Waterbody

The layer of ice that forms on top of a lake, pond, river, or stream provides some insulation that helps the waterbody retain its heat.

The area navigable by anything other than a raft gradually shrank, and parts of the inland sea were converted into fish farms. Chiku Lagoon (七股潟湖) is the largest remnant waterbody of the inland sea. Much of the rest has been filled in and built on.