Amid yellowing aspens and white-topped hills that warned of winter, this normally pleasant resort was given over last week to a bleak vision of the future.

Here, at a between-season time when the streets and shops are as deserted as an Outer Banks beach the week after Labor Day, an odd assortment of ranchers, farmers, scientists, academics and government men gathered to access the future of agriculture in the Rocky Mountains. What they saw was not good.

Except for an official optimist or two in the government delegation, they looked into the next decade and saw a dying agriculture and the diversion of the region's life blood -- irrigated water -- to coal plants, slurry lines, synfuel developments, subdivisions and military bases. Some forecast the ruin of the American West.

The most apocalyptic voice was that of historian and hell-raiser K. Ross Toole, best known for his writings on the despoliation of Montana in the days when copper was king. On Toole's mind was acid rainfall, certain to be increased by the massive coalfired power plants now under construction in the mountain West. Despite a recent acid rain scare, Toole believes that the potential impact on food and fiber still is understated. The consequences of any effort to make the Rocky Mountains the nation's main energy producer, he predicted, would be incalculable.

"The Brobdingnagian plants now built or under construction can clearly deposit acid precipitation or dry deposition [of sulfuric acid] across the bread basket," Tool said. "If the synfuels program actually becomes operative in the coal-bearing section of the northern Great Plains, the effect on the lush farmlands to the east is very frightening to contemplate."

The speech had a stunning impact. It seemed an overstatement to some, if only because few farmers have room in their imaginations for the possibility that the rich soil of the Middle West could turn permanently acidic.

But Toole's warning was supported by University of Montana botanist Clancy Gordan, who demonstrated with studies and slides the damage already done to vegetation by pollution from coal plants operating in Montana. It is Gordan's view that farmlands to the east could be spared only if "sacrifice areas" were established in the northern Rockies, where vegetation would absorb the sulfur and nitrogen oxides emitted by purposely shortened power plant stacks.

Most of the large new generating plants in the West are fitted with tall stacks that release pollutants at such high altitudes that smoke remains in the atmosphere long enough to form sulfuric and nitric acids. The region'ss prevailing winds come out of the west; thus the concern about farmlands to the east.

"It's an insidious process, and it's irreversible unless we act in time," Gordon said.

Those who work the land have a more private, down-to-scale vision of catastrophe. Consider Burrt Trueblood, who was born on the land he farms in Homestead, Idaho. He brought up a family in Alaska but has returned home. Once back, he says, "we tried to raise alfalfa seed . . . by the technique that had been very sucessful when we were kids, but it was a total failure . . . it was a disaster because there were no pollinizers."

Alfalfa, the region's most common livestock fodder, is best pollinized by the alkali bee, increasingly the victim of pesticides and now all but gone in the Rockies.

Trueblood also brought to Sun Valley a soil sample exhibit, which he showed to the director of the U.S. Bureau of Reclamation, R. Keith Higginson. Trueblood told him that it demonstrated 60 years of decline in the soil of his family farm.

There were other accessments, most of them of equal gloom. Leonard Arrington, the historian of the Church of Latter-day Saints (Mormons), observed that the farming acreage in Utah's Salt Lake County, once an agricultural marvel, had declined by half in the past 25 years and would disappear by the turn of the century if its present rate of conversion into subdivisions goes unchecked.

Thadis Box, dean of the College of Resources at Utah University and a relative optimist about the current condition of western rangeland, worried about the potential for a new Dust Bowl in the Southwest.

Arizona rancher-poet Drummond Hadley, author of a ballad about a cowboy who can't decide whether he prefers green eyes or green grass, composed another poem on the spot after listening to the government spokesmen. A verse describes people who say less with more words than anyone Hadley ever met.

It was an odd conference. The ranchers and academics drank whiskey and ate steak and watched farm films together, talking about the way it used to be and how much things cost. They did not settle anything, but left weighed down by a fear that their agriculture is in trouble, and with it a lot of other things as well.

"Almost everything that is treasured and deemed worth saving about the Rocky Mountain West has existed -- and continues to exist -- because in large measure this region rests basically on a ranching-farming economy," said Indian historian Alvin M. Josephy Jr. in a closing speech to the conference.

"That economy has played a key role in protecting and maintaining what is treasured here -- the wide-open spaces, the unpolluted air, the bounties and beauties of nature, the ease and grace and freedom of life. As that economy goes, so will go the West as it has been."