As authorities raced to reduce the level of Lake Oroville before the next storm, new evidence emerged on Monday that more than a decade ago state water officials insisted that the emergency spillway that forced the evacuation of nearly 200,000 residents this week was “a safe and stable structure founded on solid bedrock that will not erode.”

The revelations in a May 26, 2006, filing from the state Department of Water Resources to federal officials underscored how state officials rejected calls 11 years ago from three environmental groups to require that concrete be used to armor the emergency spillway at the nation’s tallest dam.

The department went on to say in its document to the Federal Energy Regulatory Commission that its geology division had closely evaluated the emergency spillway at Oroville, the tallest dam in the United States. Environmentalists’ concerns that the spillway — a 1,730-foot concrete lip along the reservoir’s edge, with only a tree-lined, earthen hillside below — could fail, triggering a catastrophe, were overblown, department officials said at the time.

“There are only one to four feet of erodable topsoil in the downstream area, and that erosion would not compromise the stability of the emergency spillway,” Michael A. Swiger, an attorney for the Department of Water Resources, wrote in the filing.

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Eventually, the federal commission agreed, and did not require the state to reinforce the emergency spillway. On Saturday, after water only about 1 foot deep flowed over it for the first time since the dam’s construction was finished in 1968, erosion carved in the hillside below so violently that the Department of Water Resources said the spillway could collapse within an hour, forcing Butte County’s sheriff to order a mass evacuation from what could have been one of the worst dam failures in U.S. history.

Monday, with state and federal dam operators trying desperately to lower the level of Lake Oroville through its damaged main concrete spillway before new storms come in on Wednesday night, Bill Croyle, acting director of the Department of Water Resources, said he was not ready to discuss earlier positions taken by his department.

“I, myself, in this incident command post, working with my partner agencies, have focused on addressing the challenges that we have at this very time,” he said at a mid-day news conference.

“We’re going to get into recommendations and concerns that were voiced in the past, but right now we’re focused on public safety,” he said, adding: “Before I answer a question like that, we need to do the homework and then we can have a better conversation on that.”

Those who raised the alarm 12 years ago said they were surprised at the time by the position of the Department of Water Resources, which was then under the administration of Gov. Arnold Schwarzenegger.

“All you have to do is look at the so-called emergency spillway and think about what you have witnessed in your backyard when you run a hose over a mound of dirt. It creates erosion. That’s what we said 12 years ago,” said Kathryn Phillips, executive director of Sierra Club California.

The Sierra Club, Friends of the River and the South Yuba Citizens League filed motions with the federal government in 2005 urging the that emergency spillway be reinforced because in a very wet winter, erosion might become so severe that the reservoir would suffer a loss of “crest control,” meaning the top would wash away, sending billions of gallons of water raging to Oroville, Marysville and other towns below.

“They just patted us on our little heads all the way along,” she said. “It’s beside me why they would think it wouldn’t erode. It seems they just didn’t want to spend the money.”

Not only were the state and federal officials opposed to armoring the spillway, the major water agencies that likely would have had to pay the costs were, too. In 2006, the Metropolitan Water District of Southern California, which provides water to 19 million people in Los Angeles, San Diego and other cities, along with the State Water Contractors, a collection of 27 water agencies that buy state water, filed a motion urging the federal government not to require a concrete emergency spillway.

On Monday, the two organizations said they didn’t take the stand over cost concerns, but contended they were simply following the recommendations of the state and federal agencies.

“I don’t recall seeing any cost estimates of it,” said Terry Erlewine, general manager of the State Water Contractors. “We just deferred to the appropriate agencies, the Corps of Engineers, FERC and DWR. We just deferred to them.”

Asked if he would support armoring the emergency spillway now, Jeff Kightlinger, general manager of the Metropolitan Water District, sidestepped the issue.

“I would defer to DWR working with the Army Corps to what is the appropriate safety standard, and then we’ll worry about the costs later,” he said, adding: “Flood control is a public benefit that is usually paid for by the state.”

Ron Stork, a policy analyst with Friends of the River, said the emergency spillway needs an upgrade — a job that likely would cost tens, if not hundreds, of millions of dollars — for public safety, and the costs should be paid by the Metropolitan Water District and the State Water Contractors, a group that includes the Santa Clara Valley Water District, Kern County Water Agency and Alameda County Water District.

“They never put it in writing, but it was common knowledge among the water community 12 years ago — DWR, local water agencies, consultants — that they didn’t want to pay the cost,” Stork said. “It’s easy to say it wasn’t needed because the experts said it wasn’t needed. If I was them, I’d own up.”

Holding his first news conference since the mass evacuations, Gov. Jerry Brown on Monday said he didn’t know about the past concerns from environmentalists about the spillway. But he assured citizens that the state was doing everything it could “to make sure we have a safe dam up there and all the other places where we have these kind of potential threats.”

The governor also on Monday sent a letter to the White House requesting direct federal assistance for some 10,000 evacuees from Butte, Sutter and Yuba counties.

“We live in a world of risk,” Brown said, referencing the collapse of the Bay Bridge after the 1989 Loma Prieta earthquake. “Stuff happens, and we respond.”

Butte County Sheriff Kory Honea stressed Monday that evacuation orders remain in place, with no set time for nearly 200,000 people in Butte County and neighboring areas to return home.

“We are working on a re-population plan that will get people back to their homes as soon as possible, as long as we can preserve public safety,” Honea said.

Meanwhile, state officials continued to race to lower the lake level, releasing 100,000 cubic feet per second of water down the main concrete spillway, as about 35,000 cubic feet per second naturally flowed into the lake. As a result, by nightfall, it was five feet below the top, and water was no longer running over the emergency spillway.

The danger, however, was that not only is a new storm forecast for Wednesday night, but the main spillway partially collapsed last week when a huge hole wore through the concrete. State water officials slowed releases through that spillway after the water, looking like Niagara Falls, carved a section of hillside away toward the main face of the 770-foot-tall Oroville Dam. If the main concrete spillway continues to crumble upstream, they may have to close off all releases or risk its collapse at the lake’s edge. Shutting off the water would send the lake level rising again. Croyle said his department is trying to lower the lake by 50 feet as quickly as possible.

Meanwhile, FERC ordered the Department of Water Resources to convene a five-member independent board of dam experts to review the condition of Oroville’s emergency spillway and the damaged regular concrete spillway, and to make recommendations about how to improve safety during the emergency and over the long term.

Trucks and helicopters Monday afternoon began dropping rocks on the compromised hillside behind the emergency spillway to slow the erosion.

The next Lake Orville area rain is projected to bring up to 8 inches of rain to the region from Wednesday night to Monday. While not as strong as the previous wet weather, officials are concerned.

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Staff writers Malaika Fraley, Katy Murphy and Mark Gomez contributed to this report.