[adevine]

I'm sure this is a stupid question, but would it be possible to build essentially a large "siphon", so that the discharge of the water didn't result in erosion directly below the spillway?

Seems like it would be pissing in the ocean (or I guess that analogy in reverse?), but when I saw the newsreels of the helicopters dropping bags of rocks into the spillway hole, that also seemed like it would take a couple thousand of those drops to make a difference.

[gpm]

They're letting 100,000 Cubic feet per second flow over the spillway. Niagra falls has an average flow rate of 85,000 feet per second.

I don't think you're going to be able to make a siphon quickly that is large and robust enough to handle a significant fraction of that.