You are referring to an oxbow lake.
Oxbow lakes form as a river deposits sediment and changes course over time.
The flood plain. Delta
a canyon, gorge or vale
In its flood plain
No, the area drained by a river is a river basin or drainage basin. The flood plain is in the lower valley reaches, where heavy rain can cause the river to flood, often bursting over flood defences, and for the water to flood over a large area.
It is the flood plain.
The narrow strip of land on both sides of the rivers is called cover flood plains
A 'flood plain' usually.
flood plain
the flood plain
River bank, flood plain, water meadow.
The flood plain
A meander in a river follows a winding course through a valley. An oxbow is a small curved lake lying on the flood plain of a river constituting the remnant of a former meander. The similarity is historical.