Considering they kept using the spillway even though it was damaged (because they hadn't repaired it as they should have due to lack of funds), and they had to use the emergency spillway (which wasn't reinforced because they didn't do it 12 years ago when they should have). I vote more than expected rainfall (216% of normal by some accounts - though they might have also been storing more water than normal due to the drought) and lack of proper funding to keep the engineering tolerances where they were supposed to be.
But I'm more inclined to suspect the problem is larger. For example that anthropogenic climate change is increasing the amplitude of the El Nino effect.
More than a decade ago, federal and state officials and some of California’s largest water agencies rejected concerns that the massive earthen spillway at Oroville Dam — at risk of collapse Sunday night and prompting the evacuation of 185,000 people — could erode during heavy winter rains and cause a catastrophe.
Three environmental groups — the Friends of the River, the Sierra Club and the South Yuba Citizens League — filed a motion with the federal government on Oct. 17, 2005, as part of Oroville Dam’s relicensing process, urging federal officials to require that the dam’s emergency spillway be armored with concrete, rather than remain as an earthen hillside.
The groups filed the motion with FERC, the Federal Energy Regulatory Commission. They said that the dam, built and owned by the state of California, and finished in 1968, did not meet modern safety standards because in the event of extreme rain and flooding, fast-rising water would overwhelm the main concrete spillway, then flow down the emergency spillway, and that could cause heavy erosion that would create flooding for communities downstream, but also could cause a failure, known as “loss of crest control.”
Agree that deferred maintenance is the primary culprit, based on what everyone is saying... The spillway should have withstood the flow of water from the extensive rainfall if operating within specifications.
While there was a lot of rain, and climate change is affecting long-term weather patterns, I think it would be misleading to associate the Oroville Dam problems to climate change.
But I'm more inclined to suspect the problem is larger. For example that anthropogenic climate change is increasing the amplitude of the El Nino effect.