Invasive Mussel Mitigation

California Department of Water Resources Senior Environmental Scientist Specialist Brianne Sakata, with the Division of Operations and Maintenance, works with California State Parks staff to collect plankton samples to study alongside invasive golden mussels from buoys in O'Neill Forebay in Merced County, California. Photo November 13, 2024.

Invasive species such as golden, quagga, and zebra mussels can thrive in the California State Water Project as they can survive in a wide range of environmental conditions, posing negative impacts to water delivery systems and the environment. Once these invasive mussel populations are established, they are difficult to control.

Golden mussels were first discovered in North America in October 2024 at the Port of Stockton when DWR discovered the mussels while conducting routine water quality monitoring. The same month, golden mussels were discovered at O’Neill Forebay in Merced County.

Unlike quagga and zebra mussels, the golden mussel (Limnoperna fortunei) can tolerate brackish water and low calcium levels, which means that more waterbodies are vulnerable to infestation. These invasive mussels can:

Alter aquatic ecosystems by filter-feeding on planktonic algae and changing the habitat structure
Impact water delivery systems by clogging small-diameter pipelines, screens, and filters
Impact recreation by fouling boat hulls and motors, and clogging intakes and cooling lines

To prevent these damaging mussels from spreading, DWR routinely monitors for these species and acts to prevent their spread to non-infested waters. DWR partners with agencies such as the California Department of Fish and Wildlife (CDFW) and California State Parks’ Division of Boating and Waterways (DBW) to inform boaters to always remember to clean, drain, and dry their boats before entering and leaving lakes, rivers, and other waterways. DWR also works with partner agencies to routinely sample for adult and juvenile mussels at 18 SWP locations.

Watercraft Inspections at SWP Reservoirs

To prevent the spread of invasive mussels, watercraft must pass an entrance inspection before launching in most SWP reservoirs. If a watercraft fails an entrance inspection or has been in an infested waterbody, it must observe a seven to eight-day dry-out period before it can be launched in SWP waters. Dry time requirements vary for lakes managed by other entities, so it is best to check before you go. SWP reservoirs with entrance inspections include:

• Lake Del Valle (Alameda County)
• San Luis Reservoir (Merced County)
• O’Neill Forebay (Merced County)
• Los Banos Creek Reservoir (Merced County)
• Pyramid Lake (Los Angeles County)
• Castaic Lake and Lagoon (Los Angeles County)
• Lake Perris (Riverside County)
• Lake Oroville (Butte County)

DWR urges other waterbody managers with invasive species prevention programs to treat watercraft originating from Bethany Reservoir, O’Neill Forebay, San Luis Reservoir, Los Banos Creek Reservoir, Pyramid Lake, Castaic Lake and Lagoon, and Silverwood Lake as high-risk and should be managed accordingly.
To prevent the spread of mussels to new lakes, mandatory exit inspections are in place at these infested and potentially infested SWP lakes:

• San Luis Reservoir (Merced County)
• O’Neill Forebay (Merced County)
• Los Banos Creek Reservoir (Merced County)
• Pyramid Lake (Los Angeles County)
• Castaic Lake and Lagoon (Los Angeles County
• Silverwood Lake (San Bernardino County)

A “clean boat” tag, which normally is attached to boats upon departure, will not be granted to boats leaving San Luis State Recreation Area (SRA) lakes. Until DWR can determine the status of mussels in San Luis Reservoir and Los Banos Creek Reservoir, all boats leaving San Luis Reservoir, O’Neill Forebay and Los Banos Creek Reservoir will not be allowed to launch in another San Luis SRA lake until a seven-day dry-out period has been observed. Boats may be permitted to launch in the same lake they last visited if exit inspection tags are in place.

Watercraft Inspections at Oroville Facilities

With the discovery of the highly invasive golden mussel in California waterways, DWR has started a mussel inspection program in Oroville 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.

All trailered vessels and personal watercraft launching at DWR’s Oroville Facilities require a properly attached seal - a wire with a blue-colored tag specific to the Oroville Facilities that connects the watercraft to the trailer or, in the case of a jet ski, seals the steering unit to the back of the vessel. Records will be kept of each inspection and will include the date, seal number, watercraft CF number, and trailer license plate number.

