May 27, 2017 0903 GMT
At least 160,000 people residing in Oroville and nearby cities were ordered to evacuated on Sunday night after water started to flow over the sides of an emergency spillway at the 230m-high Lake Oroville Dam.
The spillway - used to alleviate pressure on the dam - is severely corroded and could fail imminently and unleash flood waters, according to Butte County Sheriff's Office.
A large section of concrete at the bottom of the dam’s main spillway had already collapsed by Sunday, sending large chunks of concrete downstream.
“Immediate evacuation from the low levels of Oroville and areas downstream is ordered,” said Sheriff Kory L. Honea on Facebook. "This is NOT A Drill. This is NOT A Drill. This is NOT A Drill."
However, latest state figures showed that water levels had fallen to safe levels, meaning that little or no water was likely coming over the emergency spillway.
Joe Countryman, a member of the Central Valley Flood Protection Board and a former engineer with the US Army Corps of Engineers, said the threat of collapse due to erosion has diminished.
He warned that the risk of catastrophic flooding was still high because officials released water so quickly over the damaged main spillway that they may have further threatened its integrity, Countryman said.
This is the first time in the spillway’s nearly 50-year history that water is overflowing because of heavy rain.
On Friday, officials warned that a 9-meter hole had appeared in a section of the dam and that water levels were less than two meters from the top. However, they said there was no immediate threat.
Completed in 1968, the Lake Oroville Dam is the tallest in the US and 12 meters higher than the famous Hoover Dam.