here's a certain irony here. On other great Montana trout streams, those who might regard themselves as conscientious environmentalists have sometimes helped to impair the very landscapes they cherish. The classic example is the Bitterroot. In recent years, Montana has become a magnet for out-of-staters, both full-time and seasonal residents, drawn here in search of clean air, spectacular mountain views, and outdoor recreation. In downtown Missoula, I stopped to chat with Jeff Crouch, a young architect who designs traditionally styled log homes for these newcomers, many of them in the Bitterroot Valley. "More than half of my clients are from California," Crouch said, "and all of them fly-fish. It's the single biggest reason they come here." Many of the wealthiest newcomers have bought up properties along the riverfront.

The Bitterroot is -- or was -- as gorgeous a valley as you could contrive, the river winding its way northward between the sheer granite peaks of the Bitterroot Range to the west and the gentler Sapphire Mountains to the east. But a hundred years ago, in sharp contrast to the vast ranches of the Blackfoot, much of the flatland here was subdivided into apple orchards, a reflection of the Bitterroot's more benign climate. Twenty-acre lots became the norm, and that encouraged denser development. The mountains are still spectacular, but Route 93, which runs due north to Missoula, disfigures their beauty: Fly shops, upscale real estate agents, and espresso bars rub shoulders with billboards, mini-malls, and fast-food logos -- the ugly street furniture of any American landscape.

The proximity of Missoula, with its urban chic, is a further draw. These are not people who want to hole up in a cabin in the wilderness and fight off grizzlies. "Over the next 20 years or so," Crouch said, shaking his head, "with all this population growth and new subdivisions, we're facing a real crisis of urban planning here in Missoula. I don't know how it's going to turn out. But the Bitterroot itself is done, it's shot. We're turning it into Southern California."

Despite the presence of so many of his constituents in the Bitterroot Valley, you'd hear much the same complaint from Bruce Farling. "What you have to bear in mind," Farling said, "is that we're not a fishing club; we're a group of conservation-minded anglers." That translates into an agenda with which environmentalists would feel entirely comfortable.

The most striking thing about Farling, I thought, was his emphasis on the health of the aquatic ecosystem as a whole, rather than on trophy fish. Most visiting fly fishers are drawn to Montana by the fantasy of catching a gigantic brown or rainbow trout. But those species aren't natives; they are exotics, introduced here a century or so ago to provide sport, and Farling seems almost disdainful of them. He'd much rather talk about leopard frogs, long-toed salamanders, macroinvertebrates, and "our obligation to protect their life-support system."

Above all, he worries about the native trout of these mountains: the bull trout (federally designated as a threatened species) and the westslope cutthroat. As much as anything, he said, it is a matter of heritage -- a comment that made me recall the cutthroat's Latin name, Oncorhyncus clarki lewisi.

Global warming is the thread that ties together many of Farling's anxieties. When he talks about "protecting cold-water habitat," the emphasis is definitely on the cold. On the national level, Trout Unlimited has lobbied for clean water and the preservation of roadless areas; it has worked to repair the damage from acid mine drainage and denounced the reckless pace of oil and gas development along the Rocky Mountain front. But on global warming the Montana council has been out in front of the national organization, which has yet to take a position on the issue.

In addition to reducing trout habitat, global warming will disrupt the balance among species, favoring exotics over natives. Warmer temperatures will drive the sensitive cutthroats to higher altitudes and more restricted habitat; the non-native brown trout, meanwhile, will take over former cutthroat waters, since it can tolerate temperatures 1.4 degrees Fahrenheit warmer than those that will kill the native fish.

"There's a U.S. Geological Survey thermograph at the mouth of Rock Creek," Farling said. "They've been collecting data on stream temperatures for about 25 years now. The data isn't publishable yet, but it's clear the rise in temperature is quite dramatic."

I mentioned that I'd been fishing on Rock Creek just the day before. "What did you catch?" he asked.

"One big rainbow," I said. "Otherwise lots of browns."

He nodded; I'd proved his point.

Rock Creek, I knew, also has high levels of whirling disease, which struck Montana's trout streams in the mid-1990s. Whirling disease is caused by a microscopic parasite, Myxobolus cerebralis, which attacks a young trout's central nervous system and cartilage. The parasite's initial host is the small tubifex worm, which lives in streambed silt.

"Well," I said brightly, "at least that has nothing to do with global warming."

Farling shook his head vigorously. "Oh, but it does. Whirling disease is directly related to climate change. The worm's production of spores depends on temperature. Also, when we get more snowmelt and higher year-round river flows, it appears that the trout survival rate is higher because the spores are dispersed. When there's a drought, on the other hand . . ." He left the thought unfinished.

n my last day in Montana, I told Bruce Farling that it was time for me to fish Norman Maclean's river, the Blackfoot. Since the river was still running high from the rains, I asked his advice on a good place to go wading. He recommended a road crossing just downstream from the town of Ovando. As drift boats zipped past in the deep, fast midstream currents, I poked around in the shallow bankside riffles. The water was icy cold, and every single fish I caught was a native westslope cutthroat.