Tens of thousands of people below the tallest U.S. dam evacuated after its damaged emergency spillway threatened to give way and cause massive flooding.
Here are answers to key questions about problems at Lake Oroville dam in Northern California:
WHAT'S AT RISK?
Failure of the spillway could send uncontrolled torrents of water flooding downstream from Lake Oroville, which filled to capacity after California and other parts of the West saw some of the heaviest rain and snow in decades this winter.
Authorities say 188,000 people in the Sierra Nevada foothills would be in danger from a 30-foot-high wall of water and ordered evacuations. The Gold Rush-era towns of Oroville, with 16,000 people, and Marysville, with 12,000, are among those that could be inundated.
Specifically, water managers fear uncontrolled flows would overwhelm the Feather River downstream, which winds through downtown Oroville, and possibly trigger a series of subsequent levee failures.
WHAT ARE SPILLWAYS?
When reservoirs get too full, their operators release extra water down long channels, or spillways, designed to carry it downstream in a safe, controlled way.
Lake Oroville dam, which holds back California's second-largest reservoir, has a main concrete spillway that normally is used to release floodwaters into the Feather River downstream.
A second spillway mainly made of earth serves as an emergency backup. It also was supposed to be able to handle high flows from the dam, but it had never been used before Saturday.
The force of floodwater from the dam has damaged both spillways.
Gushing Water Damages Lake Oroville's Spillways
WHAT CAUSED THE THREAT?
After five years of drought, a wet winter has strained the system at Lake Oroville, which is receiving runoff from melting snow in the Sierra Nevada as well as from the latest in a series of heavy storms.
Dam operators noticed chunks of concrete in the main spillway last Tuesday. When workers stopped releasing water to investigate, they found that concrete patches the size of football fields had fallen out of the channel. With the reservoir nearing the top of the 770-foot-high dam, dam operators were forced to keep using the main spillway despite increasing damage to it from the rushing water.
The dam reached capacity Saturday, and dam operators had to use the emergency spillway, too. Operators on Sunday noticed water was gouging a hole in the earthen emergency spillway as well. Fearing that the emergency spillway could fail and send torrents of water rushing downstream uncontrolled, authorities ordered the evacuation Sunday evening.
[NATL-BAY]Dramatic Images Show Oroville Dam Crisis
WHAT DOES THIS MEAN FOR CALIFORNIA'S WATER SUPPLY?
Lake Oroville is the main reservoir of California's State Water Project, which supplies water for more than half the state's 39 million residents and for millions of acres of farmland in the leading agricultural state. It's not clear how damage to the two spillways will affect long-term water releases from the dam.