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Oroville Dam

Oroville Dam Incident Explained: What Happened, Why and What’s Next

Problems at two spillways at California’s Oroville Dam in February have resulted in months of analysis and reports about what went wrong and how much repairs will cost.

Written by Ian Evans Published on Read time Approx. 5 minutes
A controlled blast on June 7, 2017, breaks up the remains of the lower chute of the Lake Oroville flood control spillway in Butte County, California.Kelly M. Grow/ California Department of Water Resources

In February, damage to the spillway of the dam on Lake Oroville in Butte County, California, and erosion under the dam’s emergency spillway threatened to send billions of gallons of water cascading through dozens of California communities.

The dam did not collapse, but the damage to the spillway and the emergency spillway was enormous. The incident, and the political aftermath, have sparked tensions and concerns that the problems that led to the Oroville incident may be part of larger institutional issues.

To catch up on what happened at Oroville, and what has happened since, here’s a brief overview.


The flood in late 1955 and early 1956 was historic. It caused widespread damage in Northern and Central California, resulted in 64 deaths and more than $200 million in property damage. To prevent further flooding, the state passed a $25 million appropriations act, including money for a dam. The dam was to be located on California’s Feather River, just above the town of Oroville, about 70 miles north of Sacramento.

Between 1961 and 1968, the California Department of Water Resources (DWR) built Oroville Dam, still the tallest dam in the United States.

Since its construction, the Oroville Dam has had several incidents, including a crack in the main 3,000ft spillway in 2013 that was repaired, with a senior engineer for the DWR telling CBS Sacramento that “everything checked out.”

The 2017 Spillway Emergency

Oroville Dam on February 27, 2017, two weeks after almost 200,000 people were evacuated from their homes. The main spillway, seen here, has split in two, and the earthen emergency spillway stands to the left of it. (KCRA via AP)

In the winter of 2016–17, after heavy rain in Northern California, Oroville’s water levels rose quickly. While the water may have been a welcome sight after five years of drought, unprecedented rainfall filled the reservoir to dangerous levels. To relieve some of the pressure, the state released water down the main spillway at rates of up to 54,500 cubic feet per second (cfs). Then, on Tuesday, February 7, a crack in the spillway appeared and soon grew into a 250-ft crater.

To prevent further damage, officials shut off water to the main spillway, but behind Oroville the reservoir continued to fill. Officials were hesitant to rely on the dam’s emergency spillway, because it had never been used and would wash earth and trees into the Feather River below. To test how much it could safely rely on the now-damaged main spillway, the state released relatively small amounts of water – around 20,000 cfs. This eroded the spillway’s hole by another 50ft.

Because officials were unable to safely release enough of it, on Saturday, February 11, the reservoir’s water rose to 901ft – enough to flow over the emergency spillway, as it was designed to do.

Unlike the main concrete spillway, the emergency spillway spills water over the lip of the dam wall. The water passes down an earthen hill and into the Feather River. On February 12, as water flowed down, it began to erode the hillside. Officials soon noticed that the erosion threatened to undercut the entire dam, which could cause a collapse and send a 30ft wall of water into the valley and communities below. That same day, California officials ordered 188,00 people to evacuate the Feather River basin, which the reservoir now threatened to inundate.

To ease the dangerous erosion, officials also released 100,000 cfs of water down the main spillway, damaging it further.

But the dam held, and the reservoir eventually dropped below 850ft. Two days after the evacuation order, the state downgraded the order to an evacuation warning and cautiously allowed residents to return to their homes.

Emergency Aftermath

After the spillway collapse and evacuation, the Federal Emergency Regulatory Commission (FERC) ordered the DWR to put together a team of independent analysts to assess the damage to the dam, how the emergency happened and what steps should be taken next. Meanwhile, the DWR also put together a separate team of independent consultants.

Experts have already identified many factors that may have led to the collapse of the spillway. For example, the spillway was built on top of weak, porous soil, according to reporting by Water Deeply, which likely eroded during the wettest water year in recorded history for Northern California.

In the first independent report by DWR’s consultants, they pointed to both damage and design failures in the main spillway that would prevent the state from completing repairs before the beginning of the rainy season in November. The study directly contradicted the official DWR report, which stated that repairs could be completed by then.

State and federal officials subsequently kept another two independent reports secret, citing security risks and terrorism risks. However, public outcry prompted officials to eventually release another five redacted versions. Many documents related to the incident remain secret, including documents about asbestos that may have been released by the dam, and the risk to the public.

Like the initial report, the redacted reports detailed the extensive structural damage and failures from the crisis. Similarly, on May 5, the FERC investigators released a preliminary memo that detailed 24 factors that potentially led to the spillway collapse.

According to Martin McCann, Jr., the director of the National Performance of Dams Project at Stanford University, Oroville may have been an institutional failure as well as a structural one. Problems with the dam-building industry not paying attention to data on dam safety and structure, bad communication between groups – like engineers and geologists – and a failure to catch the signs of impending failures during routine inspections, all contributed to the Oroville Dam incident. They may point to a much larger problem.

The Sacramento Bee reported that California is borrowing $500 million from the federal government to pay for the Oroville Dam repairs, including $274 million that was previously approved by President Donald Trump to respond to the crisis. Currently, work is underway to repair the damaged spillway – just dredging the spillway debris has cost the state more than $22 million, and Kiewit Corporation won the contract to repair the spillway for $275 million, according to the Bee. The state has deployed helicopters and drones to assess the extent of the damage, and engineers in Utah even built a working, scale model of the spillway to better assess how water flows over the damaged concrete, and how best to repair it.

You can access ongoing, updated documents about the Oroville Dam on the DWR’s website or stayed tuned to Water Deeply’s Oroville Dam coverage.

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