Lake Oroville Update - May 23, 2025

Golden Mussels Found During Boat Inspection at Thermalito Forebay 

The Department of Water Resources’ (DWR) newly implemented invasive mussel inspection program for State Water Project (SWP) facilities in Oroville uncovered golden mussels attached to a pontoon boat late Wednesday afternoon in Butte County. Staff immediately contacted the California Department of Fish and Wildlife (CDFW) for further inspection and decontamination. The vessel will not be allowed to launch at a DWR reservoir until CDFW confirms that it is free of mussels. 

The pontoon had been out of the Delta waters for an unknown period and was purchased Wednesday morning in Stockton by Oroville-area residents who planned to launch the boat at Lake Oroville for the Memorial Day weekend. The boat was brought to DWR’s inspection site at the North Thermalito Forebay where golden mussels ranging in size from approximately 1/8 inch to 3/4 inch were discovered during the inspection.  

“We are thankful that DWR’s new inspection program in Oroville caught these highly destructive mussels before they could be introduced to our largest water supply reservoir,” said John Yarbrough, DWR’s Deputy Director of the State Water Project. “Oroville’s reservoirs provide water to the Feather River Fish Hatchery and several municipal water agencies for residential, business, and agricultural use, produce hydroelectric power, and provide flood control. This discovery further highlights the importance of protecting these critical functions through watercraft inspections given the ease with which golden mussels are transported from the Delta.” 

More information about the golden mussel discovery is available on DWR’s blog.   

Prepare for Water Recreating This Memorial Day Weekend    

DWR’s mussel inspection program in Oroville is underway to protect State Water Project (SWP) infrastructure. Mandatory watercraft inspections are required prior to launching at the Oroville Facilities (Lake Oroville, Thermalito Forebay, Thermalito Afterbay). DWR’s free mussel inspections are only performed at the North Thermalito Forebay. Watercraft that pass the inspection will receive a seal and can launch during designated ramp hours. 

In addition to getting watercraft inspected and sealed for the Memorial Day weekend, owners should ensure their vessel is in proper working condition prior to launching – check batteries, filters, and fluids, purchase life vests for all occupants, and remember to clean, drain, and dry your vessel prior to launching at a new water body.       

Watercraft Inspection Location/Decontamination Services 

North Thermalito Forebay at Garden Drive and HWY 70 in Oroville   

Hours of operation: Daily from 8:30 a.m. to 5:30 p.m.  

Watercraft inspections and decontamination services at DWR’s Oroville facilities are free. To pass an inspection, watercraft must be clean and 100 percent dry.  

Vessels that fail inspection can undergo decontamination where water-containing areas will be flushed with hot (120 F) water to kill mussels and their larvae. Watercraft will then receive a blue Oroville seal and may launch immediately if desired.  

If a boat owner chooses not to have their vessel decontaminated, the vessel can be sealed with a red quarantine tag and will not be permitted to launch at the Oroville facilities for a minimum of 30 days. After completing the 30-day quarantine period, the vessel is eligible for launching.  

Sealed Vessel Launching   

All trailered and personal watercraft with an intact blue seal and watercraft with a red seal that have not launched for 30 days will be permitted to launch at the following boat ramp facilities during the hours listed. Boat ramps at the Oroville facilities will no longer be open 24/7. Launching trailered boats or personal watercraft is not allowed at any other location.      

When a vessel is leaving the Oroville facilities, staff at designated launch ramps will offer to re-seal all departing watercraft with a blue Oroville seal. During the next visit, sealed Oroville vessels can proceed directly to the ramp to launch where a seal checker will remove the seal. Reinspection is not required as long as the seal is not broken.  
Re-entry seals will only be offered to departing vessels during ramp hours. Please plan ahead. 

Lake Oroville   

Ramp hours: Daily from 7:30 a.m. to 10 p.m.    

• Lime Saddle  

• Loafer Point   

• Spillway      

Extended Ramp hours: Daily from 5 a.m. to 10 p.m.    

• Bidwell Canyon     

Thermalito Afterbay   

Ramp hours: Daily from 7:30 a.m. to one hour after sunset 

• Monument Hill  

Thermalito Forebay  

Ramp Hours: Daily from 7:30 a.m. to 7:30 p.m.  

