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Cull Canyon reservoir is photographed in Castro Valley, Calif., Wednesday, Feb. 5, 2014. The reservoir's water level is being kept low due to a dam that is seismically unsafe and is filling with silt, so Alameda County is proposing a plan to notch the spillway, allowing Cull Creek to flow naturally. (Anda Chu/Bay Area News Group)

CASTRO VALLEY -- Cull Canyon Reservoir could be drained within the year, doomed by a seismically unsafe dam combined with so much silt filling it that it is only 6 feet deep.

It would cost millions to dredge the reservoir and strengthen the dam. Instead, Alameda County engineers want to notch the reservoir's weir, an underwater dam above its spillway. The notch would let the water flow downstream and restore Cull Creek to a more natural state above the spillway, with the area around it becoming a meadow.

The notch would not affect the separate popular swim lagoon run by the East Bay Regional Park District.

A visitor walks his dog at Cull Canyon reservoir in Castro Valley, Calif., Wednesday, Feb. 5, 2014. The reservoir’s water level is being kept low due
A visitor walks his dog at Cull Canyon reservoir in Castro Valley, Calif., Wednesday, Feb. 5, 2014. The reservoir's water level is being kept low due to a dam that is seismically unsafe and is filling with silt, so Alameda County is proposing a plan to notch the spillway, allowing Cull Creek to flow naturally. (Anda Chu/Bay Area News Group)

"The first storm every winter, the reservoir fills up," said Hank Ackerman, Alameda County flood control program manager. "If we were to notch the weir, it could keep water from backing up in storms."

Cull Canyon Reservoir was 30 feet deep when it was built in 1963. But about 12,000 to 20,000 cubic yards of silt and debris flow into the reservoir every year. "It's almost completely silted up," Ackerman said.

The reservoir never was very effective at flood control, he said. "It was constructed basically to put a road across over to the high school, which is now Canyon Middle School," he said. Heyer Avenue, a heavily traveled east-west road, runs along the top of the dam.

At one time, Cull Canyon Reservoir was a popular fishing site.

"East Bay Regional Park District used to stock it for fishing, but five or 10 years ago, it got shallow enough that there was a big fish kill on a hot August day, so they stopped," Ackerman said. "This last October, it dried up completely."

To dredge the reservoir would cost about $16 million and require 28,000 round trips by double bottom dump semitrailers to remove the estimated 450,000 cubic yards of sediment, he said.

Because of concerns the dam could not withstand a major earthquake, a 12-inch pipe was punched through the concrete spillway in 2006, lowering the reservoir level and easing pressure on the dam. Bringing the dam up to current seismic safety standards would cost $8 million to $10 million, Ackerman said.

If the county removed the silt, the dam would need reinforcing. "What makes it unsafe is the water behind it," he said.

The problem of silt filling the reservoir has plagued the county for years, but its options were limited until now. The dam was built with funds from the Davis-Grunsky Act of 1960, and under the grant's terms, the county had to maintain the dam. That grant expired in December, freeing up the county to modify the dam.

"We can notch it for less than $1 million," Ackerman said. "The state Division of Safety of Dams would no longer consider it a dam if it didn't hold back water."

One longtime visitor said he hates to see the reservoir go but does not see any options.

"It was always a great place to go to," said Marc Crawford, who moved to Castro Valley with his family when he was 5 years old and is now chairman of the Castro Valley Municipal Advisory Council. "It's one of those aesthetically beautiful places that is an amenity of living here." Crawford and his wife both passed by the reservoir daily when they attended Canyon Middle School.

"We spent a lot of time there. It's a shame, but there's so much silt, and I don't know if it's realistic for the county to bear that cost," he said.

Cull Canyon reservoir is photographed in Castro Valley, Calif., Wednesday, Feb. 5, 2014. The reservoir’s water level is begin kept low due to a dam
Cull Canyon reservoir is photographed in Castro Valley, Calif., Wednesday, Feb. 5, 2014. The reservoir's water level is begin kept low due to a dam that is seismically unsafe and is filling with silt, so Alameda County is proposing a plan to notch the spillway, allowing Cull Creek to flow naturally. (Anda Chu/Bay Area News Group)

The county Department of Public Works hopes to have a design for the notching by April, Ackerman said. County workers will then present the design to the advisory council and hold a public meeting, with work scheduled to start in late summer.

The project would not completely restore Cull Creek to its natural state; the spillway would remain.

"If we notch it, it would be something that would be reversible if the community later decided they wanted a dam," Ackerman said.

Contact Rebecca Parr at 510-293-2473 or follow her at Twitter.com/rdparr1.