Typically, three types of categories describe the saline characteristics of an estuary:
Stratified estuaries are salt wedge type estuaries, in which the inflowing fresh water overrides a saline (dense) tidal inflow. The saline wedge advances along the bottom until the freshwater flow forces can no longer be overcome. At this point, the tip of the salt wedge will be blunt during inflowing tide and tapered during ebb tide. Mixing occurs at the saltwater/freshwater interface by entrainment, a process caused by shear forces between the two moving layers. As small amounts of dense water are mixed in the upper layers, more fluid enters the estuary near the bottom to compensate for the loss, and more fluid leaves the estuary in the upper layers to attain equilibrium of forces. Normally the isohaline is ‘deep’ and the freshwater surface layer is almost homogeneous. Only during low fresh water inflows, the maximum salinity gradient will reach the surface. At the mouth, the salinity structure will differ from the upstream areas, because of larger tidal velocities, resulting in weaker stratification and usually a better-mixed water column.
Partially mixed estuaries
These are estuaries (normally relative shallow) in which tidal energy is dissipated by bottom friction, resulting in turbulent upward mixing of salt water and downward mixing of fresh water. As the salinity at the surface increases, the surface outflow is increasing to maintain fresh water inflow as well as the upward-mixed saline water, resulting in a compensating inflow at the bottom. The salinity structure is different from a stratified estuary because of the efficient mixing of salt and fresh water. Normally, undiluted fresh water now occurring only near the head of the estuary.
Where tidal flow is much larger than the fresh water inflow and bottom friction large enough to mix the entire water column, a vertically homogeneous density structure exists. If the estuary is wide, horizontal flow separation can result in a vertically homogeneous, but laterally non-homogeneous conditions. Vertically and laterally homogeneous conditions occur in ‘narrow’ estuaries, typically like the Milnerton estuary, in which salinity increases evenly towards the mouth. However, because of continuous changing processes (river and tidal flows), an estuary cannot be exactly categorized, but will continuously move through different stages of categories between the mouth and the head of the estuary. At the head of the estuary where tidal range is reduced and fresh water inflow dominates, a stratified condition may exist. Farther downstream as tidal amplitudes increase with subsequent turbulent mixing, partially mixed conditions occur and closer to the mouth where the tidal flow becomes larger than fresh water inflow, vertically homogeneous (well mixed) conditions will occur. These conditions will vary diurnally and seasonally.