“California Department of Water Resources (DWR) personnel and a host of collaborating agencies continued to monitor Lake Oroville spillway flows through the night. As expected, the overnight flow rate of 20,000 cubic feet per second (cfs) caused additional lower spillway erosion. Spillway flow was stopped again for a few hours this morning to allow engineers to evaluate the integrity of the structure.
There is no imminent or expected threat to public safety or the integrity of Oroville Dam.
After conferring with State and federal dam safety entities, this morning DWR will ramp up flows down the spillway to 35,000 cfs. More erosion is expected, but the releases will help operators absorb the inflow of the storm expected today and tomorrow. DWR first noticed erosion on the spillway Tuesday morning and shut off flows to investigate.
Engineers from DWR’s Oroville Field Division, Dam Safety Branch, Division of Engineering, the Division of Safety of Dams, and the Federal Energy Regulatory Commission’s Dam Safety Branch are working jointly to monitor the condition of the spillway chute to determine the maximum spillway flow rate that is safe. The spillway will be monitored continuously by on-site engineers and also with video and drones.
As a contingency, DWR also is preparing to use the emergency spillway at the dam. Lake Oroville would naturally flow over this ungated concrete crest, into a mostly unlined emergency spillway if the reservoir reaches 901 feet elevation. This would be the first time the spillway has been used in the dam’s 48-year history although the reservoir came within one foot of flowing over in January 1997.
The reservoir, with maximum capacity of 3.5 million acre-feet, now holds approximately 3.22 million acre-feet and stands at an elevation of 880 feet, which is 90% of capacity. As of this morning, inflow to the reservoir was roughly 118,000 cfs, while 13,000 cfs was being released through the Hyatt Power Plant before the spillway reopened.
In preparation for the potential use of the emergency spillway, DWR has been clearing trees, rocks, and other debris from the hillside near the dam where water will flow. CAL FIRE crews have been mobilized to clear approximately 50 to 100 acres of trees and brush that could be affected by water releases. These efforts would reduce the potential debris flow into the Diversion Pool and Feather River. DWR Crews are also placing debris booms in several key areas in the Diversion Pool. To protect the young salmon and steelhead downstream at the Feather River Hatchery from a large load of sediment and turbidity, the California Department of Fish and Wildlife began Thursday to evacuate fish from raceways to the Thermalito Annex and is exploring options for remaining fish eggs, which are in a susceptible state.
If the emergency spillway is used, flows would start gradually and reach a maximum level within the design capacity of downstream channels.
Crews will continuously monitor inflows, outflows, the spillway condition, and all aspects of the dam’s operations to protect Oroville Dam and ensure the safety of the public.
Fluctuations in Feather River flows should be expected over the next several days because spillway flows may be started and stopped as necessary to evaluate the spillway condition as outflows are increased.
Lake conditions, including lake levels, inflows, and outflows can be obtained via a recorded message at 530-534-2307.