WILLIAM BRANGHAM:

The Mercury News reports that officials ignored warnings about the fragility of the emergency spillway for years. Back in 2005, environmental groups warned officials that this other spillway — quote — "didn't meet modern safety standards."

For now, officials hope to drain enough water from the reservoir to make room for the large storm that's expected on Wednesday.

For more on the threat at the Oroville Dam, I'm joined now by Jeffrey Mount. He's senior fellow with the Public Policy Institute of California Water Policy Center. His research focuses on water resources and flood management.

Thank you very much for being here.

So, Jeffrey, I understand that the main threat seems to have receded slightly. I think people are still evacuating as we speak. What was it that officials were worried was going to happen?

JEFFREY MOUNT, Public Policy Institute of California: Well, ultimately, what the big fear was, that there would be an uncontrolled release from the reservoir.

The worst nightmare of a reservoir engineer is to not be able to control the water behind the reservoir. And the emergency spillway, had it failed and had it collapsed, would basically have lowered the lake level by almost 30 feet. And that is a tremendous amount of water in a short period of time, which would have resulted in catastrophic flooding downstream.