As ice forms in South Channel between St. Ignace and Mackinac Island, these waters are closed to navigation to allow the formation of ice bridges. Mariners are notified of the closure by Broadcast Notice to Mariners. From the Mackinac Bridge to Lansing Shoals, the straits are normally frozen over with solid plate ice by mid January. Heavy accumulations and ridging occur in the vicinity of St. Helena Island, White Shoal, and the reefs along the Upper Peninsula of Michigan. As deterioration begins in March or April, stable fast ice becomes drift ice moving with winds and currents.
Tracks cut by icebreakers become unreliable as the ice field detteriorates and shifts.
Thick shore ice may drift into otherwise open channels endanger even ice-reinforced vessels. A vessel which becomes beset in drift ice is vulnerable to grounding because of the many shoals, reefs and shallow-water areas in the Straits of Mackinac. Vessels beset in ice southeast od St. Helena Island have become endangered by drifting toward McGulpin Pointor the Mackinac Bridge pilings. The brash and drift ice between Mackinaw City, St. Ignace and Round Island remains east of the Mackinac Bridge, trapped by the ice in South Channel. The NNW winds will flush this ice out into Lake Huron when the ice in South Channel begins to break up.