11.6: Wetlands

On many shores protected from wave action, tidal wetlands develop. Tidal wetlands are flat, muddy areas covered by water during only part of the tidal cycle. Most tidal wetlands are characterized by an abundance of emergent plants: marsh grasses in salt marshes, and mangroves in mangrove swamps. Both plant types grow well in muds rich in organic matter that are inundated periodically by salt water. Grasses grow in salt marshes at all latitudes, but mangroves grow only in tropical and subtropical latitudes between about 30°N and 30°S. Where both are present, mangroves usually rapidly outgrow and eliminate marsh grasses.

Wetlands are created by the accumulation of sediments from a variety of sources. The most important are usually river-borne muds laden with organic detritus. Sediments accumulate in shallow protected waters, particularly at the edges of estuaries and on lagoon shores, until they extend above the low-tide line and a wetland forms.

A tidal wetland consists of a muddy, flat terrace between the low- and high-tide lines that is dissected by shallow channels. As the tide rises, water moves through the channels and spreads over the mudflats. The current speed within the channels is faster than the water’s speed as it spreads over the flats. As the speed decreases over the mudflats, suspended particles carried up the channels are deposited (CC4) as wetland mud. Particles of organic detritus have low density and can be fragmented to very small sizes. Consequently, wetlands tend to accumulate muds that are rich in organic matter, with much of the detritus contributed by the local marsh grasses or mangroves.

The abundant supply of detritus makes wetlands very attractive places for marine animals to feed and for the juvenile stages of many marine species to spend part of their life cycle before migrating to the sea. Wetlands provide not only abundant food, but also safety from predators. Large marine predators are not able to enter these shallow areas, and the grasses, mangrove roots, and abundant leaf litter offer many excellent places to hide from other predators. Wetlands have such an abundance of vegetation and juvenile marine animals that many birds use wetlands as their principal, or only, feeding areas.

Wetlands are extremely valuable ecologically, but generally they are not considered to have great scenic beauty. Therefore, because their proximity to the water makes them very attractive for development, vast areas of wetlands in the U.S. and elsewhere have been filled or drained. The result has been the destruction of habitat that is important for many commercially valuable fishes and shellfish species and a number of species of ducks and other birds. Filled wetlands are not ideal places to build or farm because they are low-lying and susceptible to flooding and to storm surges or storm waves. Structures built on former wetlands also are susceptible to subsidence as the former wetland sediments upon which they are built are compacted.

Even more importantly, structures built on filled wetlands are very vulnerable to earthquake damage. The sediments underlying structures in filled wetlands, if not properly compacted during the construction process, can be “liquefied” by the energy of earthquake waves, allowing the structures to collapse and sink. This is one of the reasons that the Loma Prieta earthquake of 1989 caused far more damage to buildings and freeways in Oakland and San Francisco, about 100 km north of the earthquake epicenter, than it did in urban areas closer to the epicenter. Most of the badly damaged or destroyed structures in Oakland and San Francisco were constructed in areas that were formerly wetlands.