ALANSON, Mich. (AP) — An unplanned detour on U.S. 31 in northern Lower Michigan will remain in place over Labor Day weekend as people head to an annual walk across the Mackinac Bridge.

Michigan Department of Transportation officials announced this week that the 12-mile detour on county roads would go through Labor Day. The Detroit Free Press reports a hidden stream caused pavement to collapse on the highway last week in Emmet County, causing the detour.

No one was hurt, but repairs are expected to cost $1.2 million. James Lake, a Department of Transportation spokesman for the area, says the closure is scheduled to last until Sept. 11.

The department says more than 40,000 people are expected to take part in Monday’s bridge walk.