Amemasu is an enormous whale-like monster that resides in Lake Mashu in Hokkaido, Japan. It loves to attack boats, creates earthquakes and causes other disasters.
It is said that one time the amemasu swallowed a deer that was drinking at the lake. As the deer was pushed down, its antlers cut open the great fish, killing it. The monster's corpse blocked up the lake, and put it in danger of flooding.
A god in the form of a bird warned the local people. The villagers upstream fled, while the villagers downstream did not believe the bird-god and stayed. They found the carcass and hauled it out of the lake. The water came flooding in destroying everything in its path and washed away the entire downriver area. This formed the Konsengen'ya plain.
- Amemasu is also a name given to the white-spotted char, Salvelinus leucomaenis leucomaenis.