A reservoir near the village of Brumovice, sometimes called Balaton by the locals. It primarily serves as an irrigation tank for the surrounding area.
It's a nice, quiet spot if you're looking for a peaceful walk or a place to go for a jog with a view of the water.
The entire area is fenced and public swimming is strictly forbidden. If you're looking for a place to cool off in the summer, you'll have to look elsewhere.