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At Northern California’s Lake Oroville, water levels receded Monday, stopping the overflow of water from the dam’s emergency spillway. This reduced the risk of immediate uncontrolled flooding -- but longer-term concerns remain. William Brangham speaks with Jeffrey Mount of the Public Policy Institute of California about the massive evacuation that took place and the outlook for the dam's future.
Now: The integrity of a major dam in California comes under threat, after days of historic rainfall in the region.
William Brangham has our report.
At Northern California's Lake Oroville, home to the nation's tallest dam, water levels finally receded, which stopped the overflow of water from the dam's emergency spillway.
This reduced the risk of the spillway's complete collapse, which would've triggered uncontrolled flooding and threatened tens of thousands of homes below.
At a press conference, local officials couldn't answer why the system failed.
WILLIAM CROYLE, California Department of Water Resources: We're not sure anything went wrong. I think that the system has been installed since early 1960s. It's been looked. It's been monitored.
Today, they faced a much tamer scene than on Saturday. Officials had to open the dam's emergency spillway for the first time in 50 years because of record high water levels caused by recent heavy winter rain and snow.
When water was drained from the dam's main spillway, the huge volume eroded chunks of concrete and dug a 30-foot-deep hole at its base. It was then that officials opened the emergency spillway. When that water started eroding the earthen embankment, officials feared the wall would collapse altogether.
And so, on Sunday, authorities ordered the evacuation of nearly 200,000 people living below the lake. With little notice, residents were stuck in traffic for hours trying to leave.
NANCY BORSDORF, Evacuee:
I panicked and just started putting things in my car, basically my violin, a didgeridoo, some family photos, and I didn't grab enough clothes. I grabbed some wet laundry. Can you believe that?
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.
So the basic issue here, obviously, as we reported, is too much rain, too much snow, which I know is a good thing in California normally.
But what is it with regards to the engineering that went wrong here?
Well, so, we have a couple of things.
You have to remember that California reservoirs are under tension all the time. We use them to store water. And, of course, we're just coming out of a record drought. We store water, but we also use them to regulate floods.
So you have to set aside some space behind the reservoir. And in this case, there's not much space set behind this reservoir to catch floods. So, it filled. And it filled pretty quickly. And then we had to run water down a spillway. And we were forced to allow water to go over an emergency spill which had never been tested since the Oroville was built in 1968.
And things didn't go well when the water went over the spillway. So we ended up with basically, what you can consider the fog of war in a flood event. You never quite know what is going to happen. And water has a way of testing all the shortcomings of your design and your actions. And it did so in Oroville.
So, as we reported, there is a hole that was carved out of the main spillway and concrete was basically ripped part by the volume of water.
How are they addressing that issue now?
Well, they're not addressing it now. They can't. They can't do anything about it because they are running 100,000 cubic feet per second of water down that spillway now.
They will be able to address it this summer, and they don't know now what they are going to do about it. But they have lost almost half of that spillway. Just the power of water tore it to pieces and moved it on down into the river downstream.
There is really nothing they can do today about that spillway. They have to use it.
We mentioned that there had been some concerns raised about the earthen berm that is underneath the auxiliary spillway. Why is it that that wasn't shored up? Did people just not see that this was going to possibly be an issue?
No, I'm sure people thought it was an issue at the time.
But you are always faced with, how much money are you going to spend to address something that you think is going to be a very, very rare event? And that's actually what happens in reservoir design, dam design anywhere. You can spend an infinite amount of money to mitigate the most improbable things, but there still is a probability.
I think the big thing, big takeaway here when you look at this is that the engineers probably dismissed the idea of coating that hillside in concrete, because it would be very, very expensive to mitigate something that was highly unlikely to happen. And it never happened before in the dam's history.
Well, now it happened. And, frankly, all our climate projections for the future are suggesting these kind of events might be more frequent and even more intense. So, for us here in California, this is a bit of a time to reflect on what we are asking our reservoirs to do and how we manage them and how we're going to prepare for the future.
All right, Jeffrey Mount of the Public Policy Institute of California, thank you so much.
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