A salt marsh or saltmarsh, also known as a coastal salt marsh or a tidal marsh, is a coastal ecosystem found between land and open salt water or brackish water that is regularly flooded by the tides.

A salt marsh begins when mud and silt are deposited along a sheltered part of the coastline. This is because rates of deposition are greater than transportation due to the lack of energy in the waves. Often salt marsh is found on the inside of a coastal spit.

Salt marsh at Spurn Point

The deposition builds up over time meaning that the mud breaks the surface to form mudflats. Some plants then begin to grow. The first plant is typically Cordgrass. These plants are called pioneer plants. It is tolerant to seawater and its long roots help hold the mud and sediment together.

Due to the sediment and material accumulating, it gets covered by the tide less. This and rain will leach (wash out) some of the salt. As the salt is now lower in concentration, it means more plants can start to grow in the more fertile soil. These new plant species include plants like sea asters. These are known as the second generation. Over time, bigger and hardier plants will grow, until in the end, marsh uplands form, that has trees as well as plants. The development of the plant is known as vegetation succession.