How are wetlands formed?

Wetlands on barrier islands at Cape Lookout National Seashore help absorb the energy of storms.

NPS photo

Although some wetlands can form relatively quickly, many others took thousands of years to develop. Here are some of the processes that form or modify wetlands:

• Flooding of coastal lowlands from rising sea levels has created broad coastal marshes that are protected from wave action by barrier islands or reefs. Coastal wetlands also form when rivers deposit sediment as they reach the ocean. Plants then take root and hold the soil firm against the forces of tides and waves.

• River floodplains develop through erosion processes and through deposition of sediment on adjacent lands during floods. Wetlands form on floodplains where periodic flooding or high water tables provide sufficient moisture. These "riparian" wetlands may undergo constant change as rivers and streams form new channels and when floods scour the floodplain or deposit new material.

• Glaciers helped to create wetlands in the northern states 9,000-12,000 years ago. Large wetlands formed when glaciers dammed rivers, scoured valleys, and reworked floodplains. Countless smaller wetlands formed when large blocks of ice left behind by receding glaciers formed pits and depressions in the land. Many of these depressions later filled with water if they had poor drainage or intersected the water table.

• Other forces of nature can create wetlands. Wind action in the sand hills of Nebraska formed depressions, many of which have become wetlands. Wetlands may also form in "sink holes" and other areas where percolating water has dissolved bedrock. Earthquakes can create wetlands by damming rivers or causing land to drop down near the water table or shoreline. Waterfalls often have lush wetland vegetation under and around them, sustained by the spray. Coastal processes such as currents and wave action can form, sustain, modify or eliminate wetlands over time.

• People create wetlands. Some "incidental" wetlands are formed when highway and dam construction, irrigation projects, or other human activities alter drainage patterns or impound water. Government agencies, conservation groups and individuals intentionally create and restore wetlands, and research to improve restoration methods continues.

Beaver lodge at Cumberland Gap National Historical Park.

• Beavers once played a more significant role in forming smaller inland wetlands by damming rivers and streams. Though trapping has greatly reduced the number of beavers in the U.S., recent wildlife protection measures have resulted in recovery of beaver populations. Beaver dams may last in excess of 100 years, though many are shorter-lived.