The West's snowcapped mountains -- the storehouses of the region's surface water supplies -- are yielding less runoff this spring, federal water officials say.

Most of the West's water originates as snowfall that accumulates over the winter and early spring. As it melts, usually over the course of several months, it swells streams and is eventually captured by reservoirs.

But this year, many of the region's streams are seeing below-average flows, and the snowpack is melting more quickly than usual, said Barry Wirth, a spokesman for the Bureau of Reclamation's Upper Colorado River Basin, which includes Colorado, northern New Mexico, Utah and Wyoming.

"We just haven't had the snow," Wirth said.

The most recent spring and summer streamflow forecast for the West from the Natural Resources Conservation Service shows the Colorado River Basin -- which also includes Arizona, California and Nevada -- and the Sierra Nevadas in California experiencing below-average flows. For instance, in the Colorado River Basin, most states will see flows between 25 and 69 percent of the 1971-2000 average, while some areas, including eastern Arizona and western New Mexico and parts of Nevada, will see flows less than 25 percent of average.

Last year, only the Southwestern states saw streamflows below 50 percent of average.

The only good news is found in the Northwest, where thick snow fields are filling streams and reservoirs with above-average flows.

Looking at the remaining snowpack, Arizona and New Mexico have the least to look forward to, with snowpacks as of yesterday at 22 and 20 percent of average for this time of year, according to the Natural Resource Conservation Service's SNOTEL snow and precipitation site. Washington and Oregon are the wettest with snowpacks at 140 percent and 113 percent of average, respectively.

Higher-than-average temperatures in March have melted snowpacks sooner than usual in the Colorado Basin, Wirth said.

"In a good year, the runoff runs into July, and in an exceptional year, it'll run into August," he said. "But it's starting early and it doesn't look like it's going to last a long time."

Officials say 2012 could end up being the second-driest year in the Colorado River Basin since the drought began in 2000. In response, Reclamation is preparing for less inflow into the reservoirs, which provide water for 25 million people in the seven-state basin.

Federal hydrologists predict total runoff for April through July in the upper basin will be about 3.5 million acre-feet, which is about 49 percent of the average runoff for the period of 1981 through 2010. The average for that period was 7.2 million acre-feet.

"It's in the bottom 10 percent for all the forecasts issued by the center," Wirth said, adding that anything below 4.5 million acre-feet is considered worrisome.

By September 30, Lake Powell is predicted to be at 3,644 feet -- about 68 percent full -- after peaking in July at around 3,660 feet. Last year, the sixth wettest since the federal government completed Glen Canyon Dam in 1963, the reservoir's level was a little higher in September: 3,653 feet, or 72 percent of capacity.

Colorado River Basin spared shortages

But it could be a lot worse, Wirth said. High flows from a generous 2010-2011 snowpack left Lakes Powell and Mead with more water than they often have at this time of year, helping to buffer them against the effects of this year's lackluster streamflows, he said.

"We do have pretty good carry-over storage," he said, adding that Reclamation should have no problem releasing the required 8.23 million acre-feet of water from Lake Powell to meet delivery obligations to the lower basin. "Even some of the smaller reservoirs in upper basin states, a lot of them filled and some of them spilled last year. Those gains that we made will be very, very critical to us."

Rose Davis, a spokeswoman for Reclamation's Lower Colorado Basin office, said officials there are not too concerned about the low river levels.

"The bottom line is that we have enough water to meet our obligations," she said. "We will be down quite a bit by the end of the water year in September, but we will not be anywhere near shortage."

But water users and managers alike should not forget that the region is still in a long-term drought, Wirth added.

NOAA's most recent drought monitor map, released last week, shows most of the southern half of the western U.S. dealing with drought conditions of varying degrees. Much of the Eastern Seaboard and the northern Great Plains states are also largely dry.

The most extreme drought conditions are predicted for west Texas, southeastern New Mexico and the Southeast, primarily southeastern Alabama, Georgia, South Carolina and Florida.

Stepped-up efforts by cities and states to stretch their limited water supplies during the drought have helped the region avoid shortages, Wirth said.

"I think people are just becoming better stewards," he said. "It seems like a better conservation ethic has helped us."

Click here to see NRCS's April streamflow map.

Click here to see NOAA's drought monitor map.

Reese writes from Santa Fe, N.M.