Harry Pierson, a freshman at McLean High School, slipped a canoe into the water of Accotink Creek with the resolve of a pioneer. No cozy classroom for him. "It's cold. I like being cold," he said.

In the name of science, Pierson and about 10 other students from the high school set off in teeth-gritting winds to explore the riches of a Potomac tributary in the northeast corner of Fairfax County. The Chesapeake Bay Foundation sponsored the trip.

What the students found, said William Portlock, Virginia field director of the foundation, is that the creek has "too much of a good thing."

They found out the hard way. Only 300 yards from shore, their paddles suddenly sank deep into silken mud -- mud so thick it finally brought their canoes to a halt.

It was Pierson's second time on the water, and with awkward but ambitious strokes, he managed to propel his canoe toward the middle of the creek. That is when some of his soldierly spirit vanished.

"God, we could have walked out here," he said, his canoe barely afloat over 10 inches of water.

"Why don't we just stay here and wait for them to pull us?" yelled Shannon Jarrot from her grounded canoe.

James Sproull, their science teacher and the only one from McLean High wearing thigh-high rubber boots, came to the rescue on foot. He tied a line to the bows of the stranded canoes, and pulled them along a meandering waterway that led to the middle of a marsh.

"What am I standing on here now?" he yelled to the canoeists as he stepped onto a mud bank.

Mud, naturally, the students yelled back.

"The correct word is sediment," he said. "What we are seeing here is the topsoil of Fairfax County. When you go home and see your parents put fertilizer and lime and mulch on the soil, well, a lot of that ends up right here."

As they sat cold and immobilized in the useless canoes, they heard about the days when three-masted schooners could sail up the creek to haul lumber. Then, when the land was cleared and the soil cultivated for planting, rain swept sediment upstream, which carried it to Accotink Creek.

"Estuaries are traps, and what goes in stays in," said Portlock.

That was the way many of the students felt about their situation, as they struggled to unstick their canoes from the waters' muddy bottoms. Of course, they had been told that marshes have their good side: They provide nutrients to plants and fowl, they are a home to beavers and they are beautiful when the yellow pond lilies bloom.

Even so, many of the young explorers, standing cold, wet and dirty on the shore, nodded quietly when Portlock said again: "The creek is experiencing too much of a good thing."