Evacuees from cities surrounding the Oroville Dam line up Monday for dinner at a shelter in Chico, Calif. Thousands were ordered to leave their homes after a damaged California spillway threatened to unleash a 30-foot wall of water.

OROVILLE, Calif. — Helicopters dropped giant rock-filled sandbags into place Monday to shore up a California reservoir that had threatened to breach its banks and unleash a 30-foot wall of water, but officials said an evacuation order covering nearly 200,000 people would stay in place until they are sure it’s safe to return home.

Officials defended their decision to issue the hasty order to abandon homes downstream from the nation’s tallest dam, about 150 miles northeast of San Francisco. They said it was necessary for public safety after engineers spotted a hole in an emergency spillway, which they feared could have failed within an hour.

The water level of the massive reservoir known as Lake Oroville dropped Monday, slightly easing fears of a catastrophic collapse. But with more storms on the horizon, crews raced to fortify heavily eroded soils where water flowed over the edge of the reservoir and carved huge channels in the earth as it gushed toward the Feather River.

The acting head of the state’s Department of Water Resources said he did not know if anything had gone wrong and was unaware of a 2005 report that recommended reinforcing the earthen emergency spillway with concrete for just such an event. The spillway had never been used in the dam’s nearly 50 years of operation, and it was not near capacity when it began to fail.

“I’m not sure anything went wrong,” Bill Croyle said. “This was a new, never-having-happened-before event.”

Croyle and the local sheriff sought to reassure the public that downstream communities were safe until water began spilling over the lake’s edge Sunday and a chocolate-colored torrent began chewing through the slope below it.

The Department of Water Resources said conditions were stable at noon and then tweeted an evacuation order at 4:45 p.m. warning of a possible failure within the hour, saying “this is not a drill.”

Chaos ensued as anxious residents rushed to pack up their families and abandon several communities in Butte, Yuba and Sutter Counties. The flat northern Central Valley is known mainly for its abundant agriculture, which is fed by dammed-up rivers that tumble down through the Sierra Nevada foothills.