HEREFORD, MD. -- It came as a shock to unenlightened fly fishermen when the Big Gunpowder River here was named one of the top 100 trout streams in the nation in Trout magazine's winter 1989 survey.

The comparison with revered mountain waters of the Wild West seemed heady for a little suburban creek five miles from the Baltimore Beltway and less than 20 from the city's murky Inner Harbor.

But there it was, big as life, and no one has come along since to blow the Gunpowder off its lofty perch. If anything, word is spreading and the notoriety is doing the little trout stream good.

"As a fisherman, I'd love to have it all to myself, of course," said Scott McLelland of Trout Unlimited's Maryland Chapter, which helped convert the Gunpowder from a battered relic to a pristine treasure over the past five years. "But as a conservationist, you have to figure the more people who know about it, the better.

"If people are out there using it, it's that many more eyes keeping a lookout; that many more people who won't stand for the loss of a precious resource."

True to his convictions, McLelland was among a small band of volunteers and state workers that recently surveyed the Gunpowder's piscatorial treasures. Ranging like a chest-wadered SWAT team up and down the creek, they electroshocked trout from their hidey-holes to see exactly what lurked in the cool, clear, autumn depths, then turned the victims loose, none the worse for wear.

"There he is," said McLelland when an electro-stunned 17-inch brown trout rolled up out of one of his favorite fishing haunts. "I've caught that fish two or three times this year, but I was afraid somebody might have taken him by now. I'm happy to see he's still here."

Bob Bachman, Maryland's freshwater fisheries chief, was happy too at the abundance of smaller browns, rainbows and brookies, all of which abound in the Gunpowder these days.

"In pounds per acre and number of adult fish per mile," said Bachman, "this river and the Savage {in Western Maryland} stack up with the better trout streams in the East."

Surprise, surprise. Only a decade ago, long stretches of the Gunpowder were eyesores that dried up in summer, leaving no place at all for trout to live.

But about six years ago, McLelland's TU chapter and the state Department of Natural Resources saw the potential of the creek, which is fed by cool waters from behind the dam at Prettyboy Reservoir, one of Baltimore's drinking water impoundments.

Most of the top trout fisheries in the nation are tailwater fisheries below such reservoirs, said Bachman. The only reason the Gunpowder wasn't prospering was unfriendly municipal water release schedules that left the creekbed dry in summer.

"So the Maryland chapter of TU approached the city of Baltimore to see if they could regulate the flow so there would be cold water in the river year-round," said Bachman.

Gov. William Donald Schaefer, then Baltimore's mayor, agreed and the stage was set for a resurrection. TU and DNR set to work reestablishing a trout population. The first year they planted brown trout eggs of the bitterroot strain from a Montana hatchery. Later, they stocked fingerlings and larger trout.

The results have been spectacular. About two-thirds of the Gunpowder, which runs 14 miles between Prettyboy and Loch Raven reservoirs, is now blue-ribbon trout water. The first mile has been designated a catch-and-release stretch for fly-fishing only, and Bachman said a three-mile stretch below will be proposed for similar protection.

Natural trout reproduction has proved excellent, Bachman said, and promises to get better as more spawners reach the size of McLelland's 17-incher. The biggest trout caught in electrofishing surveys so far has been a 21-inch brown, "and there are plenty in the 14-to-17-inch range, which is your top breeding fish," said Bachman.

In addition to providing prime habitat for trout, the steady supply of cool water also creates superb conditions for insects, trout's main food.

"The insect population is increasing every year, with healthy populations of mayflies, stone flies, caddis and, in the upper stretches, midges," McLelland said, adding that the best for fishing is the sulphur hatch in May.

"It's a businessman's dream because it's an evening hatch during the time of the year when days are longest."

This time of year, trout gobble ants, grasshoppers and spiders, McLelland said, and even in the dead of winter the hearty fly angler can dredge up a few trout by fishing woolly buggers, crayfish and minnow imitations near the bottom.

McLelland calls the Gunpowder the prettiest place in Baltimore County. Baltimore Sun columnist Roger Simon says it's the coolest place around on a hot summer day. Bachman says the river lately has received the ultimate accolade -- fly fishermen from trout-rich Pennsylvania are trying their luck here.

"Three years ago today," said Bachman, reclining on the grass at a little park where York Road crosses the stream, "we were sitting around saying how nice it would be if we could sit here one day and watch trout rising.

"Yesterday we did, and Howard {DNR biologist Howard Stinefelt} got his fly rod and caught an eight-inch rainbow on an elk-hair caddis."

Bachman pointed to the spot where it happened. As if on cue, a trout rose and slurped a bug, leaving behind the distinctive, expanding ring of ripples that gladdens trout anglers' hearts.

Fly fishermen interested in trying the Gunpowder with some expert guidance can book a trip with McLelland's Gunpowder Trout Scouts, a guide service. Call (301) 668-9006.