This basin beside the white-dotted peaks of the Mission Mountains is marked on all maps as an official Indian reservation, but most of the people who live here no longer are Indians -- a situation that has brought a new, complex legal and political struggle to this and many other U.S. Indian territories.

According to a new U.S. Census Bureau study, for the first time in U.S. history, more than half the residents of Indian reservations are not Indians. Of 691,070 reservation residents, 339,836 -- 49.2 percent -- are members of Indian tribes.

In the last century, new laws opening reservations to white settlement, sale of land by Indians and booming trade in timber, minerals, agriculture products and energy have drawn non-Indians into areas once reserved for American tribes.

The result is not only a rising tide of legal disputes between Indians and whites over tax revenues, court jurisdiction and job discrimination, but an important debate among Indians over whether and how to preserve what land they still have.

Some tribal areas, such as this one, have been twisted into knots over the subtle question of how to define, after generations of intermarriage, just who is an Indian and who is not.

In some places, such as this sprawling 1.2 million-acre reservation full of lumber and hydroelectric riches, the percentage of Indians holding on against massive non-Indian immigration is much lower than average. According to the census, 19.2 percent of the Flathead Reservation's residents -- 3,771 of 19,628 -- are American Indians.

Many Indian leaders see a direct connection between the natural wealth of a reservation and white-population growth. Reservations with the highest percentage of Indian residents, such as Papago (96.6 percent Indian), Gila River (95.8 percent) and Navajo (95.1 percent) are located in southwestern desert wastes.

Some leaders of the Salish and Kootenai tribes who joined to accept reservation status here 129 years ago say they think that invading outsiders have helped produce tribes stronger and more inventive than most. In recent years tribes have moved to take over management of a local power dam and have established their own post and pole plant, forest management firm and an electronics firm that has secured $3.1 million in government contracts in less than a year.

"We are more assertive and aggressive than other tribes," said Teresa Wall, at 30 the youngest of 10 members on the tribal council. Richard Acevedo, 33, the tribal prosecutor, boasts of the high education of the tribal staff, which includes five attorneys and a doctor of economics from Harvard.

The reservation was ceded to the tribes in 1855, but in 1887 Congress, over tribal objections, opened a third of the reservation to non-Indians. Good crops, strong timber demand, new rail lines and new hydroelectric plants attracted more whites in the 20th century, many of whom bought farmland from Indians who chose to give up their 19th-century allotments.

Those pressures on tribal control of the reservation have not abated. Mike Hutchin, 35, a Lake County commissioner whose family arrived here as loggers three generations ago, is pushing for formal termination of the reservation altogether.

"Lake County is 40 percent owned by the tribe, for which we get no taxes even though we provide services," Hutchin said. He and Glacier County Commissioner Fred Johnson have asked all of Montana's county governments to contribute $500 to $1,000 each for lobbying in Washington to "promote self-determination of tribes progressing toward an ultimate phase-out of all reservations and of the BIA," the U.S. Bureau of Indian Affairs.

At least, Hutchin said, the U.S. government should compensate Lake County for the $1 million in tax revenue lost each year because of tribal holdings. A National Association of Counties resolution calls for "satisfactory methods to compensate local governments" burdened with untaxable Indian lands.

Activists among the majority of non-Indians on this reservation have formed Montanans Opposing Discrimination (MOD) and its successor organization, All Citizens Equal (ACE), to fight what they consider unfair Indian privileges.

Before federal programs benefiting Indians began in the last two decades, "everybody was getting along and nobody thought much about whether they were Indian or not," said F. L. Ingraham, a local attorney representing ACE. "Now we got reverse discrimination that is almost intolerable."

ACE objects to public housing projects which bar non-Indians, civil suits against non-Indians in tribal courts, some cases of pro-Indian job discrimination and exclusion of Indian land from the tax rolls.

Ingraham said he has relatives who are tribal members and abhors racial bias against Indians. Indians a century ago may have been severely mistreated, he said, but most of the people claiming redress for those grievances now are less than one-half Indian, he said. "Some are descendents of the very white people who took their land."

George Atkinson, a gas station owner and mayor of Ronan, acknowledges, however, that "there's lots of prejudice in some areas. It's pretty obvious, both ways."

"It's very clear that there is discrimination in hiring practices" by tribal enterprises, Hutchin said. "My people just jump up and down that we have to advertise that we don't discriminate . . . while they can do it on a daily basis."

"I worked for a while up in Hot Springs," a resort operated by Indians, said masseur Howard Harvey, "but they didn't want me working up there . . . I'm not an Indian."

In an area where hunting and fishing are an obsession, non-Indians often complain about the need to get tribal permits to hunt waterfowl near local streams and reservoirs or to fish on the lake.

Local and tribal police avoid conflict by use of a few ground rules. The first officer to the scene makes the arrest, but tribal courts try only Indians, even if a white commits a crime on tribal land.

Acevedo grumbles about the county's refusal to deputize tribal police. "The people who elect the . . . sheriff do not want Indian police to be arresting them," he said.

The confederated tribes now have a little more than 6,000 members allowed to vote for the tribal council. They secure low-cost tribal loans and receive some annual proceeds -- about $200 a year for each enrolled member -- from tribal enterprises such as the lease of Kerr Dam to the Montana Power Co. The Salish and Kootenai (pronounced "Koo-tuh-nee") and their reservation may not survive if intermarriage with non-Indians continues to dilute the Indian blood that determines membership.

Before 1960, individuals with as little as one-sixteenth Salish or Kootenai blood could be enrolled as members if they were born on the reservation or were children of servicemen away at war at the time of birth. But in 1960, under pressure from the BIA, and threatened with federal termination of their reservation in part because of their widespread assimilation into non-Indian culture, the tribes agreed to impose a new requirement of one-fourth Indian blood for membership of anyone born after 1960.

Tribal leaders have begun to argue about what this will do to their future. "Those people who have lots of Indian blood are dead set against letting those they see as non-Indians become members," Wall said.

Tribal Chairman Joseph (Joe Dog) Felsman, 51, is 11 thirty-seconds Indian. Only one of his four children now qualifies for membership because the child was born before the 1960 rule-change. But the former Navy medic and steelworker said the tribal council has the power to alter membership requirements whenever it thinks it necessary.

"I expect the tribe will be here 100 years from now," he said