OROVILLE, Calif. — Nearly 200,000 people remained under evacuation orders today as California authorities try to fix erosion of the emergency spillway at the nation’s tallest dam that could unleash uncontrolled flood waters if it fails.

About 150 miles northeast of San Francisco, Lake Oroville — one of California’s largest man-made lakes — had water levels so high that an emergency spillway was used Saturday for the first time in almost 50 years.

The evacuation was ordered Sunday afternoon after engineers spotted a hole on the concrete lip of the secondary spillway for the 770-foot-tall Oroville Dam and told authorities that it could fail within the hour.

“I’m just shocked,” said Greg Levias, who was evacuating with his wife, Kaysi, two boys and a dog.

What they couldn’t fit in their trunk they piled as high as they could in their downstairs Yuba City apartment and joined the line of traffic attempting to leave the city where they had moved just three weeks ago.