The Oroville Dam sits nestled in the hills of Oroville, in Northern California. It blocks the Feather River to form a 25-square-mile lake. After a massive sinkhole developed in the dam's main spillway on Feb. 7, water officials began using the
emergency spillway. Concerns about possible erosion prompted the evacuation of more than 180,000 people on Feb. 11.
The dam at Lake Oroville, a reservoir near Oroville, California, can hold back up to 3.5 million acre-feet of water, but federal regulations require 20 percent be set aside in a spillway for flood control.
Most years, that strategy works pretty well.
But not this year. When the main spillway broke, causing the water level to creep dangerously high, officials decided to use an untested emergency spillway on Feb. 11. The reservoir was at 901 feet that day, the highest recorded level since 1985.
Oroville Dam water elevation
The near failure of that spillway led to the evacuation of more than 180,000 people downstream. In the days before the evacuation, hourly outflow sensors recorded the dramatic drop in the release of water while main spillway damage was assessed.
By midday Feb. 7, water releases were slowed to a trickle. From then, officials struggled to increase outflow without further damaging the spillway.
Oroville Dam hourly outflow
Some say it’s time to rethink how Oroville and other large dams are managed.
“Flood management is in tension with other services because a mostly empty reservoir … is the best hedge against floods,” Jeffrey Mount, a senior fellow at the Public Policy Institute of California wrote in a recent blog post.
Federal regulations require that 20 percent of Oroville Dam’s capacity be reserved for flood control purposes. Officials began releasing more water from the dam on Jan. 12, when water levels started to reach the flood control limit.
“By design, Oroville was relatively full when the latest floods arrived, reflecting its top priority (water supply) and compounding flood risk,” Mount wrote.
When the main spillway was damaged on Feb. 7, officials couldn’t properly respond to the heavy inflows. The remaining 20 percent of the dam filled quickly.
Oroville Dam daily inflows and outflows
Chris Orrock, a spokesman for the Department of Water Resources, which operates the Oroville Dam, said this month’s crisis was triggered by the crippled spillway, not tension between flood control and water supply.
“You get criticized if you send too much water down the system and it gets washed out to sea,” Orrock said. “And you get criticized if you keep too much water in the reservoir. We do what’s best for the overall state water project while still
meeting the needs of flood control for the reservoir.”
125 feet in 60 days
From December to January, the water in Lake Oroville rose by more than 125 feet. These satellite images show the change in water level. In December, pink areas show the bank and beaches. Those all but disappeared by January.
The Feather River watersheds, which feed into Lake Oroville, saw historic rainfall this year. Average rain and snowfall levels were between 180 percent and 378 percent of normal January levels, according to a Reveal from The Center for Investigative
Reporting analysis of provisional data from the PRISM Climate Group. As of Feb. 14, rain and snowmelt totals throughout the region already had exceeded normal values for the month by as much as 226 percent.
Average rainfall and snowmelt for the Feather River