Because the strong currents reverse direction every few days, a rupture of the oil pipeline beneath the Straits of Mackinac would quickly contaminate shorelines miles away in both lakes Michigan and Huron.

That's according to a new University of Michigan study commissioned by the National Wildlife Federation.Just west of the Mackinac Bridge, the 61-year-old Enbridge pipeline carries 23 million gallows of crude oil daily.

One scenario examined in the study and illustrated in a video, shows oil from a hypothetical pipeline break would reach Mackinac Island and Round Island in 12 hours and Bois Blanc Island after two days. All three islands are in westernmost Lake Huron, just east of the straits.

Within 20 days of a spill in the Straits of Mackinac, the oil would spread as far west as Beaver Island in Lake Michigan, a distance of roughly 35 miles, and as far southeast as Rogers City in Lake Huron, a distance of about 50 miles.

The area around the 5-mile-wide straits is considered ecologically sensitive and is a major tourist draw.