This zone has a characteristic steep gradient, fast flowing water and cool temperature.
It also has a good oxygen content and a mixed substrate of silt and gravel in which plants can take root.
Most of the fish species found in this zone lay their eggs in the vegetation on the river bed, this provides them with good protection and allows the eggs a good supply of oxygen given off from photosynthesis in the plants.
Fish species found in this zone lay adhesive eggs in the weeds.
Aarts, B. G. W. and Nienhuis, P. H. (2003) ‘Fish zonations and guilds as the basis for assessment of ecological integrity of large rivers’, Hydrobiologia, 500, pp. 157–178.