Lateral extent • Elevation change • Changes in creek and marsh • Changes in floristic composition • Changes in vegetation vigour

Elevation change

Following initial colonisation of a tidal flat by vegetation, saltmarshes grow vertically until they approach a surface elevation which is in equilibrium with the local tidal frame. Typically, it may take 80-100 years for a marsh to achieve an equilibrium elevation (i.e. to become 'mature'). Comparison of the surface levels of mature marshes with local tidal parameters at different locations around the coast of Great Britain has shown that the relationship varies from region to region. This reflects geographical variations in tidal range, tidal flooding frequency and duration (Pye, 2000). Generally the mature marsh surface has an average elevation which is several centimeters to tens of centimeters below the level of the highest astronomical tide (HAT). Once established, the height differential tends to be maintained if the tidal range and sediment supply characteristics remain unaltered.

Allen (1992) outlined a simple model which relates change in marsh surface elevation to a number of controlling variables:

dE/dt = dS_min/dt + dS_org/dt + dA/dt - dM/dt -dP/dt

where:	dE/dt	is the change in elevation of the sediment surface relative to the tidal datum
	t	is time
	dSmin	is the thickness of minerogenic sediment added by the tide
	dSorg	is the thickness of organic sedimentary material
	dA	is the change in amplitude of the extreme astronomical tide - increase positive
	dM	is the change in relative sea level (upward positive)
	dP	is the change in position of the sediment surface due to consolidation.

The value of this model is that it provides a means to evaluate the relative importance of changes in the different factors. In practice, the individual parameters may be difficult to quantify. Short to medium-term sediment accretion, and even changes in surface level, can be monitored, but accurate quantitative data on longer-term changes are difficult to obtain without detailed studies of marsh stratigraphy, sediment ages and engineering properties.

Measurements of short-term surface accretion invariably over-estimate the longer-term net accretion represented by surface elevation change. This is due both to shallow auto-compaction (Allen, 2000b), sometimes misleadingly referred to as 'shallow subsidence' (Cahoon et al., 1995), deep sediment deformation (especially adjacent to major estuarine channels) and diagenetic changes within the sediments.

Field monitoring has shown that vertical accretion rates on saltmarshes can be of the order of up to 10cm per year during the early stages of growth, but are often less than 2mm/yr on mature British marshes.

By definition, lowering of the surface elevation of a saltmarsh relative to the tidal frame can only occur if (a) sediment is physically eroded from the surface or (b) the rate of accretion / net surface level increase fails to keep pace with the tidal frame over time. The former requires partial or complete de-vegetation, caused by an increase in wave activity, overgrazing, biologically-induced or pollution-induced dieback of the vegetation. Failure of the surface to keep up with moving tidal frame may result from a reduction in minerogenic or organic sedimentation rates, a change in tidal amplitude or an increase in compaction rates.

'Drowning' of saltmarshes is highly unlikely unless rates of sea level rise are extremely high (>10 cm/yr) and rates of sediment supply very low. At most sites in eastern England, vertical accretion has easily outpaced sea level rise over the last 40 years (e.g. French and Burningham, 2003).