Hundreds of thousands of people remained under evacuation orders Monday in the low levels of Oroville, California, and several areas downstream from Lake Oroville as authorities try to fix erosion of the emergency spillway at the nation's tallest dam, which 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 at Oroville Dam was used Saturday for the first time in almost 50 years. The main spillway was damaged in storms this week.

The evacuation of nearly 200,000 people was ordered Sunday afternoon after engineers spotted a hole on the concrete lip of the secondary spillway for the 770-foot-tall dam and told authorities that it could fail within the hour. The potential impact would be all the way to Marysville–Yuba City and Beale Air Force Base.

Crews on Monday were busy filling large bags with rocks to help shore up the erosion concerns along the spillway. Those bags are expected to be flown via helicopters to the spillway and installed in hopes of quelling a serious flood.

The sheriff's office posted the following on its Facebook page Sunday afternoon, emphasizing that the evacuation orders were not a drill:

"A hazardous situation is developing with the Oroville Dam auxiliary spillway. Operation of the auxiliary spillway has led to severe erosion that could lead to a failure of the structure. Failure of the auxiliary spillway structure will result in an uncontrolled release of flood waters from Lake Oroville.

"In response to this developing situation, DWR (Department of Water Resources) is increasing water releases to 100,000 cubic feet per second.

"Immediate evacuation from the low levels of Oroville and areas downstream is ordered."

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

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.

Panicked and angry residents sat in bumper-to-bumper traffic hours after the evacuation order was given.

Late Sunday, officials said the evacuation orders remained in place despite the fact water was no longer spilling over the eroded area.

"There is still a lot of unknowns," Butte County Sheriff Kory Honea said at a news conference. "We need to continue to lower the lake levels and we need to give the Department of Water Resources time to fully evaluate the situation so we can make the decision to whether or not it is safe to repopulate the area."

At least 188,000 people on Sunday had been asked to evacuate areas south of the dam, according to state officials. They said the cities of Oroville, Gridley, Live Oak, Marysville, Wheatland, Yuba City, Plumas Lake and Olivehurst are all under evacuation orders. Those mandatory orders remained in place as of Monday morning, with the exception of Wheatland, which was under a voluntary evacuation mandate.

Gov. Jerry Brown issued an emergency order Sunday night to bolster the state’s response to the situation.

"I’ve been in close contact with emergency personnel managing the situation in Oroville throughout the weekend, and it’s clear the circumstances are complex and rapidly changing," Brown said. "I want to thank local and state law enforcement for leading evacuation efforts and doing their part to keep residents safe. The state is directing all necessary personnel and resources to deal with this very serious situation."

Traffic jammed up quickly on highways 99 and 70 out of the area. The southbound lanes on each of the highways eventually were closed and made into northbound lanes to aid in the evacuations.

Water began flowing over the emergency spillway at the dam in Northern California on Saturday for the first time in its nearly 50-year history after heavy rainfall.

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

Department of Water Resources spokesman Eric See said earlier Sunday that skies were clear, and the overflow was steadily slowing. It was expected to stop by midday Monday.

The lake is a central piece of California's government-run water delivery network, supplying water for agriculture in the Central Valley and residents and businesses in Southern California.

Some evacuees waited for hours to get out of Marysville and surrounding communities. Judy and John Jenkins grabbed what they could – loaded up their motor home and left home shortly after 5 p.m., and at 7 p.m. they were still stuck in traffic just a few miles from home.

"Even when we evacuated for '97, it wasn’t that bad," John Jenkins said. "This is the worst I’ve ever seen it."

A list of evacuation shelters, animal shelters and school closures can be found on the Butte County website.

The sheriff's office said residents of Oroville, a town of 16,000 people, should head north toward Chico and that other cities should follow orders from their local law enforcement agencies, according to The Associated Press.

Sheriff Honea said engineers with the California Department of Water Resources informed him shortly after 6 p.m. that the erosion on the emergency spillway was not advancing as fast as they thought, but water was still coming over the dam at that time.

"We had to make the very difficult and critical decision to initiate the evacuation of the Orville area and all locations south of that," he said. "We needed to get people moving quickly to save lives if the worst-case scenario came into fruition."

Late Sunday night, the California Office of Emergency Services (Cal OES) activated the State Operations Center to provide assistance to Butte County for the potential spillway failure. Cal OES Fire, law enforcement and inland region personnel were working with various response agencies to address all emergency management, evacuation and mutual aid needs, the office said.

DWR officials said later Sunday they plan on deploying helicopters to drop large rocks into a gouge in the auxiliary spillway to stabilize it.