where boaters like to picnic," complains one woman on the tour. "Do approve of that?"

Mitchell declines to becomes embroiled and goes on to the model's next project: a study of the Potomac and its capacity for water supply and sewage treatment.

"We're going to simulate sewage treatment plant discharges, using dyes," he explains. "Now if you really wanted to be authentic . . . " begins one man on the tour but Mitchell diverts attention to the tide of the Potomac.

"Watch the reflection of the lights on top of the water and you'll see it," says Mitchell. "It changes first near the shore. It will start to move upstream there while it's still going downstream in the center."

As the kids maneuver a narrow branch of the York River, we've come full circle.

"Well, we've just walked 800 miles," announces Mitchell.

The others disperse, but, feeling guilty about polluting the Bay, we go back to look for the brochure we dropped in. It has floated down the James River into the Baltimore and appear to be heading for the open sea. BY THE BAY

Free tours of the mini-Bay are given Monday through Friday at 10, 1 and 3. Directions: Take U.S. 50 across the Bay Bridge to Kent Island. Turn right at the first traffic light and go on about five miles to the model.