NGELES NATIONAL FOREST, Calif. — The Santa Ana sucker would not be a strong candidate as anyone's darling.

This homely Southern California fish grows to six inches from tail to unattractive snout, which it uses to scrape algae from rocks and river bottoms. Those angling for its more regal neighbor, the rainbow trout, have often regarded it as little more than a nuisance.

Until, that is, the sucker had something the trout did not: federal protection.

"It's not a bad little fish and it really is a big part of the biodiversity of this region and it deserves protection," said John G. Tomlinson Jr., an official at California Trout, an environmental group that, though it is not considering changing its name, has embraced the sucker's plight as its own. "We've discovered that what's good for the sucker is good for the trout."

Indeed, the sucker could prove to be the tool that environmentalists and sportsmen have been looking for to conserve one of the few rivers for wild trout in Southern California, the West Fork of the San Gabriel River, and some say the best of them.

Like most other rivers in the state, the San Gabriel has not flowed freely for decades. A series of dams was built in the 1930's, largely for flood control.

Environmentalists have struggled with the Los Angeles County Department of Public Works, which operates the dams, to provide steady flows of cool water year-round, precisely the conditions that the trout — or rather, the suckers — love. The problem is that in years of low rainfall, which means most years, the county says it cannot supply the volumes the environmentalists demand.

"I certainly understand their frustration on that, but Mother Nature hasn't exactly been cooperative," said Patricia Wood, a senior engineer at the Department of Public Works. "We received the water they expect us to manage in only 6 years out of the last 60 in terms of the natural flow. We have to play with the hand we're dealt."

But heavy development has pushed the remaining Santa Ana suckers into just three watersheds in this area, prompting the federal government to list the fish as threatened in April 2000. The county thus has no choice but to try to work out a plan that will provide a better habitat for the fish — and, as luck would have it, any other fish that enjoys similar conditions. All the stakeholders in the river, including several federal, state, county and local agencies, hope to conclude such a plan by the end of the year.

"You can't go in and say, `We want to protect the river because we want to catch more fish' anymore," said Jim Edmondson, the conservation director for California Trout. "That used to work. It doesn't now. We need the sucker. It is a major arrow in our quiver."

The San Gabriel River, and especially its west fork, is a striking anomaly. Lying in this national forest in the far northeast reaches of Los Angeles County, which has about 10 million residents, it is far, very far, in feeling from the urban sprawl along its lower reaches. Beyond the mustard-hued smog and subdivisions along the freeway and in the city of Azusa, at the base of the San Gabriel Mountains, the road narrows, the cliffs soar and the air, flora and fauna shift.

Heading up the west fork, for the first mile the clear, chilly water tumbles over sculptured rocks in what is still an urban tableau. Graffiti covers many boulders and signs. Garbage from fast-food restaurants and beer bottles litter the path. The weekend crowds are dense.

But once the visitor gets a couple of miles in, the scraggly trees are replaced by stands of alder, sycamore, scented bay laurel and oak. Their shade keeps the temperatures low. The steep canyon walls are frequently painted with dark splashes of algae where water seeps through the rock. And the flies flitting above the riffles of the river are food, not a nuisance.

"Look, there's one there," said Mr. Tomlinson on a recent morning, as he pointed to the ripples where a trout, hiding behind a rock, had just gobbled a bug. "You watch what they're doing and you learn just about everything you need to know about the trout here."

At one pool, a kingfisher cackled and made a fuss as several visitors approached.

"He's telling us, `That's my fishing spot,' " said Will Trefry, a California Trout board member. "He knows there's all kinds of fish here."

The river's problems begin with the unusual geology here. The San Gabriel Mountains are formed by the grinding of two plates along the San Andreas Fault. Because of this tension, they are among the world's fastest-growing mountains, in geologic terms anyway. But because they are made of a crumbly granite, they are also among the most swiftly eroding.

The granite washes away with unusual speed, and so the rivers here are filled with many times more silt than average. And all that fine sand backs up as silt in the San Gabriel River's dams, requiring constant maintenance.

The dams were originally built with special sluice gates near the bottom, so they could be opened periodically and the silt flushed downriver. That was last done in 1981, and everyone agrees that it was a disaster for the fish and other wildlife in the river.

The county agreed in 1989 to clear the silt in a more expensive but environmentally friendly way. It lowers the water level in the reservoirs, then digs out the silt and trucks it to a nearby canyon. It seemed a happy compromise until the environmentalists found the water flows lower than what had been agreed to, which prompted demands for new negotiations.

Once the Santa Ana sucker was listed as threatened, they had their way.

"I know it's not an exciting animal," said John Stephenson, a supervisor in the region for the United States Fish and Wildlife Service, "but a lot of the species we list are not very charismatic. That's not a criterion. But it is a native, and there's great interest in the scientific community."

Ms. Wood of the county insisted that, while providing the flows necessary for the sucker was a good idea, she had many masters to serve, including the cities and local agencies downriver that use the water.

Mr. Trefry of California Trout suggested the two sides were not far apart.

"Well, it's just right for them to look at it that way," Mr. Trefry said. "They have to look after their interests and their stakeholders. Our view is, if we do this right, we make them into heroes. We don't have to be adversaries. We just want the fish to do O.K."