Estuary (tidal)

The estuary of a river is the transitional zone between the freshwater river environment and the sea. They are very dynamic places as they have both marine influences, such as tides, waves, and saline water; and riverine influences, such as flows of fresh water and sediment. The inflow of both seawater and freshwater provide high levels of nutrients in both the water column and sediment, making estuaries among the most productive natural habitats in the world. Estuary

The uppermost limit is most easily defined as mean high water spring. The bottom of the estuary can be defined as headland to headland. You can also use salinity and all coastal water in the UK are euhaline, i.e. > 30 ppt.

In England and Wales for the Water Framework Directive the freshwater limits of transitional water bodies were determined in this manner: 1) At a physical barrier (e.g. a weir) 2) If there is no physical barrier then the chloride content should not exceed 200 mg/l at high water of mean spring tides and during low freshwater flow. 3) If there is no physical barrier and no appropriate data for (2) then we use the mean high-water mark as given in Ordnance Survey 10K maps.

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