• North Forebay (Non-motorized vessels only)  

More details about DWR’s mussel inspection program are available at water.ca.gov/mussels.  

Lake Oroville Reaches Full Capacity 

Thanks to warming temperatures and ongoing mountain snowmelt in the Feather River watershed, Lake Oroville reached full capacity this week for the third year in a row, offering ample water recreation and cooling opportunities for visitors. DWR continues to monitor Lake Oroville levels to optimize water storage while meeting environmental requirements and allowing for carryover storage into next year.    

With the lake at full capacity, windy periods are likely to cause water to splash onto and over the crest of the emergency spillway. This will result in minor surface wetting of the downstream side of the emergency spillway crest and the very upper sections of the splash pad. Visitors to Oroville Dam may also notice minor amounts of water flowing from drains built into the emergency spillway. Both conditions are normal and expected given the emergency spillway design. The dam and emergency spillway continue to operate as intended. 

Temporary Crane Construction Near Diversion Pool 

DWR is temporarily constructing a crane along the west side of the Diversion Pool, just north of the Diversion Dam. The crane and temporary infrastructure will enable the transfer of two new spherical valves that will be loaded onto barges and transported to Oroville Dam as part of infrastructure upgrades to the Dam’s River Valve Outlet System (RVOS). During this operation, Diversion Pool water levels are expected to fluctuate a couple feet. It is anticipated that the crane and associated work will be completed by the middle of June.  

The RVOS is a series of two parallel outlet conduits, each with two valves, that have the capacity to draw water from deep in Lake Oroville for release. The RVOS is a critical feature of the dam, providing the ability to discharge cold water to the Feather River to maintain water temperatures necessary for fish health. It also ensures the ability to maintain Feather River flows through RVOS releases during drought years, especially if reservoir levels drop below Hyatt Powerplant’s water intake structures. 

CAL FIRE Burning Grasses Around Thermalito  

For the fifth straight year, CAL FIRE Butte County, DWR, and the California Department of Parks and Recreation (State Parks) will conduct a series of prescribed burns around the Thermalito Forebay and Afterbay. Prescribed burning began May 23 along north and south Wilbur Road with additional burns planned for June 9 near the Thermalito Forebay.  

DWR’s Thermalito Vegetation Management Project was first conducted in 2021 to improve valley grassland and vernal pool habitat. Invasive annual grasses and other invasive species like the yellow starthistle degrade native grasslands and fire is a valuable tool to combat this degradation. Because invasive species have abundant seeds and dry material called thatch that build up on the soil, it is important to burn the same area multiple years in a row. When completed during the right stage in these species' growth cycle, the prescribed burns eradicate the current crop, reduce the dry residual thatch, and decrease the spread of seeds. The area has seen a significant decline in the volume of invasive grasses and starthistles since the project began. The burns also allow DWR to continue post-fire studies on vernal pool habitat. 

Oroville Recreation  

Between Jan. 30 and April 20, higher flows in the Feather River required the temporary removal of sensitive fish monitoring equipment or may have impacted counting operations. Upstream migrating fish totals through the Feather River Fish Monitoring Station between Jan. 1 and May 11, 2025 are:    

• Spring-run Chinook salmon: 3,975 

• Steelhead: 115 

• To see previous year data, visit CalFish.org. 

Current Lake Operations 

Lake Oroville is at 899 feet elevation and storage is approximately 3.41 million acre-feet (MAF), which is 100 percent of its total capacity and 122 percent of the historical average.  

Feather River flows are at 650 cfs through the City of Oroville with 1,050 cfs being released from the Thermalito Afterbay River Outlet (Outlet) for a total Feather River release of 1,700 cfs downstream. Total Feather River releases will increase to 2,500 cfs on Sunday, May 25. DWR continues to assess Feather River releases daily.   

The public can track precipitation, snow, reservoir levels, and more at the California Data Exchange Center. The Lake Oroville gage station is identified as “ORO.”  

All data as of 11:59 p.m. on 5/22/2025. 

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