At a river mouth the salinity gradient between fresh water and the sea fluctuates continuously with the state of the tide and varies with the amount of fresh water coming downstream. The salinity regime in many British estuaries can be described as 'partially mixed' and the salt content varies both vertically and horizontally. Dense salt water flowing upstream along the estuary bottom mixes with the lighter fresh water flowing downstream. In this way, an irregular and often indistinct, vertical salinity gradient forms between surface and bottom (see isohalines in Figure 8.16), and mixed water flows seaward at the surface.

Exactly where and how much mixing occurs will vary with, for instance, the state of the tide and the amount of freshwater flow. In estuaries such as the Severn and the Thames, which have very strong tidal flows, mixing may be almost complete so that there is little or no vertical stratification. At the other end of the scale, 'salt-wedge' estuaries have almost no vertical mixing and a marked halocline exists because a high river flow rate holds back the lesser flow of salt water. Usually the estuary bottom and lower shore experience wider variations of salinity than the higher shore levels.

Where the salinity is vertically stratified, the water is prone to develop deoxygenated layers (Figure 8.16), especially in summer when river flow is less. Water of intermediate density may ebb to and fro as the tide rises and falls without much mixing with well-oxygenated surface water or deeper salt water.

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