Oroville dam: Water officials race against the clock to prevent disaster as heavy rain approaches

Hundreds of thousands evacuated over fear US's tallest dam could collapse

Almost 200,000 California residents remain under threat after being ordered to evacuate their homes as officials desperately search for a fix to a damaged spillway on the nation’s tallest dam ahead of heavy rain expected later in the week.

Yet there is no timetable to fix the eroded spillway (which helps to control the flow of water) at Lake Oroville – about 150 miles (241 km) north of San Francisco – Butte County Sheriffs said.

The water level behind the dam fell, however, allaying fears that the spillway would collapse, resulting in catastrophic flooding of the nearby town.

As rain approaches later this week, the California Department of Water Resources is up against the clock to repair the spillway. In one last ditch effort, officials said they may use helicopters to drop rock loads on the spillway to fill the gap.

Residents living below the lake, home to largest dam in the US, were ordered to evacuate on Sunday. All 188,000 people still remain under those orders.

“People were just panicking,” Nancy Borsdorf said. Ms Borsdorf has been an Oroville resident for 13 years, but she is staying at a shelter in nearby Chico until it is safe to return home.

“We’ve always loved and trusted our dam,” she added. “I’m really hopeful Oroville wasn’t flooded.”

Over 200,000 people have been evacuated after a hole emerged in an emergency spillway of the Oroville Dam, threatening to flood the surrounding area (Getty Images)

The Oroville dam was completed in the 1960s. As the area received significant rainfall in recent weeks, the water level raised, causing DWR officials to drain some water through the spillway for the first time in five decades.

But water officials denied that the state neglected the spillway.

“We have a very rigorous schedule of inspections that is determined by state and federal regulators,” said DWR spokesperson Eric See. “We actually do those inspections annually.”

Mr See added that workers are still working to identify the best plan to proceed with inspections.

But as the water level falls, as does the threat of flooding.

When officials ordered the Sunday evacuation of residents living below the 770-foot dam, engineers believed the spillway structure could fail within the hour.

The water level will need to drop 50 more feet by mid-week for the threat to be fully lifted.

The Red Cross said more than 500 people arrived at an evacuation centre in Chico.

The California National Guard is on notice to deploy its 23,000 soldiers and airmen in case of a disaster.

Still, it remains unclear what caused the rift in the spillway, but DWR officials believe that it has stopped crumbling.

It emerged this week that federal and state officials ignored warnings from local groups about the impact severe weather could have on the dam’s capabilities.

Built in 1968, the structure is 230 metres high, nearly nine metres taller than the Hoover Dam, the next largest in the country.

Additional reporting by the Associated Press.