Question

How is the area of river deposits shown on the map?

Answer 1

It is shown "in the shaded region in the delta, and a couple of miles outside of the river".  

Deposition is where material being transported by a river is deposited. Deposition happens when a river loses vitality. This can be the point at which a waterway enters a shallow territory which could be the point at which it surges and comes into contact with the flood plain, or towards its mouth where it meets another waterway.