A week before boating is set to return, state officials announced they had intercepted a vessel carrying invasive golden mussels at Folsom Lake this week, the first such discovery since inspections began last month under a new emergency program aimed at protecting the reservoir’s water infrastructure.

California State Parks staff found a live infestation of golden mussels clinging to a boat Tuesday during a screening at Beals Point. The vessel, which had recently been in the Sacramento-San Joaquin Delta, was quarantined immediately to prevent the highly invasive species from contaminating the lake, state officials said Wednesday.

A collage of photos provided by park rangers showed several of the bivalve mollusks, each smaller than the size of a quarter.

The lake has been closed to trailered and motorized boats since April 14 under a joint closure by State Parks and the U.S. Bureau of Reclamation, which operates Folsom Dam. The 30-day boat quarantine was implemented after golden mussels were discovered last fall in the Delta, marking the species’ first known presence in North America, and follows similar guidelines and restrictions at other capital region waterways, including Lake Tahoe.

“Thanks to this new program and the diligent work of our inspection team, we prevented this vessel from ever touching the water, which could have had catastrophic consequences,” Folsom Lake State Recreation Area Superintendent Mike Howard said in a statement.

Howard said the inspection and seal system is critical to protecting “billions of dollars of water and hydroelectric infrastructure in addition to recreation and the fishery.”

State Parks officials have inspected nearly 5,000 vessels since the closure took effect, placing watercraft under quarantine and applying red seals for 30 days. Boats that complete the quarantine may return to the lake after seal removal by authorized staff.

Golden mussels pose a serious risk to California’s water system by clogging pipes, harming native species and damaging boats. Their rapid spread since being found in Stockton last October has triggered new policies at lakes across the region.

The discovery this week highlights both the threat and the stakes: Without the inspections, the mussels could have reached Folsom Lake, which plays a key role in water storage and flood control for the Sacramento region.

The current closure will end Wednesday. More information about the restrictions at Folsom Lake and nearby Lake Clementine is available on the State Parks’ website.