How Did the Oroville Dam Crisis Get So Dire?

Maybe the Oroville Dam was cursed from the start.

In December 1964, three years into the massive barrier’s construction, a huge flood struck the northwest, killing dozens. The dam was nearly overtopped, which could have led to its failure even before it was completed. Instead, the partially completed dam helped prevent a larger disaster by reducing the flow of the Feather River. Less than a year later, two trains working on the site collided head-on in a tunnel near the dam, killing four men in a fiery crash and damaging the tunnel, slowing down work on the project.

The dam, which sits south of Chico and north of Sacramento, was eventually completed in 1968, creating the nation’s tallest dam. It forms the head of California’s massive, byzantine State Water Project (SWP). The SWP moves water from Northern California south toward Los Angeles, an average of 3 million acre-feet per year. A drop of water that starts at Lake Oroville, above the dam, takes 10 days to move all the way to the end of the system, south of Los Angeles.

At least in theory. Controlling a system that large is never simple, and the delicate flow of the State Water Project is under threat now, and on Sunday, authorities ordered 188,000 people near the dam to evacuate. “This in NOT A Drill. This in NOT A Drill. This in NOT A Drill,” the Butte County Sheriff’s Office blared in its order. Officials say the dam itself is structurally sound, but the spillways designed to take pressure off the dam in the case of high water levels are both damaged. Dramatic videos show water pouring out of the lake and over the spillways.

There’s some bitter irony to the problem of too much water menacing the Golden State. California has suffered through a long and severe drought, at times driving Governor Jerry Brown to institute stringent—critics say draconian—water controls. This winter has seen much more snow and rain, which is good news for the parched state, but bad news for the Oroville Dam, where huge amounts of water are collecting. The lake rose 50 feet in a matter of days. Earlier in February, as operators let water over a concrete spillway to reduce the pressure, a crater appeared in the spillway. Faced with too much water in the lake, they continued to use the spillway anyway, and the damage got worse. On Friday, the crater was 45 feet deep, 300 feet wide, and 500 feet long.

There’s a backup for the concrete spillway, an auxiliary spillway that had never been used. It’s really just a hillside sloping down from the reservoir, covered in brush and trees. As the situation became more dire last week, crews starting clearing the slope for its first baptism. Managers hoped pressing the auxiliary spillway into service would give them time to patch up the concrete spillway over what’s expected to be a drier season. (That could be easier said than done: Snowpack upstream is 150 percent of normal for this time of year, meaning there’s going to be more melt headed downstream than normal.)

Initially, that seemed to do the trick: The water level in Lake Oroville was dropping, and the danger seemed to be abating. On Sunday, however, officials noticed the auxiliary spillway was starting to erode—at the same time that huge amounts of water continued to flow into the lake. The fear is that if the spillway gives out, a wall of water could push down out of Lake Oroville and toward lower ground. Workers are trying to shore up the emergency spillway with bags of rocks, including dropping them from helicopters. If it gives way, the Feather River would flood downstream, and might wash out other levees farther down the river. Meanwhile, debris from erosion also forced the  state Department of Water Resources, the dam’s operator, to shut down its power plant, which could have helped to release some additional water. And there’s rain forecast for later this week.

How did the situation get so dire? One part of that is the seesaw state of the drought, with the weather moving from dry to saturated in a matter of months. (While droughts are a normal part of the globe’s climate, scientists say human-caused climate change has exacerbated them, increasing the severity of California’s drought by as much as 20 percent.)

Dam operators can’t control the weather, but they can try to prepare for unexpected events like the sudden inundation of Lake Oroville with consistent maintenance. One question in this case is whether the Oroville Dam has been adequately maintained.

In 2005, a trio of environmental groups filed a complaint with the Federal Energy Regulatory Commission, saying the emergency spillway was unsafe, The Mercury News reports. Their worry proved prophetic: The groups said in the event of heavy rain and flooding, the hillside would wash out and produce flooding downstream. They asked that the auxiliary spillway be paved with concrete, like the primary one. But the federal government rejected the request after consulting with the state and local agencies involved in the water system, which said they did not believe the upgrades were needed.

As for the primary spillway, the state did some repair work around the area of the collapse in 2013, CBS Sacramento reports. The last state inspection was in July 2015, but workers did not closely inspect the concrete, the Redding Record Searchlight notes, instead eyeing it from a distance and concluding it was safe. Officials say repairs should cost $100 million to $200 million, once it’s dry enough to begin them.

The Oroville Dam may be the most urgent case in the country at this moment—it’s not often that nearly 200,000 people are forced to evacuate—but it’s hardly alone. In 2013, the American Society of Civil Engineers conducted its most recent quadrennial survey of the nation’s infrastructure, and it gave the U.S. a ‘D’ for maintenance of dams.

“Thousands of our nation’s dams are in need of rehabilitation to meet current design and safety standards,” the report said. “They are not only aging, but are subject to stricter criteria as a result of increased downstream development and advancing scientific knowledge predicting flooding, earthquakes, and dam failures.”

But California is by some standards among the best states for dam safety in the country. While it has an enormous number of high-hazard dams—a classification meaning that lives are at risk if they fail—they are also inspected far more regularly than in some other states. More existing dams in the U.S. were built in the 1960s, like the Oroville Dam, than in any other decade. The combination of aging dams, bad maintenance, and spotty inspections means that what looks like a trickle of worry out of Lake Oroville and over the dam’s fragile spillways could turn into a gusher of danger around the U.S. in the coming years.