Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

Mandatory evacuations remained in effect Monday afternoon for nearly 190,000 people in Northern California after a spillway serving the country's tallest dam developed a hole that threatened to release uncontrolled floodwaters, officials said.

The emergency spillway off the Oroville Dam was the second to fail in a matter of days, after the dam's primary spillway developed a 200-foot-long, 30-foot-deep hole last week.

Officials and contractors were working feverishly to get as much water out of Lake Oroville as possible by Thursday, when another round of storms is expected to sweep across the area.

The acting director of California's Department of Water Resources, Bill Croyle, told reporters that it was unclear what caused the erosion in the emergency spillway, but he said he expected significant progress in the agency's goal of dropping lake levels by 50 feet.

The dam itself was not damaged. But because its water levels are so high following heavy rain, the emergency spillway could unleash a wall of water onto communities and rivers below if it collapses.

Sean Dennis was one thousands of residents trying to make the gridlocked journey to safer ground.

"We've never seen anything like this in modern times"

"We both were kind of shocked. Nothing like this has ever happened," said Dennis, 30, a chef from Yuba City, who recalled the moment he and his wife found out they had to leave. "We just grabbed what we could."

Dennis spoke while driving his family to a hotel in Willows, a journey that had already taken five hours despite its being only 55 miles away.

Cars quickly piled up at gas stations and on routes out of the evacuation zone after the order was given Sunday.

"What was usually a 20-minute drive took two hours," said Heather Sutton, 22, a Yuba Community College student. "It was bumper to bumper. ... You can almost see the panic happening."

Sutton recalled telling her friend before they evacuated that "we need to grab photos, anything that has sentimental value." Everything else was left behind, she said.

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

The sudden evacuation panicked residents, who scrambled to get their belongings into cars and then grew angry as they sat in bumper-to-bumper traffic hours after the order was given.

Raj Gill, managing a Shell station where anxious motorists got gas and snacks, said his boss told him to close the station and flee himself. But he stayed open to feed a steady line of customers.

"You can't even move," he said. "I'm trying to get out of here, too. I'm worried about the flooding. I've seen the pictures — that's a lot of water."

A Red Cross spokeswoman said more than 500 people showed up at an evacuation center in Chico.

The shelter had run out of blankets and cots, and a tractor-trailer with 1,000 more cots was stuck in the gridlock of traffic Sunday night, Red Cross shelter manager Pam Deditch said.

Butte County Sheriff Kory Honea, who ordered the evacuations, said Monday afternoon that he had no choice but to tell people to flee as quickly as possible.

"When this incident occurred, it became apparent that we needed to act quickly," he said. "I recognize and appreciate the frustration that people who have been evacuated must feel. That was not a decision I made lightly."

The order was issued suddenly Sunday after officials spotted the damage to the emergency spillway's concrete lip. The auxiliary spillway was being used for the first time in almost 50 years, according to The Associated Press, because the 770-foot-tall dam was full to the brim and its main spillway was damaged by heavy rain last week.

It could also breach the network of levees along the way and cause problems as far away as Sacramento.

Several state water and government officials told NBC News that 1 million acre-feet of water could be released, overwhelming the Feather River and flooding communities in Butte County, Yuba City and Marysville. Kevin Lawson, deputy chief of the California Department of Forestry and Fire Protection, said at a news conference Sunday night that 188,000 people had been ordered to evacuate from those areas.

"We've never seen anything like this in modern times," a state water official told NBC News. "This is a worst-case scenario for any water management agency, a worst-case nightmare."

Croyle, meanwhile, said he wasn't aware of a 2005 filing with the Federal Energy Regulatory Commission that warned about a possible failure with the emergency spillway.

"We're not going to get into recommendations or concerns that were voiced in the past," he said.

Three environmental groups — Friends of the River, the Sierra Club and the South Yuba River Citizens League — filed the document after determining that the dam didn't meet modern safety standards. Its emergency spillway needed to be armored with concrete, the groups said.

"What nearly happened last night was what I've been fearing," Ronald Stork, senior policy advocate at Friends of the River, told NBC News. "I thought it was the time to fix it years ago."

A regional engineer with the federal agency didn't appear to think it needed fixing, however. In response to the filing, the engineer, John Onderdonk, said in a 2006 memo that it met federal guidelines.

"During a rare flood event, it is acceptable for emergency spillway to sustain significant damage," he wrote.

From Sunday night to Monday morning, the water level of Lake Oroville had decreased, allowing engineers to assess the damage, according to the AP. Officials were still releasing water through the main spillway ahead of this week's expected storm.

By midday Monday, officials said, water flows into the lake stood at about 45,000 cubic feet per second, with outflows at 100,000 cubic feet per second.

Sheriff Honea said earlier that the damage could result in a "catastrophic failure" of the emergency spillway.

"Although it brings some stability to the situation, there are still a lot of unknowns," he told reporters. "We have staff looking at the various areas that evacuations have been ordered in and making a determination as to what areas are clearly in danger and what areas may be less vulnerable."

California Gov. Jerry Brown issued an emergency order Sunday night to bolster the state's response, and on Monday he said he'd filed an emergency request for relief with the federal government and discussed the events in Oroville with a recently confirmed member of Donald Trump's cabinet.

Although Brown and other elected state officials have vowed to make California a bulwark of resistance in Donald Trump's America, the governor described the conversation as congenial. He refused to identify the cabinet member, however.