Is every single flood myth grounded in the end of the Last Glacial Period?

[Stranger in a strange land]

No. Some probably are, but not all. Also, there are a lot of misconceptions about "flood myths":

- Noah-type Flood Myths (where a Deity floods the entire world and a righteous man and his family have to take two of every animal aboard a ship) are indeed common throughout the world, but they mostly occur in cultures in the Old World that had contact with one another in antiquity, however infrequent.
- Many "Flood Myths", especially from the New World, are indeed myths about floods, but have little in common with the Noah/Gilgamesh/Decaulion story other than that they involve floods.
- You'll often hear that every culture has a flood myth, and while they're common, they aren't universal. Japan doesn't have a flood myth, as has already been noted, and some Australian aboriginal cultures have them, while some don't.