present at the very top of the tidal range, where there is minimal salt incursion.
Further downstream, towards the upper end of the estuary, where there is a freshwater influence but with higher salinity, you can find populations of coarse species such as carp, bream and roach. In the same area, it is also possible to find sea species with a high freshwater tolerance, such as mullet, bass, flounder and smelt.
The textbook descriptions of river zones are fine in theory, but the reality of the situation is that there is usually not a welldefined band, more a gradual convergence of one zone into another. It is not uncommon to find, for example, trout in the barbel zone, carp in the grayling zone or barbel in the bream zone. It is only where there are well-defined physiological requirements, such as the oxygen and spawning requirements of the salmonids, that species zonation is more rigidly defined.
The influence of river engineering muddies zonation, too, with dams, weirs, channel straightening and various flood defence or agricultural schemes all modifying flow and interfering with the natural balance of the flood plain. It is certainly not unusual to find carp and bream populations in slow, deep and turbid water above weirs, with barbel and occasional trout in the fast, well-oxygenated water below. There are also discreet pockets of microhabitat found along the length of all flowing waters.