RAPIDAN TOWNSHIP, Minn. – County and state officials gathered at the Rapidan Dam near Mankato on Tuesday to assess a still-closed bridge next to the dam that may be at risk of "partial or full failure" as a result of the dam's worst flooding in nearly 60 years.

The assessment came during a visit by Gov. Tim Walz to tour the dam, which county officials said last week was at risk of "imminent failure" but remains standing.

Erosion caused by the now-receding floodwaters is threatening the stability of County Road 9 bridge, which is just upstream of the dam, Walz and county officials said. The Blue Earth River last week swelled to the second-strongest flood ever recorded at the dam as the area received some 10 inches of rain in a week.

"The concern is that [the bridge] is going to be structurally damaged here and it will need to be replaced," Walz said.

The bridge was constructed on top of erodible sandstone bedrock, said Ryan Thilges, county engineer and public works director for Blue Earth County.

"As the river level continues to decrease and cut the riverbed elevation down, we're very concerned about the potential for partial or full failure of the bridge," Thilges said at a news conference.

Richard Tsong-Taatarii, Star Tribune
Ryan Thilges, left, Blue Earth County's director of public works, helps Gov. Tim Walz check out flooding at the Rapidan Dam near Mankato on Tuesday.

The county launched an emergency operation last week to stabilize one of the bridge's supports, but because of safety concerns officials were not able to stabilize a second support, he added.

The county closed the bridge eight days ago and is monitoring it, he said.

The statements came as county and state officials come to grips with the toll of June's devastating floods.

Blue Earth County on Monday filed a report with federal energy regulators, detailing how raging water picked up trees and other debris that lodged into the dam's steel gates and blocked the river from pouring through the opening.

That sent water over the top of the dam and around its west end, knocking out the Xcel Energy substation and eventually a nearby home.

The report outlines how Blue Earth tried to clear the dam of trees on June 25, but five contractors felt it would be too dangerous.

By the next day, the county and others decided there was no point in trying to remove debris from the gates given that most of the water was flowing around the dam anyway.