In the weeks and months to come, investigators will no-doubt probe many potential reasons for the near-catastrophic failures at Oroville Dam in February. Those will range from decisions made more than 50 years ago, to the truly extraordinary weather of 2017.

But for the moment, the emergency at Oroville Dam has largely passed. The 180,000 people who were evacuated from their homes last month have returned, and construction crews continue to put millions of tons of rocks and concrete across a badly eroded hillside under the emergency spillway. In the coming months, crews will begin to fix the main concrete spillway, which developed a gaping hole on Feb. 7.

Still, officials with the state Department of Water Resources aren’t out of the woods yet. If a series of warm storms pounds California this spring, that could send billions of gallons of water raging into Lake Oroville again.

(Graphics by Teodros Hailye/KQED)

The Sierra snowpack is at nearly double its historic average in some places, and will begin melting as the weather warms. Though officials are aiming to keep the lake level at roughly 50 feet below the lip of the emergency spillway, it has been rising again as unseasonably warm temperatures accelerate spring runoff from the upper Feather River watershed. Hydrologists warn that flows from the coming runoff season could yet again test Oroville’s patched-up infrastructure.

Another risk: As officials release water down the main broken spillway, the concrete could erode up toward the lake. If officials have to shut the gates of the emergency spillway, the lake could rise again quickly, increasing the risk of water going over the emergency spillway onto the vulnerable hillside.

Crews have been clearing debris out of the channel below Oroville Dam to reopen Hyatt Power Plant. Once it’s running, the plant pulls up to 15,000 cubic feet per second out of the reservoir to make power.

The California Department of Water Resources expects to resume use of the main spillway this week, but at half the volume compared to when operators were frantically trying to lower the lake to below the emergency overflow.

Anatomy of a Near-Disaster

The Oroville drama began quietly enough, as prolonged winter storms — especially in January — began rapidly filling Lake Oroville, the reservoir behind the dam. California’s second-largest man-made reservoir behind Shasta Lake, Oroville is designed to hold more than 3.5 million acre-feet of water.

After record runoff caused the lake level to rise 70 feet in January, operators opened the dam’s main spillway to release water and create space in the lake for expected runoff.

But when the main spillway showed signs of disintegration, engineers dialed back releases to take pressure off the crippled structure. That caused the lake to rise even faster, eventually forcing water to tumble over the dam’s secondary, emergency spillway (also known as the auxiliary spillway) for the first time since the dam was completed in 1968.

Almost immediately, erosion began to eat away the emergency spillway, all but the upper lip of which is bare earth. Whole trees began washing into the diversion channel below.

The hillside began eroding uphill, threatening to undercut the concrete lip of the emergency spillway, and on Sunday evening, February 12, Butte County Sheriff Kory Honae issued an evacuation order that sent tens of thousands of people in Oroville and other downstream communities scrambling for higher ground. (The evacuation order covered nearly 200,000 residents, but how many actually relocated is unknown.)

In an attempt to lower the lake level, engineers were forced to resume massive flows down the main spillway, knowing that the 100,000 cubic-feet-per-second cascade would likely tear apart what remained of the enormous concrete chute.

Clearly they had no choice. Officials say the main body of the dam was never threatened, but had the lip of the auxiliary spillway collapsed, essentially the top 30 feet of the lake would have emptied, sending a wall of water down the Feather River valley, and likely causing the worst U.S. dam disaster since Idaho’s Teton Dam collapsed in 1976. Cities such as Marysville and Yuba City would have been devastated.

A long summer lies ahead, with hundreds of millions of dollars in repairs to do before the rainy season starts again next October.