A and B denote zones of river channels. In those zones, the river channels out to the sea. While A seems to be far from the sea coast, B is closer to the sea.
Zone D is oceanic zone and here the salinity will not fluctuate
C is zone where the river water mixes with the sea. This is termed as estuary where there are partially enclosed coastal bodies as there is flow of river or stream into the sea. Here the river freely connects with the sea. In other words, we can say this zone is transition place for the river environment and maritime environment. If biomes are situated here, it is sure to experience a mixup of saline water and river water bringing in fluctuations in salinity.
