Oroville Dam is an earth-fill embankment dam on the Feather River in Butte County, northern California, just east of the city of Oroville in the Sierra Nevada foothills. At 770 feet (235 m) it is the tallest dam in the United States. Completed in 1968 by the California Department of Water Resources (DWR), it holds back Lake Oroville, the state’s second-largest reservoir at more than 3.5 million acre-feet (about 4.3 km³), and feeds the Edward Hyatt power plant, with 819 MW of capacity. It is the keystone storage reservoir of the California State Water Project, which supplies water to more than 27 million people. It is best known for February 2017, when its main and emergency spillways both began to fail and about 188,000 people downstream were evacuated.
February 7–14, 2017: How Both Oroville Spillways Failed
The winter of 2016–17 was northern California’s wettest in more than a century, arriving after five years of drought. Between February 6 and 10, 2017, about 12.8 inches (330 mm) of rain fell on the Feather River basin. Inflow to Lake Oroville jumped from 30,000 cubic feet per second (cfs) on February 6 to more than 130,000 cfs at midday on February 7.
That day, with about 54,500 cfs running down the concrete main spillway, operators saw an unusual flow pattern and shut the gates. Engineers found a crater roughly halfway down the 3,050-foot (930 m) chute, where the concrete lining had broken through and water was scouring the hillside beneath. Test releases on February 8 and 9 lengthened the hole from about 250 to 300 feet (76 to 91 m), while inflow peaked at 190,435 cfs late on February 9. DWR faced two bad options: keep using a damaged spillway, or let the lake rise until it poured over the emergency spillway, a 1,730-foot (530 m) concrete weir with a bare, unlined hillside below it. It reopened the main spillway, then cut releases from 65,000 to 55,000 cfs on February 10 to protect power lines serving the Hyatt plant. The lake kept rising.
On February 11, the lake reached 901 feet (275 m) above sea level and water spilled over the emergency weir for the first time since the dam was completed in 1968. The flow over the weir peaked at only about 12,600 cfs, but the hillside below it eroded far faster than expected, and the erosion began cutting uphill toward the weir itself. If the weir had collapsed, up to 30 feet (9 m) of the lake could have been released in an uncontrolled surge into the Feather River.
Butte, Yuba and Sutter Counties: The Evacuation of 188,000 People
On February 12, the Butte County Sheriff’s Office ordered the evacuation of low-lying areas along the Feather River in Butte, Yuba and Sutter counties, including the city of Oroville itself, home to about 20,000 people at the 2020 census. To take pressure off the weir, DWR nearly doubled releases down the already broken main spillway, from 55,000 to 100,000 cfs, accepting that this would tear away more concrete and hillside. By that evening the lake had fallen below the weir crest and the emergency spillway stopped flowing.
On February 13, helicopters dropped sandbags and large rocks onto the eroded slope, and about 23,000 National Guard troops were put on standby in case the spillway failed. By then, 188,000 people had been reported evacuated. On the afternoon of February 14 the order was reduced to an evacuation watch and residents were allowed home. The main dam embankment was never at risk; the danger came from the spillways beside it.
The damage carried on downstream. About 150,000 cubic yards (110,000 m³) of debris from the eroded hillside clogged the Feather River below the dam, blocking the outflow from the Hyatt plant, and the muddy water forced staff at the Feather River Fish Hatchery to move more than nine million fish to a satellite facility at Thermalito Afterbay.
Investigators later found that warnings had been raised years earlier. During the dam’s 2005 relicensing, three environmental groups asked the Federal Energy Regulatory Commission (FERC) to require concrete armour on the emergency spillway, warning that heavy use would erode the hillside. FERC judged the spillway compliant, and the lining, estimated at more than $100 million, was not built. A 2018 study found that the hillside had been described in 1960s reports as bedrock when it was in fact weathered rock. The Independent Forensic Team led by John France published a report of nearly 600 pages in January 2018 that listed 24 possible causes of the main spillway failure, among them a faulty drainage system, uneven concrete thickness and corroded rebar.
Building the Tallest Dam in the United States, 1957–1968
The dam began as part of the Feather River Project, proposed in 1951 by State Engineer A. D. Edmonston to store northern water and send it south. Floods in the 1950s led to a 1957 flood-control bill that funded a dam at Oroville, and groundbreaking began that May with the relocation of the Western Pacific Railroad out of the Feather River Canyon. The Burns–Porter Act, which created the State Water Project, passed the legislature on November 8, 1960. Main construction began in 1961, with DWR engineer Donald Thayer leading the design.
