Residents below the tallest dam in the United States, near Oroville in Northern California, have been ordered to evacuate immediately after authorities said an auxiliary spillway was in danger of imminent collapse.

"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.

Officials hoped to avoid using Oroville Dam's emergency spillway, fearing it could cause trees to fall and leave debris cascading into water that rushes through the Feather River, into the Sacramento River and on to the San Francisco Bay.

Crews prepared for several days, clearing trees and brush.

Water began running over the emergency spillway around 8am, according to California's Department of Water Resources.

It was the first time the emergency spillway has been used in the reservoir's nearly 50-year history.

Water was expected to continuing flowing over the emergency spillway for 38 to 56 hours, agency spokesman Eric See said at a news conference on Saturday afternoon (local time).

In addition to the emergency spillway, water was also flowing through the main spillway that was significantly damaged from erosion, he said.

The earth fill dam is just upstream and to the east of Oroville, a city of more than 16,260 people.

How did this happen?

Unexpected erosion chewed through the main spillway earlier this week, sending chunks of concrete flying and creating a 60-metre-long, nine-metre-deep hole that continues growing.

State authorities and engineers on Thursday began carefully releasing water from the Lake Oroville Dam, some 105km north of Sacramento, after noticing the missing chunks of concrete from the spillway.

Engineers do not know what caused the cave-in, which is expected to keep getting bigger until it reaches bedrock.

(ABC)