It's been more than a week since engineers at the nation's tallest dam noticed damage to an emergency spillway, starting a series of events that culminated with the two-day evacuation of nearly 200,000 California residents downstream from Oroville Dam.
Here's what we know:
It straddles the Feather River about 70 miles north of Sacramento in rural Northern California. Construction was completed in 1968, creating Lake Oroville behind it. The lake is a major water supplier for farmers in the agricultural-rich Central Valley and Southern California residents.
There are two primary ways dam workers drain water from the reservoir. They can control the flow of water downriver by opening a gate to the dam's main concrete spillway. They also can divert water through a nearby power plant. The dam also is equipped with an emergency spillway adjacent to the main dam.
The emergency spillway is an earthen structure — a big hill — with a 30-foot concrete wall on top. It is several feet lower than the main dam, and water will flow over it uncontrollably when the lake is over capacity. Water had never flowed over the emergency spillway until Feb. 11.
THE FIRST PROBLEM
Engineers noticed an odd flow pattern on the main concrete spillway Feb. 7 and shut off the water to investigate. They discovered a large crater in the bottom of the channel.
Despite heavy rain and melting snow, engineers slowed releases later in the week after the hole grew bigger. Compounding matters, concrete from the eroding hole clogged the power plant's exit channel and forced the plant's shutdown.
Engineers determined the hole couldn't immediately be fixed and decided to keep using the damaged spillway, but with reduced flows. The following day, it rained harder than expected, and a day later, water was flowing in almost twice as fast as it was draining.
THE SECOND PROBLEM
Water started flowing over the emergency spillway at 8 a.m. Sunday when the lake reached capacity, but officials kept telling the public there was no threat until as late as noon Sunday.
A few hours later, engineers determined the hillside was eroding faster than expected, undermining the 30-foot concrete wall. Officials feared it was about to collapse and cause a catastrophic flood.
Around 4 p.m., authorities sounded sirens and ordered nearly 200,000 residents downstream from the dam to evacuate.
Authorities turned nearby highways into one-way roads heading south. Still, massive gridlock occurred. Gas stations ran out of gas. Motels and hotels quickly sold out. Shelters swiftly opened.
Officials boosted the flow through the damaged concrete spillway in a frantic bid to lower the reservoir.
Lake levels fell enough that water stopped pouring over the emergency spillway about four hours after the evacuation order.
Crews worked to dump and cement thousands of tons of rocks to shore up the damaged spillways.
THE RETURN HOME
Butte County Sheriff Kory Honea announced Tuesday afternoon that residents could return home. He conceded the evacuation was "chaotic" but defended the decision as "better safe than sorry."
WHAT WENT WRONG?
"I'm not sure anything went wrong," Department of Water Resources acting chief Bill Croyle said of the near-failure of the emergency spillway. "This was a new, never-happened-before event."
Environmental groups demanded 12 years ago that the hillside be paved with concrete to prevent erosion. But federal regulators decided against ordering the paving after state water agencies argued that "armoring" the emergency spillway would cost too much for a structure that, at that point, had never been used.
IS IT SAFE?
Croyle said he's confident the dam and primary spillway can handle rainfall from three storms rolling into the region this week. He said more water is leaving the lake than entering it and there's enough capacity to handle storm runoff and melting snow.
The emergency spillway also has been repaired enough to handle any overflow, Croyle said. The temporary fixes will last beyond snow melting this spring, and officials will then make long-term fixes, the water agency said.
Nonetheless, the sheriff told residents to prepare for another evacuation if the situation worsens.