Catherine Mull peered down at the water bug her young daughter had ladled out of Sugarland Run's icy waters.

"Hurry, daddy!" squealed daughter Erin. "I caught a critter!"

Kerry Mull hurried across the mossy stones to get a closer look at what his wife and their 8-year-old daughter were picking out of a small net.

"Hmmm . . . . Six legs, a hooked tail. Yes," he said, comparing the insect to pictures on a bug chart. "It's a caddis fly larva." The presence of the bug was a sign to the Mulls that the water quality in the stream had improved since the family last checked it.

The Mulls are monitoring the stream as part of their commitment to care for the environment. The stream program is one of several projects the family has taken on to help preserve nature's resources.

Once a month, the Mulls trek through their grassy Herndon back yard past man-high cattails that mark the edge of Runneymeade Park to the tree-shaded riffle where they check the stream.

They report their monthly findings to the Izaak Walton League of America, a 78-year-old, nonprofit conservation organization based in Arlington, which keeps a data base of information on streams sent in by thousands of monitoring teams throughout the country.

The league's Save Our Streams program uses the information to track trends and establish statistical justification for advocating new water quality policies.

"We have a responsibility to do something about {the environment} together," said Catherine Mull, who does environmental casework part time for Fairfax County Supervisor Lilla Richards (D-Dranesville).

Catherine and Kerry Mull remember their participation in the demonstration on the first Earth Day back when they were in college.

Today, their focus is closer to home.

Several years ago, they stopped using lawn chemicals.

The following year, their lawn had more dandelions, but they also saw more goldfinches in their shrubbery.

"Not a bad tradeoff," Catherine Mull said.

The Mulls recycle all their paper, glass and plastic. They control the temperature in their house with a heat pump instead of a furnace and air conditioner.

Catherine Mull packs cloth napkins instead of paper ones in Erin's lunch box.

"She's the only child in her school to carry them," Mull said.

Evergreens planted on the north side of their house shield it from heat-sapping winter winds, and leafy deciduous trees on the south shade it in the summer.

"Small changes really. We haven't stopped enjoying ourselves," Catherine Mull said. "We're not the type to chain ourselves to trees."

But the Mulls also are not the type to duck controversy in their own community.

They openly opposed a plan last year to turn part of 58 acres of Runneymeade Park into playing fields for Herndon's many soccer teams.

"It got very bitter, neighbor against neighbor. We were accused of being anti-family," Catherine Mull said. "Sure a lot of kids would use the fields, but it's far more important to their future to preserve this area" in its natural state.

Although the fields became part of Herndon's master plan, the Town Council has not approved funds to build them and the Mulls say they believe it never will.

The Mulls didn't suddenly become an environmentally conscious family. They have phased in changes over a period of years and often for reasons that go beyond purely environmental concerns.

"It's easier that way," Kerry Mull said. "You make a change here and there, then one day you look at your life and see you're living it more responsibly."

Five years ago, Kerry Mull quit his job in Washington for one closer to home in Fairfax. He was concerned about fuel consumption and air pollution, but he also was tired of fighting traffic for hours each week.

"If you've stood on a tall building lately and seen the smog in the air," he said, "you know we're on our way to becoming another L.A."

Today, the engineer gets to his job in fewer than 20 minutes whether he drives or rides his bicycle, something he always does when the weather is fair.

The Mulls plan to plant berry bushes in their back yard next spring. They expect the new shrubbery will improve the landscape and perhaps add value to their house, but more important, the bushes will provide natural food for wildlife.

"Maybe I'll see some more fawns," Erin said.

Last year, the Mulls gave up red meat, partly for health reasons but mostly for the sake of the environment.

"We learned how much water in the U.S. is devoted to raising cattle and how much fuel is used to ship the meat to market and we had to stop," Catherine Mull said.

"But we're not religious about it," Kerry Mull said. "We'd never refuse to eat beef stew at someone's house or make an issue at a family gathering."

But the Mulls do get somewhat upset over the issue of water quality. They see a crisis ahead for Americans and most particularly for their daughter.

That's why the family began monitoring Sugarland Run last summer. The family learned how to do it in a one-session stream monitoring workshop sponsored by the Izaak Walton League.

They learned that a stream without tiny organisms such as caddis flies, riffle beatles and snails probably isn't a healthy one.

When the Mulls began monitoring Sugarland Run in July, they noticed the stream was cloudy with silt.

They traced the mud upstream to the construction site of the Fairfax County Parkway.

They had learned in their workshop that builders in Virginia are required to keep silt from flowing into streams, so they reported their findings to the Virginia Water Control Board, as did other stream monitors. Soon the silt began clearing up and now the stream runs clear.

Around Labor Day, the Mulls watched their stream go through another crisis. In the September monitoring results they found no organisms in the stream. The sudden disappearance seemed to coincide with the draining of a community swimming pool.

"We think it might have been the chlorine," Catherine Mull said.

Again the Mulls called water authorities, who sent a letter to the pool management company reminding it that Virginia prohibits the emptying of pool water directly into streams.

According to this month's monitoring results, Sugarland Run has started to recover. "The critters are coming back. They know when it's okay," Erin said. "Maybe that means the stream will still be around when I'm grown up."