SONOMA (CBS SF) — The Sonoma County Water Agency is lowering the inflatable rubber dam on the Russian River to prevent damage from raised river flows caused by heavy rain this week.

The Water Agency, also known as Sonoma Water, began inflating the dam near Forestville Monday. It became fully inflated on Saturday.

The giant, steel-belted rubber black bladder — eleven feet in diameter and eight inches thick — was custom made by a Japanese tire company. Every spring, it is inflated with water to create a temporary dam across the Russian River for use during the dry summer months, as long as the river stays calm.

On Thursday after the unusually heavy rain, the Water Agency began lowering the dam to prevent damage.

“If the river flows increase above 2,000 cubic feet per minute, then we make the call to lower the dam,” said Deputy Chief Engineer Kevin Booker.

With the past week’s rain, combined with the expected weekend storm, the flow is expected to increase to 6,000 cubic feet a minute.

“If the dam, for whatever reason gets washed out in the summer time, that would be bad,” said Booker.

It’s expected to take 1-2 days to lower the dam completely.

The dam usually remains inflated until fall. It creates a small pool of water for use in four off-stream infiltration ponds that help recharge groundwater, which is naturally filtered through sand and gravel and delivered to the Water Agency’s 600,000 customers in Sonoma and Marin counties.

Boaters must portage around the fully inflated rubber dam located downstream of Wohler Bridge, and notices have been posted warning the public not to recreate on or near the dam. Fishing is prohibited within 250 feet of the upstream and downstream sides of the dam.

The concrete sides of the rubber dam are safe, much of it brand new. Next year, the rubber bladder itself is scheduled to be replaced.

“It’s intended to last, I think, about 20 years. So this dam is nearing the end of its life span,” said community affairs specialist Barry Dugan.

It will take the Japanese a year to build the new bladder. Meanwhile, the dam is expected to be re-inflated sometime in the next couple weeks.

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