Heavy Snowmelt Fills Reservoirs in California to the Brim

California’s reservoirs are brimming with a bountiful late snowmelt that has left the state’s water storage system in its best shape in nearly a decade.

With some of the upper reaches of the Sierra still buried under as much as 6 to 9 feet of snow at the end of May, spring runoff has approached twice the norm.

The unusually wet spring and heavy snowpack put Central and Northern California on flood watch for weeks, straining its rivers and the levees that guard them. But the weather was a boon to the state’s immense plumbing works.

“If you could try to design ideal storm systems as they moved through, 2006 would come pretty close,” said Dave Paulson of the State Water Project, which captures water in the north and ferries it more than 400 miles to the dry south. “The later in the year the water comes, the more advantageous for our system.”

On average, the state project can fill about 75% of its contract demands. This year it is delivering 100%. So is the federal Central Valley Project, the nation’s largest water supplier, which irrigates California’s agricultural midriff.

“The system is chock-full of water,” said Jeff McCracken of the U.S. Bureau of Reclamation, which operates the Central Valley Project.

State officials say this is the best water storage year since the El Nino winter of 1998.

The snow was stacked up to 20 feet on some mountain sites and more important, it was a wet snow, good for feeding runoff.

“We’re talking 4 and 5 feet of water content sitting over many square miles. That was common at higher elevations,” said Dave Hart of the state Snow Survey Program.

Still, he said: “It’s not a blockbuster year by any means.”

With April through July runoff expected to hover around 180% of the average, this will be the ninth wettest runoff year in a century of record-keeping. The wettest was 1983, when runoff was 220% of the norm.

The above average snowpack, which the state also experienced last year, bucks the long-term trend under global warming.

Forecasts differ as to whether California will grow wetter or drier with climate change. But there is a general consensus that as the temperature rises, more of the state’s precipitation will fall as rain and less as snow. That will diminish the snowpack, which acts as the state’s largest reservoir, and dramatically alter runoff patterns.

Computer models suggest that the snowpack could drop by a third to a half.

“That is natural storage that we’re blessed with that we won’t have,” state hydrologist Maury Roos said.

The trend is apparent in the Sacramento River basin, the state’s most important watershed, where spring runoff has progressively declined since the 1950s, reflecting earlier snowmelt.

In the early 1900s, nearly 45% of the river’s annual runoff occurred between April and July. By 2004, that figure was down to about 33%.

In a report Roos prepared for the most recent state water plan, he said a possible 3-degree centigrade temperature increase over the next century could raise average snow levels by 1,500 feet. The effect will be more pronounced in the northern Sierra than the south, where peaks are higher.

Overall, climate change will make it harder to operate the state’s big reservoirs. It will be even more critical for dam operators to maintain empty space to control floodwaters from winter rains. But come spring, there will be less snowmelt to refill the storage.

This year, dam managers faced the opposite challenge. They had to keep releasing water throughout the spring to make room for more snowmelt, running the risk of flooding downstream river courses.

With peak runoff now past them, officials say, they’re out of the woods. “The worst is behind us in terms of concerns” about flooding, said Gary Bardini, chief of hydrology for the state Department of Water Resources.