Two diversion tunnels, each 4,400 feet (1,341 m) long and 35 feet (11 m) across, carried the river around the site. A 128-foot (39 m) concrete cofferdam, finished in May 1963, later became the impervious core of the dam. The fill was hauled along an 11-mile (18 km) railway, an average of 120 cars an hour, and much of it came from piles of hydraulic mining debris washed down the Feather River after the Gold Rush, a story told in our guide to the geography of the California Gold Rush.
The unfinished dam was tested early. In the Christmas flood of December 22, 1964, the Feather River peaked at 250,000 cfs above the site; water nearly overtopped the partial embankment, but the structure cut the river’s peak by almost 40%. On October 7, 1965, two 40-car work trains collided head-on at a tunnel entrance, killing four men and starting a 100-acre fire. In all, 34 workers died building the dam.
The embankment was topped out on October 6, 1967, after more than 40,000 train trips delivered 155 million short tons (140.6 million tonnes) of fill. It stands 770 feet (235 m) high and 6,920 feet (2,110 m) long at the crest, and it was the largest earth-fill dam in the world until Egypt’s Aswan High Dam overtook it. Governor Ronald Reagan spoke at the dedication on May 4, 1968, with former governor and then Chief Justice Earl Warren in attendance. It stands taller than the concrete Hoover Dam on the Colorado River.
Lake Oroville and the California State Water Project
Lake Oroville drowned the confluence of the North Fork and main Feather River and the old towns of Bidwell and Land, close to the bar where John Bidwell found gold in April 1848, three months after the strike at Sutter’s Mill. It is fed by the North, Middle, West Branch and South forks of the Feather, which drain about 3,611 square miles (9,350 km²) of the northern Sierra Nevada and southern Cascade Range. Only Shasta Lake, behind Shasta Dam on the Sacramento River, stores more water in California.
Water released from the dam flows down the Feather into the Sacramento River and on to the Sacramento–San Joaquin Delta, where the California Aqueduct diverts it south through the San Joaquin Valley and over the Tehachapi Mountains. Water and power from Oroville help irrigate about 755,000 acres (306,000 ha) on the arid west side of the San Joaquin Valley and supply cities across Southern California. Roughly a third of the water released is used between Oroville and the Delta.
Flood control shapes how the lake is run. In winter and early spring at least 750,000 acre-feet, about a fifth of the reservoir, must be kept empty to catch storms. In the 1997 flood, inflows topped 331,000 cfs, but operators held outflow to 160,000 cfs. Drought pulls the other way: after dry years from 2020, the lake fell to a record low of 22% of capacity by September 30, 2021.
Edward Hyatt Power Plant and the Thermalito Complex
Underground beside the dam sits the Edward Hyatt Pump-Generating Plant, completed shortly after the dam and at the time the largest underground power station in the United States. It has three 132 MW conventional turbines and three 141 MW pump-generators, 819 MW in total, using up to 16,950 cfs at full output. The pump-generators can also lift up to 5,610 cfs back into the lake when spare power is available.
Since 1969 Hyatt has worked with two off-stream reservoirs west of Oroville, the Thermalito Forebay and Afterbay, as a pumped-storage system that can generate a further 114 MW at times of peak demand. Together, Hyatt and Thermalito produce about 2,200 GWh a year, around half of the electricity from the State Water Project’s eight hydroelectric facilities and about a third of the power needed to pump water from the Delta over the Tehachapis. In August 2021 Hyatt had to shut down when the lake dropped below its intakes; it restarted on January 4, 2022.
The dam also blocks salmon and steelhead from their upstream spawning grounds. To compensate, DWR and the California Department of Fish and Game opened the Feather River Fish Hatchery in 1967, which releases about 10 million salmon smolts and 450,000 trout smolts into the river each year.
Rebuilding the Spillways, 2017–2019, at a Cost Above $500 Million
The main spillway was shut on May 19, 2017, so demolition could begin. The repair was first projected to exceed $400 million, with the $275 million main contract going to Kiewit, and the estimate passed $500 million by November 2017, driven up by power-line relocation, river dredging and weak bedrock. Phase one made the spillway usable by November 1, 2017. In phase two, in 2018–19, the temporary sections were replaced with steel-reinforced structural concrete, and a concrete cutoff wall was built under the emergency spillway to stop erosion working uphill if it is ever used again. Crews worked more than 2 million hours on the rebuild.
The new chute was tested for real on April 2, 2019, when heavy rain upstream forced releases of 8,300 cfs, raised to 25,000 cfs on April 7. A DWR assessment dated October 1, 2020 concluded that the dam was fit for continued safe operation, though FERC asked the state for a plan, due by September 2022, to cope with the heavier rain predicted in future, and periodic concrete and sealant work was scheduled to run to 2025. For the farms that depend on Oroville’s water, see our guide to California’s Central Valley.