SIOUX FALLS, S.D. (KELO) – Zebra mussels have been confirmed in the Big Sioux River near Watertown and will eventually make their way to parts of the river in Sioux Falls. 

The South Dakota Game, Fish and Parks Department announced zebra mussels were found in the river roughly five miles south of Watertown and the Big Sioux River is considered infested all the way to where it connects with the Missouri River. 

So when should people expect to find the fingernail-sized snail-like mollusk in Sioux Falls? 

GFP communications manager Nick Harrington told KELOLAND News zebra mussels move in larval stage when they free float for 2-3 weeks downstream. 

“Regardless of the flow of water, zebra mussels require hard surfaces and vegetation to attach to, which can be hard to find in the Big Sioux River,” Harrington said in an email. “Individuals downriver, such as Sioux Falls, may see small areas of colonization where suitable habitat is present.” 

Harrington said people should expect to find areas of zebra mussel colonization on hard surfaces like rocks, dams and pipes. 

“Individuals should not expect to see a noticeable change in water clarity, as zebra mussels filter organic matter, as opposed to the inorganic sediment present in the Big Sioux River,” Harrington said. 

He said additional costs because of zebra mussel infestation would be mostly infrastructure that uses the river for irrigation use like pipes and screens.

At Gavins Point dam in Yankton, the US Army Corps of Engineers installed an ultraviolet light system to kill mussel larvae floating through the system. Between 2014 and 2018, 2,500 man-hours were spent related to mussel issues impacting dam infrastructure, a news release from the Army Corps of Engineers said

The Big Sioux River is just the latest of many eastern South Dakota water bodies to be infested by zebra mussels which were first confirmed in Lewis and Clark Reservoir in 2015 and Pickerel Lake in 2020. You can find the complete list of the more than a dozen water bodies on the GFP’s website. 

Harrington said the biggest impacts of zebra mussels are on dams and pipes, increased vegetation blooms in summer months and protective footwear when swimming in infested waters. 

“GFP also reminds all boaters and water users to clean, drain, and dry every time they are on the water,” Harrington said. “In the fall, zebra mussels have attached to hard substances and can be easily detected, which is why we have included inspecting for zebra mussels in our fall sampling.”