In the first use of federal law to protect scenic vistas within a national park, Environmental Protection Agency Administrator William K. Reilly has decided to order an Arizona power plant to stop polluting the air over the Grand Canyon.

The decision, which is expected to be announced today and is subject to agency review, is aimed at thinning the persistent haze that on some winter days makes the canyon's vivid colors and spectacular rock formations all but invisible to a tourist standing on its rim.

Reilly's order would mark the first time that clean air laws generally used to protect public health have been applied solely for aesthetic purposes. The directive has implications for other national parks where views are obscured by industrial pollution, including Shenandoah National Park 60 miles west of Washington.

"Grand Canyon is to the United States what Notre Dame is to France," Reilly said earlier this week. "It's a worldwide symbol of our country's spirit and beauty."

But Reilly had difficulty convincing some administration officials that merely improving views of the canyon was worth the costs -- estimated at up to $2.3 billion -- of installing anti-pollution controls on the Navajo Generating Station 80 miles to the northeast.

He backed off the agency's original proposal to remove 90 percent of the sulfurous emissions from the coal-fired plant, accepting 70 percent as a compromise. The concession drew sharp criticism from environmentalists and National Park Service officials, who said even a small amount of pollution would cause significant visibility problems in the pristine atmosphere of northwest and north-central Arizona.

"During {the worst pollution} episodes, it's so dirty you cannot see the bottom of the canyon, much less the length of it," said William Malm, a research physicist for the Park Service. "Under those kinds of conditions there's no question that the difference between 70 and 90 percent would be very noticeable."

Navajo's operators contend that no amount of pollution control will improve the canyon's visibility because the plant contributes an insignificant amount of pollution.

"There's a higher and better use for $2 billion than to try to eliminate a very, very modest contribution that may or may not impair visibility several days a month, several months a year," said D. Michael Rappoport, a spokesman for the Salt River Project, which operates the power plant located in Page, Ariz.

No one disputes that pollution is a problem in the canyon, one of the national park system's "crown jewels" and, with 4 million visitors a year, one of the world's most popular attractions. Using various techniques, the Park Service discovered that the source of pollution varies by season, with smog from Southern California wafting in with southwesterly winds during the summer months.

In the winter, however, the wind blows from the northeast -- the direction of the Navajo plant -- and most of the haze consists of sulfates, tiny particles formed when sulfur dioxide hits the atmosphere. Navajo is the second-largest source of sulfur dioxide in the West, emitting up to 10 tons of the gas per hour.

A 1987 study by the Park Service concluded that Navajo was the largest source of canyon haze during the winter, generating up to 70 percent of pollution on the most occluded days. Although the National Academy of Sciences later questioned the percentage, it said the plant is a "significant" contributor to the haze.

Navajo's operators said their studies show the plant contributes to the haze 3 to 4 percent of the time during winter days.

The EPA has responsibility under the Clean Air Act to protect visibility within national parks. But the agency failed to implement the 1977 provision until a lawsuit by the Environmental Defense Fund forced it to formulate a program in 1984.

Complicating the EPA's first case is the fact that Navajo is partly owned by the Interior Department's Bureau of Reclamation -- a sister agency of the National Park Service -- which uses the electricity to siphon water from the Colorado River to cities and farms in central Arizona. Although the Park Service has been adamant about imposing controls on the facility, some Interior Department officials have argued that the cost to taxpayers of such a measure is not justified by scientific data.

Under Reilly's decision, a 70 percent reduction in Navajo power plant emissions would register a "very apparent" improvement in Grand Canyon visibility on seven winter days, according to government data. But a 90 percent reduction would triple the number of clearer days.

Environmentalists criticized Reilly's proposal for not requiring the tougher cuts, which they said would cost $10 million a year more, by EPA's estimates, and would result in 14,000 fewer tons of pollutants.

"That would be the best pollution control buy in the country and would provide a cushion for growth," said Robert Yuhnke, of the Environmental Defense Fund.

According to Reilly, the cost difference is so small that Navajo will further reduce emissions to earn "pollution credits" offered in the new Clean Air Act to encourage utilities to cut sulfur dioxide blamed for acid rain in the East. The credits can be sold to other utilities seeking to expand beyond prescribed emission limits.

The Grand Canyon decision has been closely watched by officials at Shenandoah, which in the Park Service's view suffers from the worst air pollution of any national park. Although the acid rain provisions of the new Clean Air Act are expected to cut down considerably on emissions from existing plants, Park Service officials are concerned about plans to build 19 power plants in Virginia.

"We could lose in Virginia what we could be gaining" under the Clean Air Act, said David Haskell, chief naturalist at Shenandoah.