WE ARE MEANDERING around a meander in quiet Muddy Creek, where the only sounds are the ripple of our canoe paddles in the water, the flute-like call of the marsh wren, and the voice of our guide, ecologist Dennis Whigham:

"That's the narrow-leaf cattail, the only kind that can stand salt water," says Whigham. "Cattails turn brown in late August, but the dead shoots are essential to get oxygen down to the roots. If the muskrats chew off the shoots, the plant won't grow . . . There's a small moth that comes into the flowering heads of the cattails. It eats the seeds and spins a web that keeps the whole mass together. Normally the heads would be gone by this time."

What happens in an estuary -- a place where fresh water meets salt water -- is one of the things the scientists at the Smithsonian Environmental Research Center, a 2,600-acre preserve in Edgewater, Maryland, are studying. Now, they are giving the public a peek at the wonderful world of the wetlands in two-hour canoe expeditions and on guided walks, both of which are available free by prearrangement.

"Now we'll start working our way down the gradient," says Whigham, leading our convoy of 13 life-vest-clad people in six canoes downstream, toward the place where Muddy Creek meets the Rhode River, an arm of the Chesapeake Bay. The cattails are bending in the breeze, and some cattail shoots are floating in the water, a sure sign that there are muskrats around. The creek hosts an active muskrat lodge, which looks like a pile of brush, and river otters live in holes dug out of the creek banks.

The scientists have also built structures in the estuary -- not homes, but a device that measures the flow of water, and a fish weir, a V-shaped trap that temporarily detains the fish swimming either upstream or downstream.

"We can document the movement of the fish," explains Whigham. "They come into the creek to breed, and the young fish stay to feed. Marshes are incredibly important as places where fish raise their young, find shelter from predators, and get nutrition."

Silverside minnows and pumpkinseed sunfish live in the creek full time. White perch and crab come to spawn, and blue crabs come to molt. Smithsonian scientists attach ultrasonic devices to some of the crabs to track their movements. Another device registers every time a crab chews, recording exactly how many bites it takes the crab to devour a clam.

Below the fish weir, the creek that began as a freshwater stream with the forest at banks' edge gives way to salt marshes and tidal flats, where the canoes must turn back. The return trip is upstream, but the tide and the wind are in our favor.

"Quite often we see kingfishers, and once in a while a bald eagle," says Whigham. "And there's a tree swallow -- you can see a flash of white belly. We have a pair of red tails that breed here, and a lot of marsh wrens. They cut off cattail shoots to weave nests, and they usually build four or five fake nests, without any young."

Later, back on dry -- or mostly dry -- land, we see the estuary from a terrestrial point of view, with animal ecologist Jim Lynch and docent Gretchen Seielstad. The two lead us down the Discovery Trail, a 1 1/2-mile journey through several kinds of environments, from forest to marsh. We start at what Lynch calls an old field, a pasture abandoned forty years ago. By rights, the area should have been reforested by now, but reforestation has been thwarted by vines.

"When we think of competition, we think of animals and birds, but plants are fighting it out for food and water, too," says Lynch, pointing out honeysuckle, wild grape, Virginia creeper and the thick, hairy vines of poison ivy.

In another place on the trail, the forest has begun the process of succession -- of regenerating in a place that was originally forest but was cleared for farm use. The first trees to succeed are pioneering trees, such as tulip poplar and black cherry, trees that grow fast and need a lot of light. Farther on, the forest canopy grows thicker, allowing only dappled light to fall on the trail. This, says Lynch, is a mature forest, where hardwoods such as oak and hickory have replaced the pioneering trees, which can't get enough light.

From the dark, cool forest, we emerge into blinding sunlight and follow a raised boardwalk across Hog Island Marsh.

"The greenbriar here is a favorite browse for deer," says Lynch. "You can see the woody plants being replaced by grasses. The last trees to go are the willow oaks. In colonial times, the salt marsh hay was used for fodder. It's a big, floating mat of vegetation, like a big peat bog. You can see pathways made by the muskrats through the marsh."

The trail also takes us past evidence of human incursions into the environment. There is a computerized field weather station, for example, which charts temperature, humidity, windspeed, sunlight and other variables, and boxes left by the scientists to collect leaves. There is also evidence of less scientific human activity.

"They're such tempting targets," says Lynch, as we gather around thick beech trees carved with every combination of letters in the alphabet. "I've never seen a beech tree that wasn't covered with initials." IN THE CANOE AND ON THE TRAIL

Guided canoe trips are offered seven days a week through October by prearrangement. No canoeing experience is necessary, as basic instruction is included. Children between 8 and 12 may sit in the middle of the canoe. Paddlers must be 12 or over. The trip lasts about two hours.

Guided Discovery Trail walks are offered weekends, year-round, by prearrangement. Children should be 10 or older. The walk lasts about two hours, and sturdy walking shoes, long sleeves, long pants and insect repellent are recommended.

Both activities are free. Reservations are required. For reservations, call Linda Chick, 301/798-4424. The Smithsonian Environmental Research Center, which is about an hour's drive from Washington, also offers a variety of activities for school and summer camp groups. Call the above telephone number for information. GETTING THERE -- From the Beltway, take U.S. 50 east toward Annapolis, exit on Davidsonville Road (Rte. 424 south), follow 424 to the traffic light at Rte. 214, turn left onto 214, go to third traffic light and take a right on Rte. 468, go .9 mile and turn left at the sign for the center.

Anne H. Oman last wrote for Weekend on running races for families.