At least 188,000 people will have to stay away from their homes as crews work to calm the potential flood danger around a dam's troubled emergency spillway in northern California Monday, emergency officials said.

RIGHT NOW: Road below top of emergency spillway washed away. Main spillway discharging to the right. #OrovilleDam pic.twitter.com/dChnHSGJvV — KenWayne (@KenWayneKTVU) February 13, 2017

The evacuations were announced suddenly after engineers discovered a hole that was eroding near the top of the 770-foot-tall Oroville Dam’s emergency spillway Sunday, sparking fears it could fail. The Oroville Dam, about 150 miles northeast of San Francisco, is the tallest in the country.

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Officials confirmed water levels at Lake Oroville -- one of the state's largest man-made lakes -- were falling Monday. Still, Butte County Sheriff Kory Honea said the situation wasn't safe enough for people to return home, and officials were working on a plan to get people back in an orderly manner. Emergency officials were also eyeing a storm predicted to hit the area Wednesday or Thursday.

The sheriff said the spillway may indeed need repairs before evacuations are lifted, but he gave no timetable. He said earlier that helicopters could plug the hole by dropping rocks into the crevasse.

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Flows into the lake were just under 45,000 cubic feet per second Monday, California Department of Water Resources officials said. Outflows remained high at nearly 100,000 cubic feet per second.

The cities of Oroville, Gridley, Live Oak, Marysville, Wheatland, Yuba City, Plumas Lake and Olivehurst were all under evacuation orders. Emergency officials stressed that the dam was structurally sound, Fox 2 reported.

The “entire” California National Guard was on alert to help in case of flooding, according to a Pentagon spokesman. Some 23,000 members of the Guard were “prepared to respond” should the dam break, Capt. Jeff Davis said.

Hundreds of cars carrying panicked and angry people were sitting in gridlocked traffic Sunday.

"The police came and told us to evacuate," said Kaysi Levias, who was with her husband, Greg, at a gas station as they attempted to get out.

Officials warned residents at the time that the spillway could fail within an hour.

"I'm just shocked," Greg Levias said. "Pretty mad."

"Not giving us more warning," said Kaysi, finishing his sentence.

Unexpected erosion chewed through the main spillway earlier this week, sending chunks of concrete flying and creating a 200-foot-long, 30-foot-deep hole. Engineers don't know what caused the cave-in, but Chris Orrock, a spokesman for the state Department of Water Resources, said it appears the dam's main spillway has stopped crumbling even though it's being used for water releases.

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Fox News' Mike Arroyo, Lucas Tomlinson and The Associated Press contributed to this report.