MAMMOTH LAKES, Calif. (AP) _ An avalanche sent snow cascading down an upper slope Mammoth Mountain on Thursday just five days after this snow-starved ski resort opened and two days after cloud seeding.

Only one skier of more than 1,000 on the slopes was knocked down by the rushing wall of snow that covered a 300-foot stretch of the ski run, but he was not hurt, said Mary Shoshone, spokeswoman for the Mammoth-June Ski Resort.

The upper run where the avalanche occurred had opened for the first time this year at 12:30 p.m. Thursday, so few skiers were likely to have been on it, she said. The avalanche occurred an hour and 15 minutes later.

Based on what witnesses said, it was believed that nobody else was involved in the avalanche, Ms. Shoshone said.

Cloud seeding operations were halted because of the avalanche, according to a company that had seeded clouds there Tuesday for the Los Angeles Department of Water and Power.

Tuesday's storm left Mammoth Mountain with at least 5 inches of new snow, according to National Weather Service spokesman Peter Wilensky in Los Angeles.

Snowslides normally occur after a heavy snowfall, Ms. Shoshone said, and the resort does avalanche control maneuvers every morning to get any potential slides out of the way before the slopes open to the public.

There was no slide when resort officials did the avalanche control maneuver on Thursday, she said.

The slack snow season in the Sierra had prompted the Los Angeles Department of Water and Power to renew its practice of cloud seeding, a tactic the agency has not used in 10 years.

The snowpack, which provides more than two-thirds of Los Angeles' water, is only about a quarter of what it was last year at this time, authorities said.

''We've gotten off to a pretty slow start,'' said Doug Gillingham, DWP project engineer for the cloud-seeding effort. ''Normally by this date we have received 50 percent of the snowpack.''

Fresno-based Atmospherics Inc. conducted the seeding. Tom Henderson, president of Atmospherics, said Thursday the avalanche means there will be no further seeding for the next few days.

In 1978, lethal mud slides occurred in Southern California shortly after seeding had been conducted by the Los Angeles County Flood Control District, prompting lawsuits and calls for tighter state regulation of weather modification experts.

In Utah, meanwhile, a ski patrolman testing for avalanches at Snowbird Ski Resort escaped serious injury when he triggered a 40-foot slide and was dug out within minutes by fellow patrol members, authorities said.

Avalanche danger is extremely high in the mountains of northern Utah following two major storms that dumped up to 4 feet of snow area ski resorts, said John Loomis, assistant mountain manager.

Collinson was treated an released at a hospital for minor injuries.