Residents below the tallest dam in the United States, near Oroville in Northern California, were ordered to evacuate on Sunday after a spillway appeared to be in danger of imminent collapse.

The abrupt evacuation orders came as authorities said that an auxiliary spillway on the Lake Oroville Dam could give way at any time, unleashing floodwaters onto rural communities along the Feather River.

"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. "This is NOT A Drill."