Wetland under Koliba in Bělské les

Apparently it was originally a tank about 100x25 meters in area, but as far as I can remember, I have never seen it filled. Rather, I'm just guessing that the main purpose of the reservoir is flood protection. In fact, instead of it, there is a wetland of the same area, through which an unnamed stream flows in meanders along its entire length, originating in the southern part of the Bělské forest above Starobělské Lourdes and in the lower part into it from the right...