In 2005, three environmental groups warned state and federal officials about what they believed was a problem with the Oroville Dam's emergency spillway - the same one that was at risk of collapsing this week, after storms caused the adjacent reservoir to swell.

The groups' concern, which seems to have fallen on deaf ears at the time, was that the emergency spillway is not really a spillway. Rather, it's a 1,700-foot-long concrete weir that empties into a dirt hillside.

That means in the event of severe flooding, water would erode that hillside and flood nearby communities in Northern California's gold country, the groups said then.

That nearly happened Sunday, when a hole in the emergency spillway threatened catastrophic floods and prompted officials to evacuate nearly 200,000 residents, who remain uncertain when they'll be allowed to return to their homes - especially with three storms lined up to drench Northern California over the next week.

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In October 2005, as the Oroville Dam was going through a re-licensing process, the three groups filed a motion urging a federal regulatory agency to require state officials to armour the emergency spillway with concrete so that in the event of extreme rain and flooding, water would not freely cascade down and erode the hillside. The upgrade would have cost millions of dollars and no one wanted to foot the bill, said Ronald Stork, senior policy advocate for Friends of the River, one of the groups that filed the motion.

This June 23, 2005 photo provided by the California Department of Water Resources shows the main concrete spillway that has also been damaged. Photo / AP
This June 23, 2005 photo provided by the California Department of Water Resources shows the main concrete spillway that has also been damaged. Photo / AP

"When the dam is overfull, water goes over that weir and down the hillside, taking much of the hillside with it," Stork told The Washington Post. "That causes huge amounts of havoc. There's roads, there's transmission lines, power lines that are potentially in the way of that water going down that auxiliary spillway."

Federal officials, however, determined that nothing was wrong and the emergency spillway, which can handle 350,000 cubic feet of water per second, "would perform as designed" and sediment resulting from erosion would be insignificant, according to a July 2006 memo from John Onderdonk, then a senior civil engineer for the Federal Energy Regulatory Commission.

"The emergency spillway meets FERC's engineering guidelines for an emergency spillway," Onderdonk wrote. "The guidelines specify that during a rare flood event, it is acceptable for the emergency spillway to sustain significant damage."

Fast-forward 11 years. The erosion of the emergency spillway became severe, with only up to 12,000 cubic feet of water per second, according to the Oroville Mercury Register. That's a little more than 3 per cent of what officials said the spillway can handle.

The water level in massive Lake Oroville rose significantly after potentially record-setting rain surged through the state following a long drought. The Oroville Dam, the tallest in the country, at 770 feet, remains stable, officials said as the water level fell several feet, to a point at which water was no longer spilling over.

But the structure of the spillways, which are designed to release water from the reservoir in a controlled fashion, have crumbled.

On Monday night, Gov. Jerry Brown (D) requested federal assistance, noting in a letter to President Trump that "officials are aggressively attempting to lower Lake Oroville's water levels, as another atmospheric river storm system is scheduled to arrive within 48 hours." The emergency, Brown wrote, is "of such severity and magnitude that continued effective response is beyond the capabilities of the state and affected local governments and supplemental federal assistance is necessary to save lives and to protect property, public health and safety, and to lessen the effects of this serious situation."

The situation has stabilised, Brown said at a news briefing in Sacramento. But the evacuation remained in place, "because there's uncertainty," he said. "Better be safe than sorry."

"My heart goes out to all those who have to live with the concern and the fear that this kind of a situation engenders," the governor said.

Asked about the 2005 motion that urged FERC to require the state to reinforce the emergency spillway, Brown suggested that he had only learned of the report after authorities frantically worked to avert the threat of an immediate flooding disaster over the weekend.

"Glad we found out about it," Brown said. "It was not part of the record before that, at least the record that I saw."

Earlier this month, a portion of the main spillway - a 3,000-foot-long structure lined in concrete - eroded because of the high volume of water spilling over from the reservoir, creating a crater-like hole. Officials with the California Department of Water Resources, which owns and operates the dam and reservoir, then decided to use the adjacent emergency spillway for the first time since the dam was built nearly 50 years ago.

Sheets of water began spilling over the emergency spillway and onto the hillside, carrying mud and debris into the nearby Feather River.

The emergency spillway appeared to be working as expected - until Sunday, when officials spotted a hole. That raised fears of a catastrophic flood that could wipe away Oroville, a town of 16,000 people, and prompted officials to evacuate nearly 200,000 area residents.

Oroville in California is empty after residents were ordered to evacuate the area. Photo / AP
Oroville in California is empty after residents were ordered to evacuate the area. Photo / AP

"Auxiliary spillway at Oroville Dam predicted to fail within the next hour. Oroville residents evacuate northward," the state water agency tweeted shortly before 5pm Sunday.

Stork said he believes none of that would have happened had officials listened to his and others' concerns and built a proper emergency spillway 12 years ago. The two other groups that filed the 2005 motion were the Sierra Club and the South Yuba River Citizens League.

"They told us not to worry. All was good. Everything was fine. It's all safe," Stork said. "First of all, they're not supposed to fail. That's not what we do in a first-world country. We don't do that. We certainly don't do that with the nation's tallest dam. An auxiliary spillway isn't supposed to cause lots of havoc when it's being used."

Construction would have cost at least $100 million, Stork said, and the state contractors in Southern California that buy water from Northern California would have been forced to pay for it. The Metropolitan Water District of Southern California, which provides water to 19 million people in Los Angeles, San Diego and other areas, and the State Water Contractors would have shouldered the cost and deemed the upgrades unnecessary, according to the Oroville Mercury Register.

"The people who are bearing the personal risk of being killed and having their homes washed away are the people of Northern California," Stork said.

Pressed during a news conference Monday about the 2005 motion, Bill Croyle, acting director of the Department of Water Resources, said he was not familiar with the conversations that happened then.

"It's the first time it's ever taken water," Croyle said of the emergency spillway. "We don't know exactly why this erosion occurred."

Lester Snow, the agency's director from 2004 to 2010, told the Mercury Register that he did not recall specific information about the debate over the emergency spillway 12 years ago.

"The dam and the outlet structures have always done well in tests and inspections," Snow said. "I don't recall the FERC process."

The crisis seemed to have been averted by Monday. Lake Oroville had dropped to 898 feet by 4am, according to the Sacramento Bee. Water flows into the emergency spillway at 901 feet.

Officials doubled the flow of water out of the main spillway to 100,000 cubic feet per second, with the hope of lowering the lake level by 50 feet to leave room for upcoming rain.

Rain is expected through the region Wednesday and Thursday, with showers lingering Friday and Saturday, according to the National Weather Service. Water levels are also expected to rise later this week and into early next week.

Officials said that they're continuing to monitor the spillways for further erosion.