Ruins of the main spillway at Oroville Dam reveal a blend of "fresh" (blue-gray) rock and "weathered" (reddish-brown) rock underneath.

Ruins of the main spillway at Oroville Dam reveal a blend of "fresh" (blue-gray) rock and "weathered" (reddish-brown) rock underneath. (Calif. Dept. of Water Resources)

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California’s Dept. of Water Resources has announced a fast-track plan to replace the shattered spillways at Oroville Dam — at least partially — by November 1, when the rainy season is expected to resume.

Meanwhile, engineers at Oroville Dam are drilling cores and conducting geological studies, hoping to better understand February’s near-catastrophic spillway failures. It’s the kind of thorough study that might’ve been lacking when the dam was built, 60 years ago.

“They did not anchor the spillway in fresh rock,” says Eldridge Moores, an eminent geologist and one of the world’s leading experts on the geology of the Sierra Nevada. (He’s the central figure in John McPhee’s 1993 book, Assembling California.)

“Fresh” is the term geologists use for rock that is fully intact and has not yet begun to break down. It is typically smooth and highly resistant to erosion.

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But much of the rock those spillways relied upon is “weathered,” the more fractured rock that is decomposing from long exposure to the elements.

“The fresh rock has been combined with the atmosphere,” explains Moores, and when that happens, you get this chemical change and you produce this softer sort of rock that’s falling apart.”

Incompetent Rock

Engineers call that “incompetent rock.” And that, says Moores, is what Oroville’s main concrete spillway was built on and what the emergency spillway was made of. In February, as intense storms filled the reservoir, both began disintegrating under the force of billions of gallons of water cascading over them.

A few miles from Oroville, Moores demonstrates by taking a rock hammer to two rocks that sit side-by-side at a spot along the Yuba River — one fresh, one in an advanced stage of weathering.

“Hear that?” he asks, as his hammer bounces off the smooth, bluish-gray fresh sample with a distinctive “tink.” Then he thumps the reddish-brown weathered rock with a series of dull thuds, which sounds less like stone than rotting wood.

“That [fresh] rock is resistant to water pouring over it,” he notes. “This is not. It’s on its way to becoming dirt. if you dump your overflow into rocks like this, of course you’re going to erode them.”



Murky Past

A 1961 planning document for Oroville Dam says the site is “blessed with a geologic 
structure and foundation rock which are suitable for the foundation.” It also suggests excavating down 18 feet to clear away “soil and weathered rock” for the main body of the dam. So it’s clear that builders working for the state were aware of the “incompetent” rock that’s prevalent at the site, but it’s less clear that similar excavations were suggested, let alone required for the spillways, which sit off to the side of the main dam.

In the 1960s, engineers built scale models of spillways to test them with various water flows and design features.
In the 1960s, engineers built scale models of spillways to test them with various water flows and design features. (Federal Energy Regulatory Commission)

Though the dam was completed nearly 50 years ago, Moores says the rock at the site would have already been substantially decomposed, as the weathering process takes place over hundreds or thousands of years.

“It was not any different when they built the dam,” he says.

It was sheer good fortune that dam operators never had to use the earthen emergency spillway until this year. But in February, with the main concrete spillway already crippled, they couldn’t empty the dam fast enough to keep pace with winter storms. The lake level rose quickly and rushed over the concrete lip, or “weir” of the emergency spillway. The earthen slope below the weir quickly disintegrated with water tumbling over it at 12,500 cubic feet per second, though design documents from the 1960s show that it could have to handle flows of 20-to-30 times that.

“I find that astounding that they would rate it like that,” says Moores. “It seems to me that even a student of geology could have told them that they were going to have an erosion problem here.”

Despite several requests, DWR did not provide an interview for this story. In a statement, it said only that the dam and spillway “met the design and construction standards of its time half a century ago.”

Detail from original drawings for Oroville Dam and spillways. The emergency spillway is shown to the left as the "1750' ungated overflow spillway."
Detail from original drawings for Oroville Dam and spillways. The emergency spillway is shown in the upper left as the “1750′ ungated overflow spillway.” (Federal Energy Regulatory Commission)

Knowing what he does about the geology of the Sierra, I asked Moores what it tells him about other dams around the state of similar design and siting.

“I worry,” he said. “That’s what it tells me about the rest of the dams.”

In late February, Gov. Jerry Brown ordered new, detailed inspections of state dams with spillways similar to Oroville’s. Those must be completed before the next flood season and must include “geologic assessments.”

On Thursday, acting DWR engineering chief Jeanne Kuttel told reporters that the new main spillway, when completed in two years, will be able to carry nearly twice the volume of water that the original concrete chute was ever called upon to do. She added that the new emergency spillway will be bolstered with retaining walls and channels made of erosion-resistant roller-compacted concrete — just in case.

“Our intent is to not to have to use the emergency spillway ever again,” said Kuttel. “However, we know that Mother Nature throws a lot at us, so we’re gonna be ready.”

How ‘Incompetent Rock’ Led to the Oroville Dam Crisis 7 April,2017Craig Miller
  • Joseph Rizzi

    Management is the real issue. Maintenance and upkeep up the main spill way was not done or was in adequate. Just like our roads government waits until it needs to be replaced or major issues for repairs or replacement. Emergency spill way was rated 10x high flow than what did flow over it for the first time since dam was built, so analysis was wrong and never tested. Think of the spill way like a water slide at a water park, what requirements would the government require a company do for safety? We should see someone get fired, retired or moved to other positions do this failure in management, but that does not happen in Govt jobs. Instead we have secrets, with the new shield of terrorism to hide behind.

  • densely

    Incompetent rock doesn’t make a structure fail unless water can wash it away. The preliminary report by outside experts said that this spillway had been repaired multiple times by removing a section of concrete, adding fill, and pouring new concrete. In other words, there were known problems with the spillway’s integrity. That experts’ report expressed alarm at the amount of water that is still draining into the spillway from the holes in its side walls, which show that water is penetrating under the structure.

    This is incompetent engineering. And management making decisions that are not well grounded in engineering reality.

  • Francis Coats

    Oh yes, blame the rock. DWR told us for nearly fifty yeas that the auxiliary spillway would act as a safe valve in the event other methods of releasing water from the lake failed, as they did this February. Now they tell us they did not believe that to be true. The sheer incompetency of the organization in putting a quarter million lives at risk is no longer incredible. It is DWR that has lost all claims to credibility.


Craig Miller

Craig is KQED's science editor, specializing in weather, climate, water & energy issues, with a little seismology thrown in just to shake things up. Prior to his current position, he launched and led the station's award-winning multimedia project, Climate Watch. Craig is also an accomplished writer/producer of television documentaries, with a focus on natural resource issues.