America's tallest dam is in danger of collapse, forcing hundreds of thousands to leave their homes.

At 770 feet (230 meters) high, the structure, built between 1962 and 1968, is the tallest US dam, exceeding the Hoover Dam by more than 12 metres.
At 770 feet (230 meters) high, the structure, built between 1962 and 1968, is the tallest US dam, exceeding the Hoover Dam by more than 12 metres. (TRT World and Agencies)

People who live near the tallest dam in the United States have been ordered to leave their homes. The evacuation is being carried out because a crucial spillway is in danger of collapsing.

The Oroville Dam lies upstream from the city of Oroville in Northern California. Nearly 200,000 people in Marysville, Yuba, Butte and Sutter counties were told to leave on Sunday after authorities said the dam could unleash floodwaters along the Feather River, which runs through the area.

"Immediate evacuation from the low levels of Oroville and areas downstream is ordered," the Butte County sheriff said in a statement posted on social media.

The state water resources department said crews using helicopters would drop rocks to fill a huge gouge, and authorities were releasing water to lower the lake's level after weeks of heavy rains in the drought-plagued state.

Local authorities have said the situation remains unpredictable.

TRT World's Simon McGregor-Wood has more.

"Right call"

Butte County Sheriff Korey Honea said at an earlier news briefing that experts assert the hole forming in the spillway could compromise the structure. Rather than risk thousands of lives, the decision was made to order evacuations.

Officials said they feared the damaged spillway could unleash a 30-foot wall of water on Oroville, north of the state capital Sacramento.

"Once you have damage to a structure like that it's catastrophic," acting Water Resources director Bill Croyle told reporters. But he stressed "the integrity of the dam is not impacted" by the damaged spillway.

Asked about the evacuation order, Croyle said "It was a tough call to make." He added: "It was the right call to make."

'Travel warning'

The Yuba County Office of Emergency Services urged evacuees to travel only to the east, south or west. "DO NOT TRAVEL NORTH TOWARD OROVILLE," the department warned on Twitter.

Evacuation centres were set up at a fairgrounds in Chico, California, about 20 miles northwest of Oroville, but major highways leading south out of the area were jammed as residents fled the flood zone and hotels quickly filled up.

The Oroville Dam is nearly full following winter storms that brought relief to the state after four years of drought. Water levels were less than 2 metres from the top of the dam on Friday.

State authorities and engineers on Thursday began releasing water from the dam after noticing that large chunks of concrete were missing from a spillway.

California Governor Jerry Brown asked the Federal Emergency Management Agency on Friday to declare the area a major disaster due to flooding and mudslides brought on by the storms.

Source: TRT World