Cultural clash on vanishing frontier; Maps & Dreams, by Hugh Brody. New York: Pantheon Books. 298 pp. $16.50.

In the summer of 1978 a young anthropologist named Hugh Brody was sent by the Canadian government to live among the Indians of northeast British Columbia. His task was to study how the hunting culture of the Beaver Tribe would be affected by the proposed Alaska Highway Natural Gas Pipeline. He would make land-use maps of all the land necessary to the Indian economy, and encourage the Indians to map all the land they had used for a variety of economic pursuits during their lifetimes.

At first the Beaver were reluctant to share any economic information at all. They knew that the white man's oil and gas interests were encroaching ever deeper on their traplines and hunting areas. Was Brody out to gather ''facts'' which would be misinterpreted and used against them?

As an anthropologist the author was quick to appreciate the subtlety and diversity of the hunting economy. He admired how flexibly the Beaver and their neighbors had been able to preserve the core of their ancient culture through nearly two centuries of white exploitation and misrule. As Mr. Brody gained their trust, the Indians made land-use maps of their own, and the basis for this important new book began to emerge.

Mountains of data seemed to fall into clear and depressing patterns, and it became increasingly difficult, indeed dishonest, to remain emotionally aloof from his findings, Brody felt. Accordingly, ''Maps & Dreams'' is written in an unusual format. The even-numbered chapters are packed with hard evidence: maps, statistics, and history. Perhaps most valuable is the author's analysis of one of white society's favorite dreams: the ever-expanding frontier. He gives us a short history of how whites have dreamed about and exploited an ever-shrinking wilderness; and the white man's idealized or degraded, but usually convenient, stereotypes of the Indians who always seemed to be in the way.

To counter this tendency to color our views of native people with our own ignorance and emotions, Brody has written the odd-numbered chapters as personal accounts of his year and a half on the reserve. He takes the reader moose hunting, fishing, and trapping as he shared the Beavers' yearly cycle of harvesting a living from the subarctic.

The Indian hunters are drawn as individuals, yet each has something to offer the reader's understanding of their hunting culture. Brody holds that it is only through our realistic appreciation of the viability of the hunting economy and the dignity of its culture -- which has flourished quietly in British Columbia for perhaps 20,000 years -- that it can be saved from our ignorance and our greed.

If ''Maps & Dreams'' seems weighty at times, it is because Hugh Brody has much to say, and there is not much time left to say it.