The bridge at Winona could remain closed for several weeks, said Kristine Hernandez, a transportation department spokeswoman.
The Highway 43 bridge, which was built in 1941, is the only Mississippi River crossing at Winona, and its closure is expected to cause significant disruptions.
The Winona bridge is 2,289 feet long and carries two lanes of traffic and around 11,600 vehicles per day. The roadway becomes Wisconsin Highway 54 on the east side of the river.
Officials began an inspection of the gusset plates on the steel truss bridge Monday as part of its statewide look at all bridges with gusset plates. Gussets hold together a bridge's beams and a design flaw with them is suspected in the deadly collapse of the Interstate 35W bridge in Minneapolis last year.
In addition to rust and corrosion, officials said there appears to be some buckling of a gusset plate in one location on the deck truss portion of the Winona bridge. Inspections have not yet been completed on the main span. Inspections of the gussets will continue through Friday.
Officials will then determine how much traffic weight might be allowable. That analysis is expected to take several weeks and the bridge will remain closed during that time.