[maxerickson]

The area around either spillway may continue to erode away. The main dam structure is currently not in danger. They don't want to run the main spillway at all but they also don't think they can rely on the emergency spillway.

It would still be a catastrophe, the outflow will go from inches per hour to feet per hour.

[ChefDenominator]

Okay, this makes more sense. The problem is that the spillways will fail in the sense that the downstream capacity cannot handle the volume of flow, so will flood those regions, as well as possibly flood areas not typically even near water as it carves its own paths.