Here's one video showing the danger. https://youtu.be/tkOBmiyTfEQ?t=8m20s https://youtu.be/tkOBmiyTfEQ?t=8m20s Basically, when the dam over-tops the water wil

Here's one video showing the danger. https://youtu.be/tkOBmiyTfEQ?t=8m20s https://youtu.be/tkOBmiyTfEQ?t=8m20s Basically, when the dam over-tops the water will cut a channel into which a little more water will flow and cut a little deeper into which more flows...and on and on. It's a run away failure and cannot be stopped. Here's a quicker example of how just a tiny breach at the top will drain the entire dam: https://www.youtube.com/watch?v=hmKLxoAPU1c https://www.youtube.com/watch?v=hmKLxoAPU1c Note in the first video it's the over-topping that causes the initial erosion/breach rather then the artificial one in the second video, but the mechanism of further failure is essentially the same. I'm not sure the exact construction of the Oroville dam but it's clear the emergency spillway was concrete lined at the top likely with this sort of failure in mind. I'd be surprised if the main bulk of the dam didn't have at least some measures to stop this kind of failure but someone with knowledge of it's construction can chime in on this point.