They may need to fill Moonbeam Canyon to access the lower part of the spillway during replacement. They are blasting out the rock to construct that equilevel parallel access-way as I recall.
The problem is, if there is a substantial rainfall before they get the lower spillway finished, enough that they have to use the upper spillway, then they will have a real problem on their hands. I guess maybe that there’s no other way to do it, and I can understand that. Some risks are necessary, and if there’s one thing that they know how to do, it’s dredge the feather river.