Perfectly round island in north Sweden/Lapland

A perfect circular grass island on a sandbank in the Rahpaselet, the valley and river delta of the Rahpaädno river in Sarek national park, Sweden. I am still searching for an answer how this circle of grass could have been formed, please let me know if you have an idea how it could have happened.
I could imagine that the phenomenon is connected to the formation of an ice disk in winter but I cannot think of a reason why this should result in a circular grass area.