There is a large river called the Amur River on the Siberian continent with a total length of 4,444 km. Its extensive length becomes apparent when compared with the total length of the Japanese islands.
This is the mouth of the Amur River. The extensive size becomes apparent when compared with Lake Biwa, the largest lake in Japan.
Fresh water flowing from the Amur River spreads over the surface of the Sea of Okhotsk and creates a thin layer of water which has a low salt content.
The water between the surface and 50 meters below has a low salt content. The water then acquires a high salt content. This creates a double-layered sea.
The low salt water on the surface maintains the convection flow in this 50 meter area. The water gets colder and freezes.