1. There are a couple of things going on with the Great Lakes history (I live in Michigan). There is evidence of a few flood events. One, as the glacial ice sheets retreated, there was a point where it appears there was a lake being held back by an ice dam, and when it released, it raised lake levels significantly. This appears to have been in and around what is now the Georgian Bay area on the east side of Lake Huron.
The water run off was historically from North to South via rivers, including the downstream Mississippi. The comet, which landed in Lake Erie was enough to raise the water level again, in a different area of the system, and create what is now the St. Lawrence seaway, now causing the Great Lakes to primarily drain West to East. It still does drain North to South, as well, at the southern tips of Lake Michigan (near Chicago) and Lake Erie (near Toledo).
Furthermore, as of today, the land underneath the Great Lakes is rising in the north and dropping in the south, a land tilt, the Earth's crust still rebounding after the glacial retreat, making the water levels lower in the north and deeper in the south, as well as more land being exposed in the north and loosing shoreline in the south.
2. It is difficult to say what actually happened with those frozen mammals up north. Why did they appear to die suddenly? Well, it was during the retreat of the glaciers. In other words, a time of global warming. Just like today, and we can look at the plight of the caribou herds right now to perhaps gain some perspective. The grasses and lichens they feed on are not growing with the abundance to support grazing herds, leading to migratory herd death due to starvation. The permafrost is also melting causing the release of ancient bacteria, like anthrax, as well as viruses, and methane pockets that can wipe out entire herds quite efficiently and quickly. Then you also have extreme weather events where a deep cold weather trough quickly comes down and deep freezes everything, causing deaths.
