Thanks, again, smiddy and eyeused2badub and jwleaks, for commenting. Helpful to hear from a "neighbor" (Oroville) and someone whose land (Oz) I visited and loved.
Latest on poor engineering, failure to maintain properly, lack of understanding the environment, geology, etc.:
Colson, who worked on other parts of the dam, said the design of the spillway never fully analyzed the potential for cavitation, which occurs when roiling water creates air pockets that cause high-pressure intrusion into cracks and fissures. The action can chew through thick concrete and even steel.
Dam spillways have a long history of problems with cavitation. The spillways at Hoover Dam have been used twice, and both times they sustained heavy damage, similar to what happened at Oroville. Cavitation caused concrete to fail and resulted in scouring of the surrounding rock. Glen Canyon Dam, also on the Colorado River, sustained massive damage in 1983 when its underground spillways were put to use. Dams in Pakistan and Brazil have also proved the potential for cavitation to damage spillways.http://www.latimes.com/local/california/la-me-ln-oroville-spillway-damage-20170220-story.html