Officials race against rains to prevent California dam disaster

Officials in California were racing against the weather Tuesday, struggling to shore up the Oroville Dam's emergency spillway before more rains pummel the area and place the structure under even greater stress.

Engineers have been trying to lower the water level in Lake Oroville, which lies behind American's tallest dam. But more rains are forecast for Thursday.

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Lake Oroville dam -- emergency spillway and evacuation
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Lake Oroville dam -- emergency spillway and evacuation
TOPSHOT - The Oroville Dam spillway releases 100,000 cubic feet of water per second down the main spillway in Oroville, California on February 13, 2017. Almost 200,000 people were under evacuation orders in northern California Monday after a threat of catastrophic failure at the United States' tallest dam. Officials said the threat had subsided for the moment as water levels at the Oroville Dam, 75 miles (120 kilometers) north of Sacramento, have eased. But people were still being told to stay out of the area. / AFP / Josh Edelson (Photo credit should read JOSH EDELSON/AFP/Getty Images)
OROVILLE, CA - JULY 20: In this handout photo provided by the California Department of Water Resources, Full water levels are visible behind the Oroville Dam at Lake Oroville on July 20, 2011 in Oroville, California. (Photo by Paul Hames/California Department of Water Resources via Getty Images)
California Department of Water Resources personnel monitor water flowing through a damaged spillway on the Oroville Dam in Oroville, California, U.S., on February 10, 2017.
FOLSOM, CA - JULY 20: In this handout photo provided by the California Department of Water Resources, Full water levels are visible behind the Folsom Dam at Folsom Lake on July 20, 2011 in El Folsom, California. (Photo by Paul Hames/California Department of Water Resources via Getty Images)
OROVILLE, CA - AUGUST 19: The Oroville Dam spillway stands dry at Lake Oroville on August 19, 2014 in Oroville, California. As the severe drought in California continues for a third straight year, water levels in the State's lakes and reservoirs is reaching historic lows. Lake Oroville is currently at 32 percent of its total 3,537,577 acre feet. (Photo by Justin Sullivan/Getty Images)
65,000 cfs of water flow through a damaged spillway on the Oroville Dam in Oroville, California, U.S., February 10, 2017. REUTERS/Max Whittaker
California Department of Water Resources crews inspect and evaluate the erosion just below the Lake Oroville Emergency Spillway site after lake levels receded, in Oroville, California, U.S., February 13, 2017. Kelly M. Grow/ California Department of Water Resources/Handout via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. EDITORIAL USE ONLY.
California Department of Water Resources personnel monitor water flowing through a damaged spillway on the Oroville Dam in Oroville, California, U.S., on February 10, 2017.
A damaged spillway with eroded hillside is seen in an aerial photo taken over the Oroville Dam in Oroville, California, U.S. February 11, 2017. California Department of Water Resources/William Croyle/Handout via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. EDITORIAL USE ONLY. THIS PICTURE WAS PROCESSED BY REUTERS TO ENHANCE QUALITY. AN UNPROCESSED VERSION HAS BEEN PROVIDED SEPARATELY.
A damaged spillway with eroded hillside is seen in an aerial photo taken over the Oroville Dam in Oroville, California, U.S. February 11, 2017. California Department of Water Resources/William Croyle/Handout via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. EDITORIAL USE ONLY. TPX IMAGES OF THE DAY
Riverbend Park is seen under flood water in Oroville, California on February 13, 2017. Almost 200,000 people were under evacuation orders in northern California Monday after a threat of catastrophic failure at the United States' tallest dam. Officials said the threat had subsided for the moment as water levels at the Oroville Dam, 75 miles (120 kilometers) north of Sacramento, have eased. But people were still being told to stay out of the area. / AFP / Josh Edelson (Photo credit should read JOSH EDELSON/AFP/Getty Images)
TOPSHOT - Crews work on a damaged section of the Oroville Dam in Oroville, California on February 13, 2017. Almost 200,000 people were under evacuation orders in northern California Monday after a threat of catastrophic failure at the United States' tallest dam. Officials said the threat had subsided for the moment as water levels at the Oroville Dam, 75 miles (120 kilometers) north of Sacramento, have eased. But people were still being told to stay out of the area. / AFP / Josh Edelson (Photo credit should read JOSH EDELSON/AFP/Getty Images)
TOPSHOT - A home is seen marooned as the surrounding property is submerged in flood water in Oroville, California on February 13, 2017. Almost 200,000 people were under evacuation orders in northern California Monday after a threat of catastrophic failure at the United States' tallest dam. Officials said the threat had subsided for the moment as water levels at the Oroville Dam, 75 miles (120 kilometers) north of Sacramento, have eased. But people were still being told to stay out of the area. / AFP / Josh Edelson (Photo credit should read JOSH EDELSON/AFP/Getty Images)
