The water level dropped and created a floating ice shelf
Flooding is a common phenomenon. Rivers and streams from all countries often experience it as a natural result of large rain storms that quickly drain into them. During winter, the top layer of the excess water can freeze before it retreats into the soft ground of the floodplain if temperatures are below 0°F.
Once the water sinks into the floodplain or is pumped out through an intricate system of canals, the ice stuck around the trees can keep floating and form floating shelves like the one we see in the picture. After a few days, that ice melts alongside the frozen top layer in the water.