After several weeks of low water level in Wisla river, this is how the once bottom looks like.
But - there've been a rainy weather in the South of Poland for the last 4 days, even big and dangerous flood in many regions. The flood wave on Wisla river is supposed to reach Warsaw this coming Saturday and they say it will be big and sudden.
This cracked place will be overflown for sure.

And this is the answer to my last mystery macro photo:
http://www.pbase.com/image/100796575