Major eruptions of Mount St. Helens, which darkened skies and spewed volcanic ash across the Pacific Northwest last spring, caused more than $1 billion in damage to rivers, roads, bridges, and the timber, agriculture and fishery industries in the Washington and Oregon region, according to a government report released yesterday.

But, while the blast, ash fallout and mudflows from the volcano blew away 120,000 acres of forest, knocked down bridges and roads, blocked the Columbia River, killed fish and destroyed crops, there was one positive aspect, the report said, and that is tourism.

Washington State tourism officials predicted that their mountain could soon rival Mount Lassen in California and Volcan Irazu in Costa Rica in popularity as a volcano attraction, "enhancing the Northwest's tourist appeal," according to the report by the International Trade Commission to the House Ways and Means Committee. The committee had asked the ITC study the export-import impact of the volcano's eruption.

For example, the U.S. Forest Service said the volcano last month attracted more than 4,000 visitors each day.

Hotel and motel receipts rose during July and August after a severe drop in May and June, following the eruptions beginning May 18, the report said. The drop in hotel and room receipts of 26 percent in Spokane and 64 percent in Yakima, Wash., during May and June were caused not only by ash fallout and negative publicity from the eruptions, but bad weather, high gasoline prices and the national economic recession, the report said.

The eruptions, which left 31 persons dead and 33 missing, originally were thought to have caused $2.7 billion in damage. Smaller eruptions took place on May 25, June 12, July 22 and Aug. 7. The volcano is located in southwestern Washington about 45 miles north of Portland, Ore., and about 100 miles south of Seattle.

Some areas such as agriculture, fishing, shipping and forestry -- the area's leading industry -- sustained shortterm

"If volcanic activity continues for a significant period, there may be a reduction in investment and a lower rate of growth in the region," the report said. "However, if Mount St. Helens returns to a dormant state, the long-term impact will be minimal, with a probably growth in tourism due to the unique nature of the mountain."

Other possible effects are intangible such as local residents' possible long-term psychological problems, emotional stress from the frustrating clean-up and insecurity about the future, the report said.

Scientists, however, are unable to predict the probability, magnitude and timing of the volcano's continued activity or what psychological effect the eruptions have had, the report added.

The report estimated losses of $695 million in timber and related industries, $192 million affecting agriculture, $95 million in the fishery business and $44 million to dredge the Columbia River, $112 million to repair damage to public roads and bridges, and $75 million to clean ash from roads.

Agricultural losses were equivalent to a 3.5 percent of the 1979 output, the report said.