Close up look at Oroville Dam damage

Exposed boat ramps at Lake Oroville Dam February 2009

Three local environmental groups filed a motion with the Federal Energy Regulatory Commission in 2005 urging federal officials to reinforce the Oroville dam's emergency spillway with concrete, the Mercury News reported.

A day after 188,000 people were evacuated from the towns surrounding Northern California's Oroville Dam, officials sounded a note of cautious optimism about containing the threat of flooding.

Department engineer and spokesman Kevin Dossey told the Sacramento Bee the emergency spillway was rated to handle 7,000 cubic metres per second, but it began to show weakness on Sunday after flows peaked at 350 metres feet per second.

A 200-foot long, 30-foot-deep hole had formed in the emergency spillway, according to reports.

Meanwhile, the Butte County sheriff said in a statement posted on social media: "Immediate evacuation from the low levels of Oroville and areas downstream is ordered, This is NOT A Drill".

Authorities issued the evacuation order on Sunday, saying that a crumbling emergency spillway on Lake Oroville Dam in north California could give way and unleash floodwaters onto rural communities along the Feather River. Bechtel is the largest privately-held construction company in the United States.

Officials from the California Department of Water Resources said they are considering using helicopters to drop loads of rock on the eroded spillway at Lake Oroville, about 240 kilometres north-east of San Francisco.

Lake Oroville is one of California's largest man-made lakes, and the 230m high Oroville Dam is the nation's tallest.

What nearly happened this year is a stark reminder of how much we depend on dams, levees, bypass channels, seawalls and other human efforts to keep destructive tendencies of nature at bay.

Officials issued mandatory evacuations for almost 190,000 people in the three counties Sunday. By late on Sunday night the situation had improved, but the evacuation order has yet to be lifted.

The hospital is outside the flood zone below the dam on Oroville Lake and sits on a hill.

"We need to do everything we can to maximize our ability to move water our of this reservoir - not just for the coming storm but for the coming storms", said Bill Croyle, acting director of department of water resources.

Evacuees have been ordered to head north towards Chico, where roads are jammed with traffic, hotels are full to capacity and Red Cross stations have been set up to deal with the influx of evacuees. The state water resources announced on Sunday that the evacuation orders were not a drill. They hope to patch the hole before more rain falls in the region on Wednesday and Thursday.

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