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.
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.
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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.