Mackinac Falls is a drowned 100-foot (30 m)-high waterfall formation under the waters of Lake Huron. The formation, which lies approximately 1 mile (1.5 km) east of Arch Rock on Mackinac Island, was the former outflow point for water flowing eastward from postglacial Lake Michigan. The formation was discovered on August 16 2007.

Postglacial Straits of Mackinac

During the period that immediately followed the end of the Wisconsin glaciation, the geographic features that would become North America's Great Lakes significantly changed in shape and size. Glacial melt, catastrophic erosion events as dammed-up waters found new outlets, and the post-glacial geomorphic rebound of this section of the earth's crust, caused various freshwater lakes in this area to form, drain away, and re-form.

During one of these periods, dated 10,000 years before the present, what is now the upper Great Lakes drained towards the Atlantic Ocean through a deep channel that passed eastward through Georgian Bay and what is now the small city of North Bay, Ontario. This channel was very efficient at draining water; the Great Lakes drainage basin was as big then as it is now, and collected a considerable amount of rainfall, but the upper Great Lakes were physically smaller. In particular, the overall level of Lake Michigan was much lower than its current level of 581 feet (177 m) above sea level. A shrunken Lake Michigan collected water from many now-drowned rivers throughout its bed, and the water drained eastward through a gorge, the now drowned Mackinac Channel, towards a similarly shrunken Lake Huron.

The Mackinac Channel generally followed the pathway of the current Straits of Mackinac. When the flowing water reached Mackinac Island, it found an impasse; the limestone breccia of the current island partially dammed the gorge, which formed a horseshoe curve north around the island.

Just east of what is now Mackinac Island stood the shores of postglacial Lake Huron. Lake Huron's level was approximately lower than Lake Michigan, and it too was shrunken. The Mackinac Channel gorge discharged its water into postglacial Lake Huron through Mackinac Falls.

Later, the North Bay channel was blocked by further postglacial geological changes, and both Lake Huron and Lake Michigan were forced to drain southeastward into what is now Lake Erie. The levels of both Lake Huron and Lake Michigan rose towards their current level, and the two lakes merged hydrologically with each other. Mackinac Falls was completely submerged.

The formation today

Waterfalls are short-lived geological formations. Their successful operation causes them to erode themselves out of existence, and they degenerate into rapids. When Mackinac Falls was submerged, however, it ceased to erode. The formation "froze" underwater in the form that it had been when it disappeared under the water of Lake Huron.

The lip of what was once Mackinac Falls today lies under 110 feet (33 m) of water. The base of the waterfall formation lies approximately 210 feet (63 m) down. The waterfall was discovered on August 16, 2007 by the Great Lakes research vessel Pride of Michigan as it took careful soundings of the lakebed east of Mackinac Island.

Under current geological conditions, the waters of four North American Great Lakes drain through Niagara Falls, which is 167 feet (51 m) tall. By contrast, Mackinac Falls drained one Great Lake and was 100 feet (30 m) tall.

The formation tomorrow

Further research should develop a precise outline of the formation's morphology. It should be possible to use computer-generated imagery (CGI) to develop an illustration of what Mackinac Falls looked like when it was in operation.

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