Riverbend Park is seen under flood water in Oroville, California on February 13, 2017. Almost 200,000 people were under evacuation orders in northern California Monday after a threat of catastrophic failure at the United States' tallest dam. Officials said the threat had subsided for the moment as water levels at the Oroville Dam, 75 miles (120 kilometers) north of Sacramento, have eased. But people were still being told to stay out of the area. / AFP / Josh Edelson (Photo credit should read JOSH EDELSON/AFP/Getty Images)
Water is released from the Lake Oroville Dam after an evacuation order was lifted for communities downstream from the dam in Oroville, California, U.S. February 16, 2017. REUTERS/Jim Urquhart TPX IMAGES OF THE DAY
A road is closed by the flooded Feather River outside town below the Lake Oroville Dam after an evacuation order was lifted for communities downstream from the dam in Oroville, California, U.S. February 16, 2017. REUTERS/Jim Urquhart
Rocks are hauled to the Lake Oroville Dam after an evacuation order was lifted for communities downstream from the dam in Oroville, California, U.S. February 15, 2017. REUTERS/Jim Urquhart
A helicopter flies rocks to the Lake Oroville Dam after an evacuation order was lifted for communities downstream from the dam in Oroville, California, U.S. February 15, 2017. REUTERS/Jim Urquhart TPX IMAGES OF THE DAY
Trucks carry rocks as water is released from the Lake Oroville Dam after an evacuation order was lifted for communities downstream from the dam in Oroville, California, U.S. February 14, 2017. REUTERS/Jim Urquhart
Staff with the California Department of Water Resources watch as water is released from the Lake Oroville Dam after an evacuation was ordered for communities downstream from the dam in Oroville, California, U.S., February 14, 2017. REUTERS/Jim Urquhart
OROVILLE, CA - FEBRUARY 15: Reconstruction continues in a race to shore up the emergency spillway at Oroville Dam in Oroville, Calif., on Feb. 15, 2017. (Brian van der Brug/Los Angeles Times via Getty Images)
OROVILLE, CA - FEBRUARY 16: Clouds drift in the sky after morning showers at the Oroville Dam overlook where helicopters are ferrying sand and rocks to the dam's emergency spillway reconstruction project in Oroville, Calif., on Feb. 16, 2017. (Photo by Brian van der Brug/Los Angeles Times via Getty Images)
OROVILLE, CA - FEBRUARY 16: California Water Service district manager Toni Ruggle (CQ) surveys the Feather River at Riverbed Park downstream from Oroville Dam in Oroville, Calif., on Feb. 16, 2017. For decades, city officials have used the steps at the park to gauge the height of the river. Water up to the fourth step is 100,000 cubic feet per second. (Photo by Brian van der Brug/Los Angeles Times via Getty Images)
OROVILLE, CA - FEBRUARY 16: A mallard floats past a picnic table along the swollen Feather River at Riverbed Park downstream from Oroville Dam in Oroville, Calif., on Feb. 16, 2017. (Photo by Brian van der Brug/Los Angeles Times via Getty Images)
Cots are washed and set to dry at the Silver Dollar Fairgrounds evacuation center in Chico, California, U.S., on Tuesday, Feb. 14, 2017. State officials rushed to repair an emergency spillway for the Oroville dam -- just 150 miles (241 kilometers) north of San Francisco -- that is threatening to submerge an entire region of northern California after a recent deluge of rain. Photographer: Michael Short/Bloomberg via Getty Images
OROVILLE, CA - FEBRUARY 14: A dog sits in the back of a pickup truck as a helicopter picks up a bag of rocks at a staging area near the Oroville Dam on February 14, 2017 in Oroville, California. More than 188,000 people were ordered to evacuate after a hole in the emergency spillway in the Oroville Dam threatened to flood the surrounding area. (Photo by Elijah Nouvelage/Getty Images)
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Nearly 190,000 people remain out of their homes for fear of catastrophic flooding. Airbnb, the home and apartment rental service, has waived all fees for people who live in the affected area, and is offering homeowners a way to offer shelter for free.

Many people have fled their homes with little more than the clothes on their back, photographs and other items of sentimental value.

The emergency spillway developed a hole Sunday, raising the risk of collapse. The dam's primary spillway developed a 200-foot-long, 20-foot-deep hole last week.

On Monday, California Gov. Jerry Brown wrote to President Donald Trump requesting emergency federal assistance for three counties in the northern part of the state.

"I have determined this incident is of such severity and magnitude that continued effective response is beyond the capabilities of the state and affected local governments," Brown wrote.

The dam itself has not been damaged. But because the water levels are so high, the emergency spillway — which appears to be eroding — could unleash a wall of water onto the communities below if it collapses.

Several state officials have told NBC News that 1 million acre-feet of water could be released onto areas that are home to 188,000 people.

"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."

The possibility of disaster was raised 12 years ago by three environmental groups that filed a document with the Federal Energy Regulatory Commission that warmed that the emergency spillway might fail.

A spillway is a structure that allows a controlled release of water from a dam. The water is released so that it does not pour over the top of the dam — or even destroy it.

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