I happened to give a little presentation about this the other night, so here's the deal. To prove inverse condemnation, the plaintiff will have to show:
1. Ownership of a protected property interest
2. That was taken or damaged by the government's actions
3. Diminution in value of the property resulted from government's actions
4. Direct, immediate, substantial interference with use and enjoyment of property
5. Government's actions were intentional and were substantially certain to cause the harm that resulted
6. Damage was greater in extent than otherwise would have been
The battle lines will be drawn on #6 because the others are pretty much a lock. I've also seen the theory that the government will argue that it didn't know precisely which houses would be flooded, but I don't think that's a winner.
If they can argue the damage would have been worse without the controlled releases, that could get them off the hook. But "they had no choice but to release the dams" is not a winning argument.