Iowa biologists fear that invasive zebra mussels that clog pipes and crowd out native species could be working their way down the Missouri River from South Dakota, the Sioux City Journal reported. In South Dakota, zebra mussels were recently found on boats at Lewis and Clark Lake near Yankton, which feeds into the Missouri River. Zebra mussels can be a nuisance to boaters and cities because they can attach themselves to boat motors, structures or water treatment facilities. On the plus side, they can help clear water because they are filter-feeders. The mussels have been found in both Clear Lake and East Okoboji Lake in Iowa.