Silt Aside, Reservoir May Last 850 Years

The 200,000 Ventura County residents who rely on Lake Piru for drinking water and irrigation supplies will have to look for another water source--in about 850 years.

A bottom survey of the lake that is conducted about every 10 years shows its capacity has decreased 1.3% since 1985, said Jim Kentosh, manager of operations and maintenance for the United Water Conservation District.

Silt contained in runoff from the 4-mile-long lake’s 432-square-mile watershed has reduced its capacity to 87,187 acre-feet, Kentosh said. An acre-foot of water is sufficient to supply a family of four for one year.

“That’s the normal evolution of a lake,” Kentosh said. “At this rate, if I do my math right, the lake will last 846 years.”

The bottom survey, conducted through a combination of sonar and aerial photography, is used for long-range planning efforts, Kentosh said.

At current sedimentation rates, the lake’s sole outlet pipe, which sends water to Oxnard, Fillmore, Santa Paula and elsewhere, should remain usable for the next quarter-century, depending on the weather. Heavy downpours that wash unstable soil from burn areas into the lake’s water sources are responsible for most of the sedimentation, Kentosh said.

The lake, which has an average depth of about 72 feet, formed in 1955 after construction of the 200-foot-high Santa Felicia Dam.