an old river
flood plain
Meander
The flood plain
You are referring to an oxbow lake. Oxbow lakes form as a river deposits sediment and changes course over time.
there is 896 square kms of flood plain
The flood plain. Delta
A floodplain.
The land formed by deposited soil at the mouth of a river is called a delta (named for the Greek letter, which is similarly triangular in shape).
a canyon, gorge or vale
It is a flood plain.
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.