Disaster officials, already shaken by floods and huge mud slides, braced today for a month of turmoil throughout western states as record snowdrifts in the Sierra Nevada and Rocky Mountains began melting into overburdened streams and rivers.

"June is going to be a very, very threatening month," said Dale Robertson, a Federal Emergency Management Agency official in San Francisco. He said flood-threatened residents of the agency's western region "have yawned a lot" at efforts to provide them federal flood insurance, even with the Sierras holding 215 percent of the usual winter snowfall.

So far two persons have been reported killed by mud slides and flooding in Utah and Nevada over the weekend, with many homes destroyed and thousands forced to evacuate threatened areas.

High temperatures, which melted snow and caused disastrous Utah flooding, dropped 10 degrees today, but National Weather Service forecaster David Sanders said that, if predicted new thunderstorms hit Salt Lake City before water levels drop, the results could be "very bad."

A record 66 1/2 feet of snow fell near the Sierras' Donner Summit this year, with much of the range's snow pack still more than 20 feet deep. Don Neudeck of the California water resources department said this year's snow melt would be the worst in 14 years, pouring more than 8 trillion gallons of water--about one-fifth the volume of Lake Tahoe--into California's Central Valley.

State officials said rivers in the San Joaquin, or southern, portion of the valley have been at near flood stage for two months as water control officials have tried to empty upstream reservoirs in preparation for the summer snow melt.

Baker Conrad of the Council of California Growers said crop damage from the winter's heavy rains and the anticipated snow melt is estimated now at $500 million, but a faster than expected melt could increase that figure and seriously hurt the nation's most productive agricultural area. Record rainfall throughout the winter over the entire West has left water tables extremely high and the ground too soaked to absorb much more moisture. Recent high temperatures accelerating the snow melt have caused serious flooding not only in Utah and Nevada, but in parts of California, Wyoming, Washington, Colorado and New Mexico.

With the snow melt come dangerous mud slides, according to Larry Ferral, deputy hydrologist in charge at the National Weather Service's California-Nevada river forecast center in Sacramento.

Water running off the mountain sides wets, lubricates and loosens the ground below. The soaked earth becomes 60 percent heavier than normal, resulting in huge, sometimes catastrophic mud slides. Ferral said resorts and small mountain communities usually build on scenic slopes where the danger is greatest, "and there is really nothing that can be done to stop the slides."

An 18-foot wall of mud and water thundered down Slide Mountain near Reno Monday on the eastern slope of the Sierra, forcing thousands to flee a popular picnic and camping area. A 20-foot mud slide also hit the Salt Lake City suburb of Farmington, and 1,100 were evacuated temporarily from the central Utah town of Fairview when another mud slide threatened.

Six weeks ago, a mud slide in Spanish Fork Canyon, 60 miles south of Salt Lake City, created a lake inundating the town of Thistle. The major rail line through the area remains cut off.

Salt Lake City officials have converted three major city streets into makeshift canals to channel the melted snow runoff. The Great Salt Lake, which has inundated much surrounding land, is now estimated to have made its largest seasonal rise since records were first kept a century ago, and is not expected to begin falling until July.

Peterson, who administers the federal flood insurance program in California, Arizona, Nevada and Hawaii, said his agency put out an official warning April 1 of major flooding during the snow melt. National figures show, however, that only 7 percent of those who could benefit from the federal flood insurance have made use of it, he said. The five-day waiting period after starting a policy made it crucial that flood-threatened individuals and businesses sign up now, he added.

Neudeck said the northern portion of California's Central Valley has so much annual rainfall it can handle the summer snow melt, but the runoff capacity in the southern part of the valley is only one-tenth as much. Lewis Brautigam, deputy agricultural commissioner in Kings County, said the winter rains have flooded 83,000 acres of prime cropland in the basin of mostly dried up Tulare Lake. The loss of cotton, alfalfa, wheat and barley so far is estimated at more than $45 million, he said, but the melting snow could make it much worse.