oxbow lake; billabong
A crescent-shaped lake formed when a meander of a river is cut off from the main channel. The Japanese name literally means 'crescent moon lake'.
An oxbow lake is a crescent-shaped lake formed when a river changes course.
Compound of 三日月 (mikazuki, 'crescent moon') + 湖 (ko, 'lake'), from the shape of such lakes.