IIRC:
The spillway of the dam was completely UNDERMINED by ground squirrels, which are prevalent in N Calif foothills. They can burrow for many feet inn many directions. Squirrel condos are common.
When the rains did come, those burrows collapsed, and the spillway had open ground under it. That threatened to collapse the spillway.
All the ENVIROS who moved to outer regions refused to let the squirrels be eradicated.
Interesting. My theory back in the day was tree root intrusion and lack of upkeep. The ground under the spillway surface was determined to be less than desirable. That certainly would allow ground critters to follow the tree roots under the spillway...