An interview with Peter Gleick about what might lie ahead for America’s tallest dam — and the people living in its shadow.

By Eric Holthaus

The Oroville Dam spillway overflows with runoff in Oroville, California, on February 14th, 2017. (Photo: Monica Davey/AFP/Getty Images)

Though the initial emergency at Lake Oroville seems to have passed, a new surge of rainfall is on the way — enough to threaten renewed concern at America’s tallest dam.

Residents were allowed to return to their homes on Tuesday, but serious danger remains. Crews at the beleaguered dam are working around the clock to stabilize and reinforce the emergency spillway in anticipation of up to a foot of additional rainfall that’s predicted over the next 10 days, likely to again boost lake levels. But the scale of action — truck after truck of giant boulders dumping 1,200 tons of rock per hour — was small in comparison to the immense scale of erosion that has already taken place. There’s a real risk that the lake could again overtop.

Here’s how we got to this point:

Over the weekend, amid the rainiest start to the rainy season on record in northern California, Lake Oroville spilled its banks.

As of late Saturday, the reservoir elevation at Lake Oroville was 902.57 feet above sea level, at 101 percent capacity. At one of the largest reservoirs in the United States, this had never happened before — and close observers were watching with bated breath.

“Water is really erosive. They use water to cut steel.”

On Sunday, it became clear that the hillside supporting the emergency spillway — which had never been used in the dam’s 48-year history — was starting to crumble. For the first time since the 1992 Los Angeles riots, the entire 23,000-member force of the California National Guard was put on alert. Anticipating a potential worst-case scenario, officials ordered the evacuation of nearly 200,000 people — who have since been allowed to return home, though an “evacuation warning” remains in place.

Since Sunday, the lake’s level has dropped more than 20 feet, temporarily easing pressure on the dam. The main spillway, despite significant damage, is operating at near full capacity — a flow rate equivalent to about 60 percent of the Mississippi River at St. Louis. That’s well short of the 50 feet authorities had been hoping the lake level would drop by before the rains arrived, however.

To be absolutely clear: The main dam itself is not at risk, though that’s not necessarily a comfort to those living in its shadow. The weakest part of the dam complex is the emergency spillway — a 20- to 30-foot concrete structure on an adjacent hillside. During this week’s overtopping event, massive erosion began to undermine that structure. If the lake overtops again, the erosion there could continue.

To understand what might happen in this scenario, I contacted Peter Gleick, chief scientist at the Pacific Institute and a frequent visitor to Lake Oroville. He cautioned that he does not work as an engineer (though he does have an undergraduate degree from Yale University in engineering and applied science), and emphasized that the scenario we discussed was very unlikely to occur without an additional overtopping event, and even then was still unlikely.

We spoke earlier this week, when Lake Oroville was still exceeding capacity — but the same basic scenario would apply if the lake again overtops.

What happens next?

This could go no further. They could draw the lake down and reduce the pressure on the emergency spillway and avoid a big uncontrolled release. Or, it could be an unbelievable catastrophe, we don’t know.

In a worst-case scenario, what would happen if they lose the emergency spillway?

If they lose the emergency spillway, if it erodes at the base and some part of the emergency spillway collapses, they will lose 20 to 30 feet of water off the surface of the reservoir. That’s about 350,000 acre feet. [Editor’s Note: One acre foot is equal to about 325,000 gallons, about as much water as a family of four uses each year.] It’s like cutting a hole 20 feet high in the hillside.

That water will cascade down that hill, and that’s a lot of water. That by itself will flood [the city of] Oroville and cause serious downstream damage. The worry is that that water itself will then further erode what it’s running over. There’s a lot of bedrock there, but what if there’s a channel that isn’t bedrock? If they lose more than 20 feet of the height, if it erodes a bigger gap, they lose that additional amount of water.

Water is really erosive. They use water to cut steel. If it’s in a little narrow channel, that’s a worry.

Is the dam itself at any risk in that situation?

Nobody is saying the dam is going to collapse. But if the hill collapses or the hill erodes, then you get the same loss of water.

The worst-case scenario is you lose the emergency spillway and it just gets worse and worse, a chain reaction.

So, an intact dam may be standing next to a big hole and partially empty reservoir by the time this is over.

We don’t really know what will happen.

How much water is 350,000 acre feet?

A lot. But it’s only about 10 percent of the lake’s total capacity. Remember, this is one of the largest dams in the world.

Total urban water use of California is about seven million acre feet per year, so about 20 times 350,000 acre feet — 350,000 acre feet is enough to irrigate about 100,000 acres of cropland in the Central Valley.

But remember also that this is water that would be lost anyway. One of the primary functions of Oroville Dam is to provide space for floodwaters. This time of year, they try to keep flood space open for exactly this reason. There’s a lot of snow in the mountains upstream — we will still get more storms this winter.

The only reason this reservoir is flooding now is because the main spillway failed so they reduced its outflow, and, in doing so, the lake filled up. When this is all over, they’re going to spill this water anyway because they need to keep flood space for spring runoff. That 350,000 acre feet is not exactly usable water that is going to be lost if there is a disaster. It’s water that would flow down the river in a wet year anyway.

What’s the long-term impact to California’s statewide water delivery system as a result of what’s happening right now at Lake Oroville?

Let me preface this by stressing that I’m not saying the worst-case scenario is going to happen. It’s unlikely, but God, who knows. The whole thing has been unlikely.

Oroville is a critical component of the state-funded State Water Project. It’s the biggest reservoir in the State Water Project. The state’s biggest reservoir is Lake Shasta, part of the federally funded Central Valley Project. Both move water from mountains to coast and north to south.

If we were to lose Oroville, a very substantial amount of agricultural land would have to be pulled out of production and deliveries to some big cities in the south, including Los Angeles, are likely to be threatened. It’s that important of a component of the system. We already have difficulty in meeting demands for water.

If we’re lucky and there’s no big erosion event, and there’s no uncontrolled release, and they’re able to draw the reservoir down enough, at the end of the season they’re going to have to fix the main spillway. The initial cost estimate for that is already $100 million to $200 million.

In that scenario, the most likely scenario, they can still run water through the turbines down the river. They can still deliver water to farmers in the Central Valley and the cities in Southern California. It just becomes a really expensive construction project.

So without Oroville, the state’s water landscape would be fundamentally changed.

You want to store water because we get it all in the winter and you want it all in the summer. You want to release it slowly when there’s a demand for it. If there’s no additional severe problem, they’ll still be able to do that.

With climate change, with more extreme events — drier drys and wetter wets — more big storms, it becomes more difficult to operate a system that was built for a climate we no longer have.

As for what happens next, like everyone else, I’m watching the news feeds.

This interview has been lightly edited and condensed.