Saltmarshes form on the upper parts of intertidal mudflats on sheltered coastlines where fine sediment is deposited by the sea. They are occupied by plants that can tolerate periodic covering by seawater. Saltmarshes are typically found on sheltered coasts such as estuaries, inlets and behind barriers such as islands or shingle spits. Most of England’s saltmarshes are concentrated in the major estuaries of eastern and north-west England, with smaller areas in the estuaries of the south coast. Because of variation in the sediment type, saltmarshes of the muddier east coast tend to be made up of different species from those of the sandy sediments more characteristic of the west coast. In saltmarshes that are used for seasonal grazing, the species composition may also be affected by the grazing regime.

Saltmarshes change naturally as they develop. Marsh vegetation grows as far as the upper limit of high tide, where it is still periodically covered by saltwater. As tidal water covers the saltmarsh it deposits silt around plant stems and roots, which increases the height of the ground. As more silt is deposited, the marsh is less frequently covered by the sea. Different species of saltmarsh plant have different tolerances to saltwater. As sediment accumulates, the marsh becomes higher, and the vegetation at a certain location will change reflecting the zonation of saltmarsh, typically in four main zones: pioneer, lower, middle and upper saltmarsh. The upper
saltmarsh zones include species that produce colourful displays of flowers, such as sea-lavender, thrift and sea aster, as well as nationally rare species such as matted sea-lavender or sea-heath.

In 1999, England had 71% of the total saltmarsh area of the UK,some 32,500 ha, the majority of which is within SSSIs. Some of these sites have been recognised as being of international importance, and have been submitted to the European Commission as candidate Special Areas of Conservation for their saltmarsh communities. In addition, many sites are included within Special Protection Areas because of their importance for birds, as roosting, feeding or nesting areas. Around one quarter of the saltmarsh (by area) in these designated sites is in unfavourable condition. The reasons for this decline range from coastal defence works, to erosion as
a result of rising sea level. If losses continue at present rates, without further re-creation, the targets agreed in the UK Biodiversity Action Plan for saltmarsh will not be met. Natural events can affect the development of saltmarsh. For example, storms may change the wave exposure of an inlet by removing offshore barriers, resulting in saltmarsh erosion. Long-term changes in the deposition of sediments in an estuary may erode saltmarsh from one area, while marsh development takes place in another.

In addition to these natural changes, where they occur, are changes caused by human activities. The natural succession of saltmarsh is affected by changes to sediment supply (for example by the effects of dredging and dumping of dredge spoil), damage from grazing, changes in wave action (for example by coastal defence work) or more directly by reclamation for development or agriculture. Alongside these are the changes caused by changes in sea level, resulting in the erosion of saltmarsh, often with limited potential for the marsh to migrate landward because of sea defences - a situation known as coastal squeeze.