How do freshwater marshes form in low areas of the landscape?

Freshwater marshes form in low-lying areas of the landscape through a variety of geological and hydrological processes where water collects or is prevented from draining away. These systems typically develop in environments where relatively impermeable soil or geologic layers cause water to pond.1

Specific mechanisms for their formation include:

• Glacial Activity: In regions like the northern Great Plains, retreating glaciers left behind irregular, poorly drained landscapes. These depressions fill with water from snowmelt or rain, creating temporary or permanent marshes.2
• Basin Depressions: Some marshes form in basinlike depressions that trap water. If a river draining such a basin is impeded—for example, by resistant rock or dams—water backs up to create marshy conditions.2
• River Dynamics: Marshes frequently develop at river mouths where deltas form. As river flow slows, sediment settles and creates poorly drained, waterlogged land.2
• Surface Water Accumulation: In areas near sea level, such as the Florida Everglades, abundant rainfall may remain on the surface rather than draining, supporting extensive marsh habitats.2
• Succession: Shallow lakes or ponds can gradually fill with sediment over time, transitioning into marshy environments as they become shallower.23