ENTERPRISE, Ore. (AP) _ Archaeologists plan to excavate what one describes as ''a prehistoric city'' next summer along the Snake River in Hells Canyon.

The U.S. Forest Service dig will focus on seven sites along two miles of the river's Idaho shoreline at Pittsburg Landing, a popular boat-launching area, said Bruce R. Womack, chief archaeologist for the Hells Canyon National Recreation Area.

''This is more than a site,'' he said. ''You can think of it like a prehistoric city, in a sense,'' although all of the sites may not have been occupied simultaneously.

Pittsburg Landing, one of the few relatively flat areas at river level inside the gorge, was a natural spot for prehistoric encampments, Womack said.

Hells Canyon contains nearly 1,000 known archaeological sites, including primitive picture writings and rock carvings called pictographs and petroglyphs, ancient Indian rock shelters, burial sites and pit house villages dating back 7,000 to 10,000 years, according to the Forest Service.

Womack said a crew of up to 12 people will be involved in the 4-month dig and an identical dig in summer 1989. Much of the effort will focus on ancient house pits, semi-subterranean homes created by digging 3 to 4 feet into the ground.

Evidence collected in preliminary work during the past winter suggests the sites probably have not been occupied for 350 to 500 years, Womack said.

Had Indians lived there in more recent times, evidence of horses, beads, trade goods and glass would be present, he said.

''They are prehistoric sites, as far as I can tell,'' Womack said. ''The existence of six sites was not known even before this winter.''

The archaeologists hope to learn if the Indian pit-house dwellers at Pittsburg Landing were hunters or fishermen, how long they lived there, the time of year they usually spent in the canyon, and how many lived there, Womack said.

Archaeologists also want to know how deposits of nearly 1 foot of ash on Hells Canyon 5,700 years ago from the eruption of Mount Mazama in southern Oregon affected the Indians' food sources. The volcano's eruption is what created Crater Lake.

It is believed that the Snake River was a transitional area between the tribes of the Great Basin, such as the Shoshones, and the Columbia Plateau tribes, including the Nez Perce, Womack said.

Three years ago, the Forest Service estimated that relic hunters and vandals had defaced or damaged 20 percent to 30 percent of the canyon's known archaeological sites.

''I don't think it has diminised,'' Womack said of the illegal activity. ''It's shifted away from the river. They are not doing it where we can see them so easily.''

In the past relic hunters often were unaware that they were breaking the law when they excavated burial sites and pit houses. ''The people who are doing it now know it's illegal and are in it for the money,'' Womack said.