The California Department of Water Resources announced Tuesday that water levels at Lake Oroville could quickly rise to the "trigger elevation" of 830 feet, the point at which DWR plans to open the spillway gates and release water down the 3,000-foot-long concrete chute.
DWR said it has begun ramping up water releases from other outlets, including the dam's hydroelectric plant, in an effort to keep water below that 830-foot threshold.
The state's announcement generated ripples of anxiety in the Oroville community, where memories remain fresh of the near-disaster on the spillway that prompted the emergency evacuation of 188,000 downstream residents in February 2017.
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"I think it runs a gamut from fully trusting them to not trusting them at all," said Butte County Supervisor Bill Connelly. "I think the majority of the people are in the latter (category). … They're going to be very nervous."
The notice that the spillway could be used was made out of an "abundance of caution," said DWR spokeswoman Erin Mellon. She said it's entirely possible the storm might not fill the lake to the trigger point.
"Forecasts change and are dynamic, and we need to be prepared for the extreme," Mellon said. "If we learned anything from last year, it's that forecasts can change."
The main spillway fractured in half in February 2017, crippling the structure and triggering a mass evacuation of the region several days later. The spillway mostly has been rebuilt, as work crews spent last summer and fall filling the giant crater that developed after water cascaded down the battered chute. The amount of concrete used to fill and overlay the spillway would be enough to build a sidewalk that ran from Sacramento to Los Angeles.
State officials say they're confident of the structure's integrity, but more reconstruction work lies ahead this summer to make it safer.
The reconstruction will cost a total of $500 million, although that figure includes work on the adjacent emergency spillway, which also suffered damage in last year's emergency.
The National Weather Service said a "pineapple express" storm should begin rolling into the Sacramento Valley late Thursday, bringing at least 3 inches of rain. Because temperatures will be warm, the water levels at Lake Oroville could rise quickly from snow melting in the vast Feather River watershed above the lake.
"The inflows are high … because it's a warm and very wet type of system," said Alan Haynes of the federal government's California Nevada River Forecast Center.
Lake Oroville's water level is at 794 feet, having risen nearly 70 feet in the past month. The reservoir is at about 60 percent of the reservoir's capacity. At 830 feet of elevation, the lake would hold 2.5 million acre-feet of water and would be 72 percent full.
DWR has been deliberately running Lake Oroville lower than usual in an effort to avoid using the spillway. The department hasn't used the spillway since last May, when it was closed off to begin phase one of the reconstruction.
DWR officials say the spillway should be able to handle water releases of as much as 100,000 cubic feet per second — more than enough to handle most storms. When the final reconstruction is completed at the end of this summer, the spillway should be able to release as much as 270,000 cubic feet per second.
The spillway was releasing water at about 50,000 cubic feet per second when it fractured. An independent team of forensic investigators blamed "long-term systemic failure" at DWR and federal regulatory agencies for the crisis, saying the spillway was poorly designed and built in the 1960s and improperly maintained since the dam opened in 1968.