Gov. Jerry Brown on Monday asked the Trump administration for federal assistance in responding to the potential failure of the Oroville Dam emergency spillway.
In a letter to the president, Brown asks for help for the three Northern California counties affected, saying aid is needed to assist the 188,000 residents of Butte, Sutter and Yuba counties who were ordered to evacuate Sunday.
Meanwhile, as the lake level continued to drop Monday, those evacuees from Oroville and areas south of the dam may not be able to return to their homes until the barrier at the nation's tallest dam is repaired, a sheriff said Monday.
Butte County Sheriff Kory Honea gave no timetable for the work. Officials from the California Department of Water Resources on Monday began stabilizing the emergency spillway using helicopters to drop loads of rocks on the eroded sections at Lake Oroville, California's second-largest reservoir located about 150 miles northeast of San Francisco.
"They'll be able to come in, grab them and drop where they need to if that's the decision, the route we want to take," said Chris Orrock of the state Department of Water Resources.
People who live on higher ground in Oroville were concerned about their friends and neighbors who were evacuated.
"We have a lot of people working 24/7," said Tara Atteberry. "We see rocks coming back and forth, doing a great job to keep us safe, and we really appreciate their help."
The water level behind the dam dropped, slightly easing the fears of a catastrophic spillway collapse. But with more rain expected later in the week, time was running short to fix the damage ahead of the storms.
While crews worked to shore up the damaged spillway, Congressman John Garamendi responded to questions about maintenance to the dam during several years a drought. The biggest question was: Why wasn’t it taken care of?
"The state was using a half a billion dollars trying to build twin tunnels, but they weren’t paying attention to what was going on here," Garamendi said.
As the day began, officials from the California Department of Water Resources inspected an erosion scar on the spillway at the dam.
Authorities ordered evacuations Sunday for everyone living below the lake out of concern that the spillway could fail and send a 30-foot wall of water roaring downstream.
"We grabbed our dog and headed to higher ground — away from the river," said Kimberly Cumings, who moved with her husband, Patrick, and 3-year-old daughter to Oroville from Fresno a month ago for a new job. They were eating at a restaurant when the evacuation order came.
A driver with a large vehicle and three children of her own gave them a ride to the Red Cross evacuation center at the Silver Dollar Fairgrounds in Chico.
"You can't take a chance with the baby," Patrick Cumings said of their decision to flee.
The water level in Lake Oroville rose significantly in recent weeks after a series of storms dumped rain and snow across California, particularly in northern parts of the state. The high water forced the use of the dam's emergency spillway, or overflow, for the first time in the dam's nearly 50-year history on Saturday.
The threat appeared to ease somewhat Monday as the water level dropped. Officials said water was flowing out of the lake at nearly twice the rate as water flowing into it.
Sunday afternoon's evacuation order came after engineers spotted a hole on the concrete lip of the secondary spillway for the 770-foot-tall Oroville Dam and told authorities that it could fail within the hour.
Nancy Borsdorf described a scene of chaos on her way out, including drivers abandoning cars as they ran out of gas.
"People were just panicking," said Borsdorf, who was at a shelter Monday in Chico.
"We've always loved and trusted our dam," she said, having lived in Oroville for 13 years. "I'm really hopeful Oroville wasn't flooded."
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Asked if the spillway was supposed to handle far more water, the acting head of California's water agency said he was "not sure anything went wrong'' on the damaged spillway.
Bill Croyle said sometimes low-flow water can be high energy and cause more damage than expected. His comments came after officials assured residents for days that the damage was nothing to be concerned about, then ordered everyone to get out in an hour.
The water level in the lake rose significantly in recent weeks after storms dumped rain and snow across California, particularly in northern parts of the state. The high water forced the use of the dam's emergency spillway, or overflow, for the first time in the dam's nearly 50-year history on Saturday.
With more rain expected Wednesday and Thursday, officials were rushing to try to fix the damage and hoping to reduce the dam's water level by 50 feet ahead of the storms.
Panicked and angry residents sat in bumper-to-bumper traffic hours after the evacuation order was given Sunday. Roads below the spillway remained closed Monday morning, according to the Butte County Sheriff's Department.
Sheriff Honea said Monday afternoon the evacuation will not end right away. Officials are working on a plan to allow residents to return home when it's safe.
At least 250 California law enforcement officers were posted near the dam and along evacuation routes to manage the exodus of residents and ensure evacuated towns don't face looting or other criminal activity.
A Red Cross spokeswoman said more than 500 people showed up at an evacuation center in Chico, California.
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 fleeing the potential flooding, said Red Cross shelter manager Pam Deditch.
A California Highway Patrol spokesman said two planes would fly Monday to help with traffic control and possible search and rescue missions.
U.S. Rep. Doug LaMalfa urged his constituents to sit tight early Monday, tweeting: "I know people in shelters will be restless, let's see what the morning assessment is of the emerg spillway and shoring up efforts, hang in!"
The erosion at the head of the emergency spillway threatens to undermine the concrete weir and allow large, uncontrolled releases of water from Lake Oroville.
Those potential flows could overwhelm the Feather River and other downstream waterways, channels and levees and flood towns in three counties.
Aside from residents, all 578 inmates from the Butte County Jail were safely relocated to Santa Rita Jail in Dublin, according to the Butte County Sheriff's Department.
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The California National Guard put out a notification to all of its 23,000 soldiers and airmen to be ready to deploy. It marked the first time an alert for the entire California National Guard had been issued since the 1992 riots in Los Angeles after a jury acquitted four police officers in the beating of Rodney King.
The Associated Press contributed to this report.
Follow @nbcbayarea for the latest updates on the Oroville Dam Spillway.