Saltmarshes are dynamic features which naturally experience change in extent (area), elevation, internal morphology and floristic composition over time.

The box below sets out the information relating to historical saltmarsh change that should be obtained to assess the form and function of a marsh system, in order to undertake and understand the likely consequences of management.

Information is provided in this guide on baseline data collection and analysis.

Some of the marshes currently in existence are very old features, having been established up to several thousand years ago (e.g. in the Holocene), while other are very young features, only a few decades old. In general, marshes older than about 100 years have attained (a degree of) equilibrium relative to the local tidal frame (which is related to mean sea level), but younger marshes are still in the process of moving towards an 'equilibrium' elevation.

Changes affecting saltmarshes take place on a variety of temporal and spatial scales, partly as a result of the dynamic nature of saltmarsh processes and partly due to changes in external forcing factors, such as sea level, tidal range, wave climate, sediment supply and human activities. It is, therefore, important to identify the scale of natural fluctuation, as distinct from any net directional change, before any intervention measures are considered and implemented (i.e. management measures). If net directional change is confirmed, it is then important to establish the causes of that change.

There are five major types of change associated with saltmarsh habitat. That is, change in:

• Lateral extent (area);
• Elevation (due to accretion or erosion);
• Creek density and morphology (caused by creek lengthening, new development or truncation) and/or change in the ratio of vegetated to unvegetated area;
• Floristic composition (plant species change due to their temporal position in succession processes, grazing and changes in sea level relative to the vertical sedimentation rate; and
• Vegetation vigour.

Understanding and determining the nature of all of the above types of change is important, both with respect to maintaining the intrinsic ecological (conservation) value and the flood-defence importance of saltmarsh. The best standard of defence is provided by a wide, high saltmarsh, with low creek density, so that the saltmarsh acts as a solid berm colonised by tall and dense vegetation (Brampton, 1992; Pye and French, 1993). A reduction in marsh width, due to erosion or reclamation, will reduce its capacity to absorb (and partially reflect) wave energy. Similarly, vertical erosion, internal dissection and heavy grazing (which greatly reduce vegetation height) will reduce the capacity of the marsh to reduce wave energy and potential overtopping of sea defences behind. A reduction in plant vigour, such as the dieback of Spartina in southern England, may also have a potentially deleterious impact on the efficiency of wave and current dissipation across a saltmarsh.

Physical factors that may contribute to saltmarsh erosion include:

• Sea-level rise;
• An increase in the tidal range;
• Tidal asymmetry;
• Increased storminess; and
• Channel migration.

An increase in saltmarsh area most commonly occurs when vegetation progressively colonises adjoining areas of mudflat or sandflat, resulting in seaward progradation of the marsh or if a new natural barrier (offshore bar or barrier beach) develops. However, in some circumstances, an increase in area may also occur if the landward boundary of the marsh moves further inland. This can occur on (so-called) natural coasts when sea level rises and there is a low-lying or gently rising hinterland with no natural or artificial barrier to prevent landward extension. In the latter case, the seaward limit of the marsh may also move landwards or, if sediment supply is sufficiently large, remain static or even prograde. Of these scenarios, landwards translation of the entire marsh zone is most likely. If however, movement of the landward boundary of the marsh is prevented by steeply rising ground or an artificial defence, 'coastal squeeze’ occurs and there is likely to be a reduction in marsh area.