On the southern end of Lake M/H there is a loss of continuity in the crater ellipse. If one looks at shallow Lake Erie, the far western edge is the shallowest, while the east flowing water is deeper on that portion. If one examines the length and angle in relation to the latitude, you could theoretically slide it westward and it would complete the water filled crater at the southern end, making a complete framing of the enclosed crater similar to Lake Manicouagan. It is totally within the realm of possibilities that glacier debris conveyored by southward flowing glaciers filled in the old southern connection path of Lake M/H with its large CFM rates during the many glacier-melting years. Lake Erie now removes any rainfall in the Superior/Michigan/Huron drainage basin and sends it over the erosion resistant Niagara Escarpment at Niagara Falls. Lake Erie, in essence, is a giant reservoir held back by the encircled Niagara Escarpment that disappears below grade on the southern end due to the nature of that impactor’s shape, speed, and composition.  However, on the far northern end, the Escarpment is absent because it met any glacier flows head on. Being of a limestone construction, the wall was at a 90°angle to any glacier advancement from the south shores of Rift Lake Superior. The current minor elevation difference between Lakes Superior and M/H would have assisted the glaciers to attack this soft wall, head on with a strength similar to marble (a number of 3-4 on a increasing scale up to 10 with granite being 6-8). Thus, its absence on the North end today could be explained. Looking down the west side of Lake Michigan or the east side of Lake Huron, the Escarpment can be seen with elevations of nearly 100′ above the terrain. There are sporadic breaks where the glaciers have broken through but, for the most part, the direction of the Escarpment is parallel to the glacier flow. The glaciers just moved on both sides of them acting as huge, yet fluid, lateral supports.

On the southern end of the impact crater Michigan/Huron, the elliptical shape is not closed with water. The land in this southeastern portion of the open ring is relatively flat. If you were to slide the relatively shallow Lake Erie to the west, the 2 ends of this lake would close and complete this crater. Glacier loess transported to this region could have filled in the original lake with soil and created the current shape. During heavy glacier melting, the high flows and scouring could have relocated Lake (River) Erie to its present position.
The shallowest of the Great Lakes: Lake Erie, which averages 62′. 

The Niagra Escarpment. The very northern end of where Lake Michigan joins Lake Huron is nearly non existent due to glacier attacks head on. 

Another satellite view of the impact crater: Lake Michigan/Huron looking southwest. 
 A completely enclosed impact crater lake in Quebec. English translation is Clear Lake.

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