SANDPOINT — Idaho State Department of Agriculture and U.S. Army Corps of Engineers staff gave a public presentation Thursday describing the invasive species that threaten Lake Pend Oreille and their organizations’ efforts to control them. 

USACE representatives said the corps will chemically de-weed about 20 acres of lakebed in April that is currently exposed due to Lake Pend Oreille’s annual off-season drawdown. Corps staff will evaluate the areas this summer before determining whether to follow up with an in-water treatment in July or August.

When it comes to invasive species, “the best, cheapest and most effective policy is always prevention, rather than trying to play catch up,” said ISDA program specialist Ryan Hanna.

Each year, ISDA conducts an extensive survey of Lake Pend Oreille in which staff sample thousands of sites to examine the proliferation of non-native aquatic weeds. Invasive plants like flowering rush can harm water quality and stifle ecological diversity if they take hold; to contain their spread, agencies treat affected areas with herbicide and urge boaters to take steps to avoid accidentally transferring species to other bodies of water. 

Paul Cvitanich, ISDA program specialist, emphasized the importance of good boat hygiene in light of a new threat emerging in southern Idaho. In September 2023, quagga mussel larvae were discovered in the Snake River, marking the first occasion the invasive species had been discovered in the state. 

The non-native animals are known to damage irrigation systems and dams; after surveyors found mature mussels one year later, ISDA began applying a chemical treatment to the river in October 2024 to suppress their spread. 

“Quagga and zebra mussels have really terrible effects on ecosystems,” Cvitanich said. “If they were to establish a widespread presence in Idaho, the problems would be $100 million-plus each year.”

Cvitanich encouraged boaters to comply with watercraft inspectors at local stations and to take advantage of inspectors’ offer to wash boats for free to ensure they’re not harboring invasive hitchhikers. 

In the fight against invasive weeds already present in Lake Pend Oreille, mitigation strategies have helped contain their spread. 

ISDA section manager Jeremy Varley said that treatment efforts over the last six years have correlated with a reduction in the abundance of one invasive species that surveyors assess each year. 

“We are finding less and less Eurasian watermilfoil in the lake as we continue to progress with our control efforts,” he said. 

Attendees of the event urged the presenters to ensure their agencies give residents ample notice whenever chemical treatments are applied in the lake. Some in the audience expressed a desire to see the agencies prioritize mechanical techniques like hand removal by divers over herbicides. 

Varley responded that manual removal of invasive species is effective for isolated patches, but that it isn’t practical for treating the prolific growth seen in parts of Lake Pend Oreille. 

“It comes down to the question of, ‘What's going to be the most effective tool for that job?’” Varley said. “We try to make sure we're making the best choice for those sites.”