As I understand it the root cause of this situation is the primary spillway being unusable when the dam was almost full. If they were able to safely use it they could have just lowered the water level. At the same time heavy rain is the most likely time for a sinkhole to form so it makes me wonder why this was the backup plan in the first place.
The main spillway became damaged. The dam operators
chose to stop using that spillway. They did this for a few reasons: To allow them to inspect the damage. To prevent further (expensive) damage to the spillway. To prevent further mud from being eroded into the river, which would harm the fish hatcheries downstream. And, I believe, to hopefully prevent some power towers that were close to the main spillway from getting taken out.
Once they determined that the spillway was so fucked that it couldn't really get more fucked, and that the water was running pretty clean through the damaged areas, and they moved lots of the baby fish further downstream, they started using the main spillway again. Lightly. Only letting a little water through. The new plan was to let water come over the emergency spillway, once the lake rose that far.
They ran around below the emergency spillway (weir) with bulldozers, clearing out trees and debris so said debris would not end up in the river.
Water then started going over the emergency spillway (weir), and more alarmingly, over the parking lot and picnic area to the left of the weir. Erosion started occurring below the weir, which was expected. Erosion started occurring below the parking lot, which wasn't expected. And then, really bad erosion started happening
somewhere in there which threatened to work its way up the hill and undercut the weir, or maybe the parking lot, which could cause the entire thing to wash away.
At this point they freaked the fuck out and called for evacuations. "Exercising an abundance of caution." Covering their asses, in other words. Crazy and doomed plans involving helicopters were discussed. Claims were made that the weir was an hour away from failing. None of that happened, but they
did decide to open up the main spillway again because, fuck, we're totally gonna get fired if mainstreet Oroville floods.
The dam is not in danger. The main spillway is not in "danger", since it's already pretty fucked and will need to be rebuilt regardless. The power lines may be fucked, but who cares because the hydro plant was shut down some time ago. The baby fish are either rescued or doomed at this point. The emergency weir may or may not be fucked, but if it goes, it'll only be
semi-catastrophic because "only" 10-20 feet of water depth would escape before the lip of the hill was scoured right down to the underlying rock. Hopefully. You don't want to risk some other weakness (read: a crack in said rock) being exposed.
At least that's what I understand. The various news agencies are spewing a mountain of bad info, so it's difficult to get the straight skinny.
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