A tidal marsh (also known as a type of "tidal wetland") is a marsh found along rivers, coasts and estuaries which floods and drains by the tidal movement of the adjacent estuary, sea or ocean. Tidal marshes experience many overlapping persistent cycles, including diurnal and semi-diurnal tides, day-night temperature fluctuations, spring-neap tides, seasonal vegetation growth and decay, upland runoff, decadal climate variations, and centennial to millennial trends in sea level and climate.
Tidal marshes are formed in areas that are sheltered from waves (such as beside edges of bays), in upper slops of intertidal, and where water is fresh or saline. They are also impacted by transient disturbances such as hurricanes, floods, storms, and upland fires.
The state of tidal marshes can be dependent on both natural and anthropogenic processes. In recent periods, human practices, small and large scale, have caused changes in ecosystems that have had a significant impact on the preservation of tidal marsh ecosystems. Some smaller scale changes include headward (i.e. upstream) erosion and coastal development. Large system changes include pollution and sea level rise (from climate change). These changes are all putting pressure on tidal marshes.
Tidal marshes can be found in two main places: coasts and estuaries. Coastal tidal marshes lie along coasts and estuarine tidal marshes lie inland within the tidal zone. Coastal tidal marshes are found within coastal watersheds and encompass a variety of types including fresh and salt marshes, bottomland hardwood swamps, mangrove swamps, and palustrine wetlands. Estuarine tidal marshes are found in estuaries, areas where freshwater streams flow into brackish areas.
They can be categorized based on salinity level, elevation, and sea level. Tidal marshes are commonly zoned into lower marshes (also called intertidal marshes) and upper/ high marshes, based on their elevation above sea level. A middle marsh zone also exists for freshwater tidal marshes.