Bottom line, a naturally weak sub-grade area, prior to construction.
Old time highway pavers have told me that the areas that give them the most long-term problem are natural streambeds (normally dry) and fence lines.
I've been meaning to get back to you on discussion of the "borrow" area excavation for the main dam. The article link you referenced included a personal interview of one of the operators of a large bucket excavator. In looking at old Oroville postcards for some history photos, I ran across these two. Pretty impressive machine. The exotic conveyor belt transport system is just as impressive.
Large bucket excavator carving into old dredge tailings. Note the leftover "grooves" on the surface - these match the satellite imagery of the "borrow fields".
Mobile swing conveyor machine. Transfers bucket excavation material to a fixed main conveyor line.
