Doesn't matter what they are for - anyway I think they were placed there just to help make this great image - lovely contrasts and a nice mood. Like the swan there in the background too. The trees traced against the sky are simply lovely.
"Where there is perhaps a province in which the photograph can tell us nothing more than what we see with our own eyes, there is another in which it proves to us how little our eyes permit us to see." ~Dorothea Lange
I just think that they are basements for leveys, like for fishing purposes, it is not a base for a bridge, the iron stands not out, wide enough. My best guess is a base or for a levey or a square wooden platform wich is often seen at trout farms.
I love the angle you captured this at!
Nice work!