I am looking for the coldest lake, pond or reservoir in Austria suitable for swimming (i.e. where swimming is legal) during the peak summer months (July and August).
The location doesn't matter as long as it's publicly accessible (no matter if it takes some hiking to get there).
To provide an example, Frillensee located in Germany close to the Austrian border is said to be the coldest lake in Central Europe but I wasn't able to verify this beyond hearsay.
The coldest I can remember (without having measured it back then) is a little lake called Seeleinsee in the Hagen Mountains. My bet is that the lake at the bottom of the sinkhole at Funtensee, the coldest place in Germany, should also be reasonably cold. Both of these examples are located in Germany though (albeit it close to the border with Austria).
Are there similar occurrences of lakes in Austria?