HONOLULU (AP) — Heavy rains prompted a warning to 10,000 Honolulu residents that they might need to evacuate their homes as water levels in a dam rose to dangerously high levels, but officials said the century-old earthen reservoir wasn't in immediate danger of collapsing.

Workers siphoned and pumped water out of the dam Thursday to bring levels down a day after Tropical Storm Olivia dumped rain on Oahu and Maui.

"This is not a dam breach situation. We're nowhere close to that," said Ernie Lau, the chief engineer for the Board of Water Supply, the agency that manages the dam.

Water levels in the dam rose 4-to-5 feet (nearly 1.5 meters) overnight. Meteorologists downgraded the storm to a tropical depression as it moved away from the islands and warned lingering moisture could bring more rain.

Plans call for a mandatory evacuation if the water reaches 1 foot (30 centimeters) under the top of the dam, Lau said.

The water was 5 feet below the top of the dam on Thursday. It was also 18 inches (45 centimeters) below a spillway. Areas downstream would flood if water goes over the spillway, Lau said.

Lau said the agency wants people who live near the dam to know that they would want to evacuate if the dam were to fail. But he stressed that wasn't imminent.

The dam is Nuuanu Dam #1, a reservoir built in 1905 in a residential neighborhood near downtown Honolulu called Nuuanu. It has the capacity to hold 21 million gallons (80 million liters).

Lau said workers began siphoning water away from the dam before Olivia reached the islands. But these efforts were unable to keep pace with the 7.3 inches (18 centimeters) of rain that fell in Nuuanu during the storm. On Thursday morning, Lau decided to begin pumping water out of the dam with the help of Honolulu firefighters.

Lau said workers made some headway and water levels came down by 2 to 3 inches. He said they'll make more progress if the rains decrease. Pumping will continue over the next week, he said.

The agency is the water utility for nearly 1 million people in Honolulu and surrounding towns on Oahu.

Andrew Pereira, a spokesman for the city, urged residents to be aware of the situation.

"While it appears the rain is subsiding, we are taking a cautious approach and asking residents to remain on alert," he said in an email.

Sherwod Chock, an artist who has lived in the neighborhood for nearly 50 years, said officials have never warned about high water levels in the dam before.

"It looks it like it can hold it, but you never know," he said. As for whether he had plans if the dam breached, he said: "I think God's going to take care of us."

The dam is one of multiple dams in Nuuanu that are used for flood control. Water levels at a bigger dam, Nuuanu Dam #4, are much lower. This dam also has an additional method for releasing water in a controlled fashion that Nuuanu Dam #1 lacks.

Meteorologists say moisture from the storm will linger until Friday.

Olivia was a hurricane earlier this week but slowly weakened as it approached Hawaii.

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