Great Lakes Shark
Occasionally, there are reports of sharks getting into the great lakes, presumably via the St. Lawrence river. Reports of sharks in the great lakes tend to be bull sharks, because they are one of the few shark species that can survive in freshwater, although reports of basking sharks have also been recorded.
In 1955, a young boy named George Lawson was attacked by a shark in Lake Michigan. People saw the dorsal fin of the animal and the outline of it in the water. A man named John Adler managed to pull him up on his boat where it was found he was missing his leg below the knee. He was later treated for his injuries. In 1960, author and environmentalist Farley Mowat claimed to have seen a shark’s dorsal fin in Lake Ontario. In 1968, Quebecois fishermen found a small four foot long, 20 pound shark in their nets west of Montreal in the St. Lawrence. In June of 1978, workers in a factory in Detroit came across the carcass of a shark inside the plant’s water-intake pipe. Presumably it got there from the Detroit River, between Lake St. Claire or Lake Erie. In 2005, bull shark teeth were found in Minnehaha Creek, Minnesota which drains into Lake Superior.
Unlike some cryptids, many scientists think it's plausible bull sharks could swim upstream from the St Lawrence or even the Mississippi River. Bull sharks have been observed in other large bodies of water, like Lake Nicaragua or the Zambezi River.