California Gov. Jerry Brown on Monday evening requested federal assistance with the Oroville Dam emergency spillway crisis as mandatory evacuation orders remained in effect for about 188,000 residents downstream from the nation's tallest earthen dam.

"I respectfully request that you issue an emergency declaration for direct federal assistance for the counties of Butte, Sutter and Yuba, as a result of the potential failure of the Lake Oroville Dam emergency spillway," Brown said in a letter to President Donald Trump.

Added Brown, "As a result of the potential for catastrophic flooding, approximately 188,000 residents from Butte, Sutter and Yuba counties were forced to immediately evacuate their homes for life and safety. Officials are aggressively attempting to lower Lake Oroville's water levels, as another atmospheric river storm system is scheduled to arrive within 48 hours."

In asking for assistance, the governor said Oroville Dam emergency is "of such severity and magnitude that continued effective response is beyond the capabilities of the state and affected local governments and supplemental federal assistance is necessary to save lives and to protect property, public health and safety, and to lessen the effects of this serious situation."

Also late Monday, workers scrambled to place giant sacks of rocks into portions of the eroded emergency spillway using heavy-lift helicopters. They also planned to use grout in damaged areas of the emergency spillway to prevent further erosion.

State and local officials worked into the night Sunday to evacuate thousands of residents downstream from the dam after a hole in an emergency spillway raised fears of flash floods. Oroville Dam — California's second-largest dam — is located about 70 miles north of Sacramento.

State officials have denied allegations there was lax safety at the Oroville Dam despite a report of previous warnings about the emergency spillway.

The hole found Sunday in the emergency spillway — essentially a natural hillside of soil, rock and brush — led engineers to shift major water flows away from this unlined channel. The main spillway is damaged from significant concrete erosion but was being utilized to release water from the swollen dam Monday, according to the California Department of Water Resources.

There have been concerns about Oroville Dam safety since the erosion of the primary spillway was discovered Tuesday . The crippled primary spillway last week was unable to release a sufficient amount of water to keep up with inflows from the plentiful rains in the surrounding area.

State and federal officials failed to heed safety warnings about Oroville more than a decade ago, according to the Mercury News. The report Sunday said three environmental groups warned the Federal Energy Regulatory Commission about the vulnerability of the hillside emergency spillway.

The Oroville Dam was completed in the late 1960s when Ronald Reagan was governor of California.

FERC confirmed last week it had several engineers onsite at the Oroville Dam but Monday the agency didn't respond to requests for comment.

Earlier Monday, DWR officials denied that the state had ignored earlier concerns about the Oroville Dam's emergency spillway or had been lax in inspections there.

"We have a very rigorous schedule of inspections that is determined by state and federal regulators," said DWR spokesman Eric See. "We actually do those inspections annually."

DWR's acting director Bill Croyle defended the agency's handling of the Oroville situation and the emergency spillway's safety when questioned at Monday's press conference.

"I'm not sure anything went wrong," Croyle told reporters. "That system has been monitored. This is the first time it's ever taken water over the system."

The emergency spillway at Oroville Dam was activated Saturday for the first time ever in the dam's 48-year history after the dam reached above its capacity following a barrage of rain in the Northern California region.

However, state engineers on Sunday discovered significant erosion had occurred back towards the face of the emergency spillway at Oroville Dam after huge water outflows, meaning the structural integrity of the dam's auxiliary spillway was at risk. That led state and local authorities Sunday to order an immediate evacuation of communities downstream from the dam.

"If the emergency spillway were to fail, it would send a 30-foot wall of water downstream, resulting in catastrophic flooding," Brown said in his Monday letter to the president.

If there were an uncontrolled discharge of waters from the dam, it would flow downstream to the Feather River and tributaries and flood communities in the eastern Sacramento Valley.

On Saturday, Oroville Dam reached its elevation capacity of 901 feet, which automatically triggers the emergency spillway. At noon on Sunday the elevation topped 902 feet and by Monday at 7 p.m. after major outflows from the primary spillway the elevation was down to around 893 feet, according to the DWR data website.

Engineers were urgently working to lower the lake level by 50 feet in order to prepare for future inflows of water due to storms and precipitation.

Butte County Sheriff Kory Honea told reporters at the press conference Monday there was progress with "no water flowing over the emergency spillway...and the lake levels are continuing to fall."

Even so, the sheriff said the evacuation order will remain in effect as experts were still trying to assess the damage at the Oroville Dam emergency spillway.

"We need to make sure that before we allow people back into those areas that it is safe to do so," said Honea. "I recognize that this is displacing a lot of people. I recognize what a hardship it has placed on our community."

Oroville Dam is located in the foothills of the western Sierra Nevada mountain range. The pressure is on state officials to resolve the spillway crisis at Oroville since the heavy snowfall in the Sierras will be melting in the spring and bring more water to area reservoirs.

Rain is forecast in the Oroville area as early as Wednesday night from incoming storms. The arriving storm systems are expected to be significant and remain in the Northern California region through next week.

"This is a series of storms coming in and we could see potentially 4 to 8 inches of precipitation," said Idamis Del Valle, a meteorologist with the National Weather Service office in Sacramento.

DWR's Croyle reiterated Monday that the "dam itself is sound. We have some little pieces that are critical to the operation of the dam that have been impaired."

Also, state officials have stressed that the dam itself is a separate structure from the emergency spillway.

The cost to repair the primary spillway was estimated to be as much as $200 million, state officials said over the weekend. With the significant damage to the emergency spillway, the price tag on repairs is likely to go much higher.

Brown's request for direct federal assistance to the president could help cushion the blow of the repair costs. The state indicated Sunday it might fix the existing spillways or build new ones.

California's governor spoke to emergency response officials at the State Operations Center late Monday, then told reporters afterword he spoke to a Trump cabinet official about the request for assistance but Brown wouldn't divulge the name of the cabinet member other than to say they had been confirmed.

The governor also was asked about Trump's threat to take money away from California could impact the state's request. "I'm sure that California and Washington will work in a constructive way," Brown said. "That's my attitude. There will be different points of view, but we're all one America."

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