Wetlands for the most part take thousands of years to form, some by glaciers, some by sea level changes, and some by repeated flooding of rivers depositing silt. Erosion is a big shaper of wetlands. Along the East Coast, as the mountains eroded, their bits of rock, silt, and organic matter settled at the mouths of rivers, creating a deep flat plain called the coastal plain. Coastal marshes formed here and upstream on the rivers. Tides and floods move the sediment. This produces a movement in wetlands and types from one area to another.
Soil build up is normal through geologic time, and marshes fill over hundreds of years to become swamps, swamps fill to become wet forest, and then dry forest. Change in a river flow may change the dry forest back to a river bank marsh. Along ocean beaches, a break in a sand barrier may wipe out a marsh behind it, but the soil of the marsh builds up another location turning it to marsh. Hurricanes, fire, and people can all have an impact on wetlands.
National parks in Florida present a history of how ancient wetlands have gone from being valued, destroyed, then rebuilt and protected, at least in parks.
The lessons learned have informed policy in other areas. A case study is presented later on the changes in policy and structure of a marsh at National Capital Parks, East.