Hi janetjanet998, The downslope movement of this lower fractured section of the spillway is what DWR is concerned with as it had a prior movement estimated from 3 to 6 inches. Essentially, the existing concrete slab anchor bars are straining to keep the slab & sidewall section from tumbling or fracturing down into the plunge pool (waterfall area at the broken end of the upper spillway). What is not known is if the underlying rock is unstable and is moving with the concrete slabs. Perhaps this is why DWR chose to drill the array of big Rock Bolt Anchors further up from this point, as they may have believed that the rock was too poor or unstable in the lower section. This strategy of putting the Rock Bolt Anchors just upslope from this failing area may have been to keep the next upper section of the spillway slabs protected from breaking away in a domino effect as the "pulling away" section could "take it with it". See the image.
Potential "Break Away". Movement detection of failing section of the damaged spillway. Emergency Repair Rock Bolts were not placed in this section (blue line downward). Rock Bolts placed in the next array of slabs above this "pulling away" seam. Thus the balance of the quality of rock + existing anchor rods determines how this section responds to spillway flow stresses (i.e. will it suddenly "fail"). This may be why the spillway is limited to 35,000 cfs.
Considering that the slabs below the anchored area are partially damaged anyway, I don’t think they wanted to waste any money trying to save them. Besides, as you said, the rock beneath them is suspect (incompetent, or “ moonbeam” rock).