These photos, by Kati Tuukkanen, were taken at Lieto, 16 km from Turku, on the lower reaches of the Nautelankoski rapids.
Bertel Vehviläinen, chief hydrologist at the Finnish Environment Institute, told the newspaper Kaleva that the unusual ice formations that have been fascinating so many people over the past few days arise from the combination of very cold temperatures and flowing water.
"First the water has to freeze just a bit and be kept moving by turbulence in the flow. It then freezes evenly around the edges, creating a rounded shape," explains Vehviläinen.
The continuous movement of these ice formations in flowing water prevents them from clumping together to form a single sheet of ice.
The same kind of ice disks are sometimes also seen along the seashore where waves keep ice in motion in the same way as the turbulence in flowing rivers and streams.