Nearly 200,000 people living near the Oroville Dam in Northern California have been ordered to immediately evacuate the area after heavy rainfall weakened a spillway and threatened to unleash raging floodwaters onto neighbouring towns.

California Governor Jerry Brown issued an emergency order to facilitate the evacuations. "It's clear the circumstances are complex and rapidly changing," a statement on Twitter read. "The state is directing all necessary personnel and resources to deal with this very serious situation."

Authorities ordered residents living near Oroville – 150 miles north-east of San Francisco – to immediately evacuate the area after a massive hole in the dam's emergency spillway was found.

The Butte County Sheriff's office wrote on Facebook: "A hazardous situation is developing with the Oroville Dam auxiliary spillway. Operation of the auxiliary spillway has lead to severe erosion that could lead to a failure of the structure. Failure of the auxiliary spillway structure will result in an uncontrolled release of flood waters from Lake Oroville." The statement concluded with the warning: "This is NOT A Drill."

A decision was taken to evacuate thousands of people after experts warned that the hole in the spillway could cause the dam to collapse. Bill Croyle, director of Water Resources, said at a press conference: "Once you have damage to a structure like that it's catastrophic." He said helicopter crews were dropping rocks into the gap in the spillway. Authorities are also releasing water to lower the lake's level and prevent the channel from overflowing.

"I would rather be safe than sorry. I would rather have people move out of the area hopefully to safety," Sheriff Kory Honea said.