Saltmarshes form part of estuary, coastal bay and often barrier beach systems. In order to develop and survive, they depend on processes and inputs from these larger systems. Saltmarshes are generally composed of mud or fine sand that settles out of suspension.

For this to occur, the suspending water must move relatively slowly, with only low levels of turbulence. Once settled on the bed, the sediment can only accumulate and saltmarsh develop if the particles are not re-suspended by wave or current action. The necessary low energy conditions for saltmarsh development are normally encountered within the shelter of coastal bays, such as Poole Harbour or The Wash, behind barrier islands such as Scolt Head Island, or in estuaries such as the Solway, Dee or Blackwater.

In each case, the location of the saltmarsh is primarily determined by the shelter afforded by the large scale coastal morphology. However, within each of these environments, a wide range of energy levels can be found and thus the distribution and morphology of saltmarshes (see figure below) is determined not only by these coastal forms, but also by more local factors such as the tidal dynamics, sediment transport pathways, locally generated waves and the presence or absence of vegetation.

A geomorphological classification of saltmarshes (from Allen, 2000)