Thank you very much.
That vodyanoy seems really similar, there could be a common origin or something like that.
Yes, the lake is Balkhash! I've tried searching for "Balkhash", "monster" and "10th of July", but nothing pops out... maybe searching in Russian could bring some results?
Other Japanese sources just describe the monster as a demon/fairy/maiden who lives in ice... I've also tried searching through lists of mythological beings, but still no clue.
Thanks for your efforts!
Riccardo
Edit: Lake Bajkal in Siberia could also have something to do with our monster.