Blue Oroville launch seals are only applied if the watercraft passes inspection. To pass inspection:

Mussels cannot be present.
All water-containing areas of the watercraft and trailer must be 100 percent dry. This includes the live well, bilge, ballast tanks, cup holders, low depressions in the hull, anchor storage area, coolers, life jackets, bait buckets, ropes or lines, or any area in the watercraft or trailer that can hold water.
Watercraft with ballast bags/tanks, bilge, and motors must have a working battery and be operational to complete required inspections. Be sure to bring your vessel’s keys!

Any watercraft with visible mussels will automatically fail, receive a red seal, and be reported to the California Department of Fish and Wildlife (CDFW) for a mandatory decontamination and quarantine period. These vessels will not be permitted for re-inspection or launching at the Oroville facilities without approval from CDFW.

Watercraft Inspection Location

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.

Starting Nov. 3: Daily from 8:30 a.m. to 4:30 p.m.

Watercraft inspections at DWR’s Oroville facilities are free. Vessels that fail inspection can undergo decontamination or return for a follow-up inspection the next day. To pass, they must be clean and 100 percent dry.

View the location map.

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.

Starting Nov. 3: Daily from 8:30 a.m. to 4:30 p.m.

Decontamination services at DWR's Oroville facilities are free. Watercraft with ballast tanks or other components that cannot be drained will automatically fail an inspection. Owners have the option to have their vessel decontaminated at a DWR-designated decontamination station where these water-containing areas will be flushed with hot (approximately 130 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.

Launching Locations and Hours

Boat ramps at Oroville facilities will no longer be open 24/7 with the launching of sealed watercraft limited to the following locations and hours. Watercraft must be out of the water by ramp closing time or they will be locked into the facility overnight. Please plan ahead.

Prior to launching, DWR’s seal checkers will remove your vessel’s seal. The seal should not be removed by the vessel owner.

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.

Lake Oroville

Ramp hours: Daily from 7:30 a.m. to 6:30 p.m.

Spillway

Ramp hours: Monday-Thursday 7:30 a.m. to 6:30 p.m.; Friday-Sunday 7:30 a.m. to 8 p.m.

Bidwell Canyon

Ramp hours: Monday-Thursday 5 a.m. to 6:30 p.m.; Friday-Sunday 5 a.m. to 8 p.m.

Lime Saddle

Ramp hours: Daily from 5 a.m. to 6:30 p.m.

Loafer Creek/Loafer Point

Thermalito Afterbay

Ramp hours: Daily from 1.5 hours before sunrise to 1 hour after sunset

Monument Hill

Thermalito Forebay

Ramp Hours: Daily from 8 a.m. to sunset

North Forebay (Non-motorized vessels only)

Clean, Drain, and Dry

When recreating on California’s waterways, always remember to:

Clean off aquatic plants and animal material from your boat, trailer, and equipment before leaving the ramp or parking lot
Drain ballast, bilge, live-wells, and other water-containing devices. Never drain back into the waterway
Dry equipment before launching into another body of water.

View the invasive mussels rack card in English and Spanish.

How to Identify Golden, Quagga, and Zebra Mussels

Description: Mussels are two-shelled invertebrates. They grow attached to hard surfaces and form colonies.
Shell Shapes: Their shells are generally D-shaped, triangular, and are smooth or shallowly ridged.
Color and Patterns: Golden mussels range in color from light yellow to dark brown. Zebra and quagga mussels range in color from light cream to dark brown and usually have stripes.
Size Range: Adult mussels range from about 1/2 inch to 2 inches.
- Young mussels may only be a few millimeters long and may be hard to detect without magnification.

State Water Project Detections Map

Map of California’s golden mussel State Water Project sightings as of September 2025.
For online map of detections, visit the California Department of Fish and Wildlife website.

Additional Resources

In California, it is illegal to import, transport or possess quagga, zebra, or golden mussels. Report any mussels you find to the local marina or park ranger and to the CDFW hotline at (866) 440-9530 or email: invasives@wildlife.ca.gov . For more information on invasive species laws and regulations, visit CDFW's website. Boating and inspection information are available at the Division of Boating and Waterways website.

Quagga and Zebra Mussels

Check out a history of quagga and zebra mussel sightings in California.

For more about quagga and zebra mussels, go to:

DWR’s Quagga Mussels: Clean, Drain, and Dry Video

Department of Fish and Wildlife website

Golden Mussel

For golden mussel information and how to report sightings, go to:

California Department of Fish and Wildlife website

Golden Mussels and Watercraft Inspections Fact Sheet

Golden Mussel DWR photography

Golden Mussel videos

Golden Mussel News Releases