By Steve Gorman

LOS ANGELES, April 10 (Reuters) - Meager precipitation and a premature spring thaw caused by unusually mild temperatures last month have left the U.S. Western mountain snowpack, a key source of fresh surface water for the region, at record low levels, the government reported on Friday.

Melting of winter snows began much earlier than usual this year, from the Sierra Nevada range in California to the lower elevations of Colorado's Rockies, leaving much of the Western snowpack greatly diminished or gone by early April, when it is typically at its peak.

As a consequence, Western states will experience reduced stream-flows from mountain runoff this spring and summer, leaving reservoirs - already well below average capacity in several states - that much more depleted, the report said.

The latest snowpack snapshot was contained in an April survey issued by the U.S. Agriculture Department's Natural Resources Conservation Service.

The issue drew wide public attention in drought-stricken California last week, when Governor Jerry Brown stood on dry grass at a snow-monitoring station in the Sierras to announce the lowest snowpack level on record there and ordered an unprecedented 25 percent mandatory cut in statewide water use.

The scant mountain snowpack extends this year to a dozen Western states, said Michael Strobel, director of the National Water and Climate Center in Portland, Oregon.

"It's pretty extreme," he said.

He added it was unclear whether the trend is a result of natural short-term fluctuations in weather patterns or longer-term changes in climate.

As in parts of the Sierras, the snowpack by April was nearly gone in Nevada, Utah, Arizona and much of New Mexico, while declining to well below normal across the rest of the region.

In Oregon, more than half of all mountain measurements taken on April 1 recorded bare ground, with snowpacks in the Cascades having peaked 40 to 90 percent below typical levels over the winter.

"Almost all of the West Coast continues to have record low snowpack," government hydrologist David Garen said. "March was warm and dry in most of the West. As a result, snow is melting earlier than usual."

The only pockets of normal or near-normal snowpack were found in the higher elevations along the crest of the Rockies and in the mountains of interior Alaska, the report said.

Reservoir storage as of April 1 was below average in at least five western states - Arizona, Nevada, New Mexico, Oregon and Utah. (Reporting by Steve Gorman; Editing by Sandra Maler)