Coarse fish reach or lowland course zone
This zone corresponds to the lower course of the river. Here the river is deep and slow moving. Its sluggish flow results in the deposition of silt forming muddy bottom on which many hydrophytes can-grow. In this zone compared to other zones, the oxygen content is less and the temperature is more variable. Although some of the fish, characteristic of the upper reaches of the river may be present in this zone, but conditions here are not suitable for them to complete their life cycle successfully. For example, salmon and trout require silt-free gravel in which to lay their eggs and cold, well oxygenated water for their young offspring.
Conditions are now more suitable for other fish species such as cyprinids, the roach, the chub and the bream. These fishes can tolerate low concentration of dissolved oxygen and higher water temperature and need water plants on which they can lay their eggs. In the lowest reaches of a river running into an estuary the sea occurs to flow under, during the summer months.