Officials have ordered the evacuation of as many as 160,000 people in northern California as the emergency spillway of one of the largest dams in the United States threatens to fail.
The auxiliary spillway at the Oroville Dam, about 125km north of Sacramento, had suffered severe erosion, the National Weather Service said in an alert - and its failure "would result in an uncontrolled release of flood waters from Lake Oroville".
Evacuees hurrying away from low-lying areas jammed local highways and petrol stations and made last-minute hotel bookings, the Sacramento Bee reported. The wide-ranging evacuation orders covered an area with more than 160,000 residents, the paper said.
In issuing its initial evacuation orders, the state's Water Resources Department tweeted that the emergency spillway could fail "within the next hour."
In the ensuing hours, the department sent more water down the dam's main spillway, in an effort to "avert more erosion at the top of the auxiliary spillway," the department said in a statement.
A massive hole that opened up last week in the dam's main spillway led to the use of the emergency spillway - the first time it had been used in the dam's 48-year history, the water department said.
The Oroville Dam itself - the United States' tallest, at nearly 235 metres - "is sound and is a separate structure," the water department said.
It is the first time that Lake Oroville has experienced such an emergency in the dam's near 50-year history.
California, which has faced a drought in recent years, has seen heavy rain and snowfalls this winter.