Reservoirs Benefiting From Storms’ Deluge : Rainfall: Eleven days of downpours have increased runoff and boosted ground-water levels in Ventura County. A mudslide Sunday closes part of California 150.

The string of storms that drenched Ventura County for nearly two weeks swelled the area’s major reservoirs and substantially raised ground-water levels, water officials said Sunday.

Since the storms began Feb. 5, Lake Casitas on the county’s west side has risen by nearly 15,000 acre-feet--almost 60% of the amount used by the Casitas Municipal Water District in a year. An acre-foot of water is considered enough to supply a family of five for a year.

Lake Piru on the county’s northeast end, which is used to help replenish ground water beneath the Oxnard Plain, has increased by more than 30,000 acre-feet, and authorities expect the lake to continue rising.

For at least a few days, however, the county will have a chance to dry out, forecasters said. The brunt of a new storm, which had been expected to bring several more inches of rain beginning Tuesday, is now expected to pass north of the county, they said. Only a few scattered showers were reported Saturday night and Sunday, primarily in Rose Valley, Oxnard and Thousand Oaks.

But the effects of 11 days of heavy rainfall continued to be felt Sunday. After a mudslide about 2 a.m., the California Highway Patrol said California 150 between Lake Casitas and Carpinteria would be closed for at least a week. And two Oxnard youths trying to cross the rain-swollen Santa Clara River on their bicycles had to be rescued by helicopter Sunday afternoon when they became stranded on a sandbar.

As mountain streams continued to send runoff into reservoirs, water officials said Lake Piru may fill to capacity. They said as much as 20,000 additional acre-feet of runoff that has been trapped upstream, behind the dam at Pyramid Lake, will eventually be allowed to flow down Piru Creek to Lake Piru.

“There is a real possibility the dam could spill,” said James T. Gross, ground-water manager for United Water Conservation District, which operates the Lake Piru reservoir. Not since 1983 has the water level been high enough in Lake Piru to spill over the dam, he said.

After more than five years of drought, ground-water basins were rapidly declining when the rainy season began Oct. 1, Gross said. But with the recent storms, which dumped more than a foot of water in some parts of the county, ground-water levels have risen by between 20 and 75 feet, he estimated.

That will help push back the underground intrusion of seawater, which has been encroaching on freshwater supplies for decades, Gross said. The rainfall seeping into underground basins will also decrease the nitrate levels in areas such as El Rio that have been plagued with water-quality problems, he said.

“I think a lot of those intense drought effects will be alleviated,” Gross said.

But the water supply in the county’s east end, which depends on reservoirs in Northern California, has not benefited as much from the series of storms. Northern California reservoirs, though on the rise, have reached only about 55% of normal levels, said Maurice Roos, chief hydrologist for the California Department of Water Resources.

Since the wet weather began, Lake Shasta and Lake Oroville have risen by a few hundred thousand acre-feet, he said--a tiny portion of the giant reservoirs’ total capacities.

While Ventura County’s rainfall is ahead of normal, Northern California has received only about two-thirds of its normal precipitation for this time of year, he said.

“Certainly the outlook is brighter than it was a week ago and we are thankful for that,” Roos said. “But we are still about 10 or 11 inches of rain below what we should have for this time of year.”

Terry Schaeffer, a National Weather Service meteorologist in Santa Paula, said the storm due Tuesday will help Northern California but will probably bring only scattered showers in the south. No significant rainfall is in sight for Ventura County, forecasters said.

Daytime temperatures were expected to stay in the upper 50s and mid-60s, with nighttime lows in the 30s and low 40s.

State Department of Transportation officials said Sunday that all state highways in Ventura County had been reopened except for the six-mile section of California 150 that was closed by the massive mudslide early Sunday. The slide dumped about 20,000 cubic yards of mud and rocks on the highway just west of the Lake Casitas Recreation Area, said Dave Servaes, regional director of Caltrans.

Five dump trucks and one loading tractor were being used to cart off the mud and rocks to a site a mile away, Servaes said. But the cleanup work is slow going because each dump truck can only haul up to 12 cubic yards, he said.

After dispatching his workers for the past several days to clear mud and rocks from highways throughout the county, Servaes said he had to hire a private contractor to do most of the work. The cleanup is expected to take up to two weeks and cost about $150,000.

“We’re already stretched to the limit,” he said of his 116-member crew. “We can’t take anymore.”

Amtrak passenger trains and Southern Pacific freight service will remain shut for at least five days until inspectors can make sure bridges are safe.

With a good bit of sunshine and virtually no rain Sunday, some Ventura County residents set out to see what the storms had wrought. Dozens of fishermen decided to try their luck in the muddy waters of Lake Casitas.

“It’s just a little uncomfortable,” said Larry Bangs, who docked his boat because of brisk winds but continued to fish along the banks of the swollen lake.

“If it was snowing and he was in a cast, he would still be out here on the lake,” said Bangs’ girlfriend, Jianna Gentilli.

Bruce Schraeder, who operates a bait and tackle shop on the lake, said business was only a little slower than expected for a three-day holiday weekend.

“The weather is holding back a few people,” he said. “But weather like this makes catfishing good. The catfish like to feed on all the debris. They like it when the water is murky like this.”

But murky water apparently fooled two Oxnard youths who were trying to cross the Santa Clara River on their bikes. About 1:35 p.m. Sunday, authorities received calls from several people who spotted the teen-agers stranded on a sandbar. A sheriff’s helicopter rescued the pair.

A third teen-ager made it across the river with his bike, but he was completely soaked, Ventura County Fire Capt. Ranger Dorn said. The names of the youths were not available.

Dorn did not know how deep the water was but he said it was moving quickly.

“I guess they found the water a little deeper, a little colder, a little swifter than they thought,” said Ventura County Sheriff’s Deputy Chris Spangenberg, who landed the chopper on the sandbar.

Times correspondent Peggy Y. Lee contributed to this story.