* quickly dredge as much as possible and dump it on land for later removal
* use sheet piling to divert the dam tailwater around the foot of the spillway
I would love to add sheet piling to the mix. Normal strength sheet pile won’t drive into areas of solid bed rock and heavy sheet piling that you could almost cut the rock with are going to penetrate rock layers that are solid enough without the cut that you want to rely upon them later, IMHO.
You idea has another merit however. In the river bed, upstream of the dispersion flow area on the original spillway path, a sheet pile wall in the river bed would keep debris from heading upstream once repairs are made.