TWIN FALLS, Idaho — Quagga mussel larvae have been found in the Snake River near Twin Falls, state Department of Agriculture officials confirmed Sept. 18.

Previously, neither quagga nor zebra mussels were believed to be present in the Columbia River Basin, in which the Snake River is the largest tributary. The mussels can cause serious damage to water quality and water delivery infrastructure including drinking water, irrigation and hydropower systems.

The larvae, or veligers, were found in the Snake River water column through the department’s monitoring program, not as a result of a watercraft making it though a roadside inspection station undetected, Lloyd Knight, the department’s deputy director, told Capital Press. The monitoring program takes about 1,500 samples per year. Department inspectors also look a rocks, docks and dams.

The veligers cannot be seen with the naked eye, in contrast to the adult mussels that may be found attached to watercraft, he said.

Ideally, any finding — whether through monitoring water or inspecting objects — occurs “quickly enough so we can do something about it,” Knight said.

As for the recent finding of quagga mussel larvae in part of the Snake River, department officials plan to define and quantify any infestation, which “will lead to control, eradication or treatment.”

Where the larvae originated and how they got into the Snake River near Twin Falls are unknown now, said Nic Zurfluh, who leads the department’s invasive species, noxious weed and range bureau. The current focus is to “find where they are and determine the next steps forward.”

Operation of the state’s roadside inspection stations will continue as scheduled, he said. Adjustments could be made in coming days or weeks depending on findings from additional sampling.

Multiple samples of larval-stage quagga mussels were found in the Centennial Waterfront Park area by the department’s early detection monitoring program.

The findings mark the first time a rapid-response plan has been put into action for quagga mussels in the state. That includes notifying impacted entities, carrying out containment measures, conducting delimiting surveys and evaluating for potential treatment options.

ISDA’s invasive species program “is prepared to respond to these findings quickly in an effort to contain mussel population growth in such a vulnerable area,” Chanel Tewalt, department director, said in a statement. “We greatly appreciate the public’s cooperation and plan on opening the area as soon as possible to minimize the impact to businesses that rely on the Snake River.”

“This is a very high priority for Idaho and for me given the gravity of the risk,” Gov. Brad Little said in a statement. “An unchecked spread — which we are doing all we can to stop — has the potential to cost Idaho hundreds of millions in actual and indirect costs.”

“Thankfully, we caught the mussels early and have already started a robust response,” he said. “We need everyone to support these efforts.”

People are encouraged to clean, drain and dry watercraft before entering or leaving an Idaho stream, river or lake, officials said.