(Reuters) - A 30-foot (9-meter) hole has appeared in a section of the tallest dam in the United States that is expected to worsen, but there was no immediate threat it will fail, endangering thousands of area residents, California state officials said on Friday.

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State authorities and engineers on Thursday carefully released water from the Lake Oroville Dam in Northern California as water levels in the reservoir rose due to heavy rain and snow.

There was no imminent or expected threat to public safety or the dam, the Butte County Sheriff’s Office said, and the California Department of Water Resources said the structure was sound.

Still, authorities advised people living along the Feather River below the dam to gather important belongings and consider shelter if an evacuation warning is issued.

The earthfill dam is just upstream and to the east of Oroville, a city of more than 16,260 people 65 miles (105 km) north of Sacramento. At 770 feet (230 meters) high, the structure, built between 1962 and 1968, is the tallest dam in the United States, besting the famed Hoover Dam by more than 40 feet (12 meters).

Water levels at the dam on Friday were over 894 feet (273 meters), less than 7 feet (2 meters) from the top, said Doug Carlson, a spokesman for the Department of Water Resources.

On Tuesday, officials began noticing large chunks of concrete missing from the dam’s spillway. Erosion eventually caused a 200-foot-long (60-meter-long), 30-foot-deep hole to form near the center of the spillway, a structure used to control the release of water.

As the spillway continued to crumble, an emergency spillway was being considered, the department said on Twitter.

The department said it preferred not to use the emergency spillway because it would dump water onto trees and put debris into the Feather River, a source of water for parts of California.

Video on the sheriff department’s Facebook page showed about 35,000 cubic feet of water per second being released down the enormous slide into the river, but officials said the additional flow would not necessarily cause flooding.

“Flooding is based on total flow to the Feather River,” the Butte County Sheriff’s Office said in a statement. “The current forecasted total flow is not expected to exceed 75 cubic feet (2 cubic meters) per second, which is less than the flow in 2006 and half of the flow in 1997.”

(This story corrects conversion in first paragraph to 9-meter instead of 90-meter.)