TheBakersfieldCalifornian.com

Now is not a good time for a big earthquake on the Kern River Fault.

For the first time since 2006, when the U.S. Army Corps of Engineers placed a safety restriction on the amount of water stored behind the two earthen dams at Isabella Lake, the reservoir has exceeded that safety zone.

Officials say a certain amount of wiggle-room is built into the restrictions and the violation of the limit is not cause for immediate concern. However, they have directed the Kern River water master to release more water downstream.

"We have asked them to increase the outflow as much as possible without damaging the channel downstream," said John Johannis, chief of water management for the Army Corps.

Just three months ago Johannis cited flow projections predicting the lake level this summer would not exceed 350,000 acre feet, well below the maximum fill restriction of 360,000 acre feet, which is about 63 percent of the lake's capacity.

But a later analysis of the snowpack raised those flow projections.

By last weekend, what was left of the season's massive snowpack combined with extremely hot temperatures and the arrival of monsoonal thundershowers to send more water into the lake than was anticipated.

Trying to predict what nature will do is always dicey, said Kern River Water Master Chuck Williams.

"There are too many variables," he said.

Johannis said Williams has done a terrific job of managing the lake level in a "strange year."

"He should be commended for that," Johannis said.

DAM RISKY

It was five years ago that the now 58-year-old Isabella dam system was identified by the Corps as one of the most at-risk facilities in the nation.

Not only was water found to be seeping beneath the dam but the geological foundation on which the auxiliary dam was constructed was determined to be made of an alluvial rock material that could be "potentially liquefiable" in a strong earthquake, causing the collapse of the dam and allowing a massive wall of water to race through the Kern River Canyon toward Bakersfield.

When several exploratory trenches were dug across the Kern River Fault, which runs directly below the auxiliary dam, radiocarbon dating and other testing methods showed the fault to be active.

According to the Army Corps' analysis, the fault has moved three times in the past 10,000 years, the last time about 3,300 years ago. In addition, the trenching project revealed that at least six seismic events have occurred on the fault within the past 35,000 years.

To top it off, the Corps determined that the spillway on the main dam is too small to handle the kind of catastrophic storm that only comes once in 10,000 years, the kind of event engineers refer to as the "probable maximum flood."

SIX DECADES OF PROTECTION

As a result of the studies, the Corps has been developing options to improve the dams, which were completed in 1953. One alternative would include raising the crest of both dams by three to four feet. It would also feature massive improvements to the weaker auxiliary dam, such as a 100-foot buttress downstream, an upstream rock buttress and treatment to strengthen the foundation.

Before the dams were built, periodic flooding in the Bakersfield area was not uncommon. At a public scoping meeting held last spring in Kernville, Veronica Petrovsky, senior manager for the dam improvement project, said the dams have prevented flooding downstream on numerous occasions.

Now that the dams have been deemed to be at-risk of failure, officials inspect them regularly to detect potential stability problems as early as possible. Last year, the Corps shored up a low spot on the auxiliary dam's crest to reduce the risk of overtopping, which can damage the earthen structure.

Public meetings have been held last year and this year to keep residents informed and to allow a forum for questions.

While safety is a concern downstream, many who live near the lake want to make certain recreation and tourism are taken into consideration when work on the project finally begins.

Since 2007, the Army Corps has received more than $24 million for the project. The Final Dam Safety Modification Report is expected in early 2012. Construction isn't expected to begin until at least 2014.

HOW DEEP WILL IT GET?

The water level in the lake, located about 40 miles northeast of Bakersfield, is not expected to drop out of the restricted zone for another week to 10 days, Johannis said.

According to the guidelines of the fill restriction, the lake's water level may be allowed to temporarily encroach into the restricted area in wet water years like this one, or during unusual weather phenomena.

However the water level must be lowered to the maximum allowable elevation, the guidelines say, "as quickly as possible without exceeding channel capacities or causing excessive damage downstream of the dam."

Williams said that depending on weather and other factors, the rising lake level could reach 370,000 acre feet before it begins to recede.

But it's important to keep it in context, he said.

Yes, it means the lake level could rise about one foot above the fill-limit. But that means the water level would still be 19 feet below the spillway on the dam.

"I think we have wiggle room," Williams said.