As for future repairs/rebuilds, I wonder if they will simply build a monster catch basis lined with 15' concrete where the current cascade is falling at the end of the remaining regular spillway? That's a mighty deep cavern now that would take endless amounts of substrate to re-fill. It's probably why the problem occurred there in the first place. Might make sense to use the natural bedrock formation/elevation drop as the basis for the new spillway. It perhaps would depend on the quality of the bedrock from the break out to the outflow channel. Regardless, I betcha the new fix won't scrimp on rebar and quality/quantity of concrete.