CEDAR RIVER WATERSHED — Tendrils of mist rising off Chester Morse Lake and a rolling basin of snow-draped forest offer a stunning alpine scene at the primary source of Seattle's water supply.

It's hard to tell by looking that the 90,000-acre watershed could be threatened by climate change.

Two years ago, the snowpack was just 25 percent of its normal level. Last year, it was 30 percent above average in some places. This season, the wild and cold weather as winter arrived was later neutralized by warm rain that left the snowpack 10 percent above normal as of this week.

To some climate scientists, those big variations are signs of trouble.

So Wednesday, Seattle Mayor Greg Nickels took a helicopter ride to a measuring station high in the remote watershed to emphasize a twin message he has been trying to send for more than a year: that the Seattle water supply is secure, but changes still are needed in the way the city manages it — and the way customers use it.

"2004-2005 was a tough one; for me, that was the 'Aha' moment for climate change," said Nickels, wearing gloves and boots to tromp through the snow 4,000 feet above sea level. "We are learning how to manage this when we have these wide swings."

Is supply adequate?

The mayor points to projections developed with the University of Washington that show the Seattle water supply should be adequate for the next 50-60 years even if the snowpack declines by half.

But those projections come with significant caveats. For one, it assumes that between 2025 and 2045, several Eastside and South King County cities will totally phase out their use of Seattle water. For another, it assumes that management will be improved and that conservation efforts will continue.

And despite the optimistic projections, the city still assumes that two years out of every century it will get hit with such severe drought or other extraordinary events that it will need to ration water or take other measures to make it through.

Richard Palmer, an expert in water resources at the UW, agrees that there generally should be enough water for the city and the region for decades to come — but only with vigilant conservation and with the phase-out of suburban use of Seattle's watershed.

Palmer also adds that continued population increases in Seattle are likely to strain the system more than climate change will.

Although some local scientists have squabbled over the actual extent, they tend to agree that Cascade Mountain snowpacks have declined since the middle of last century. Many believe climate change will continue that trend.

And climate change poses other challenges. Experts say the water supply will increasingly come down as rain rather than snow. So reservoirs will be more important than ever. And there is growing concern the existing drainage system won't be able to handle the extra rain.

Officials say a big part of the strategy is conservation. Seattle Public Utilities, the city's water department, hopes to offer as many as 100,000 free low-flow shower heads this summer — $1 million worth.

Meanwhile, eight Eastside and South King County cities that belong to the Cascade Water Alliance are expected to finalize real-estate and water-rights deals by June to eventually draw much of their water from Lake Tapps in Pierce County, said Michael Gagliardo, the alliance's general manager.

"It's turned out to be a hugely complex transaction," he said. But he said he believes it will happen.

This winter, at least, above-average snowpack has been welcome for skiers. For Crystal Mountain, this season is in the top 10 for revenue, said spokeswoman Tiana Enger. The Summit at Snoqualmie's opening day came nearly a month early this season, although snowfall has tailed off, said spokesman John Pretty. Both ski areas are scheduled to close April 15.

"It looks like spring has definitely arrived," Pretty said.

Nick Perry: 206-515-5639 or nperry@seattletimes.com