Estuarine or Tidal Fringe Wetlands

A vegetated estuarine or tidal fringe wetland is often referred to as a marsh, whereas an unvegetated area is referred to as a mud or sand flat.  This type of wetland is found between the open saltwater of the bays or Gulf and the uplands of the coastal plain and barrier islands.  These wetlands may occur in small strips just 10 to 20 feet wide or may be several miles wide and occupy thousands of acres.  Marshes are almost always in protected areas along bay shorelines or on the bay sides of barrier islands.  Without protection, wave energy is too great for salt marsh vegetation to get established, which is why we seldom see salt marshes on Gulf-facing beaches.

These marshes are also found on the back or bay sides of barrier islands, and also may extend inland a few miles along some of the major and minor streams that drain into the Gulf.

Most Texas estuarine wetlands are in river valleys that flooded when sea level rose between 18,000 and 4,000 years ago.  When sea level was lower, the coastal rivers cut deep valleys into the coastal plain sediments.  Most of our salt marshes have formed around the bays that resulted from the flooding and filling of these ancient river valleys. 

Estuarine wetland soils can range from clayey to sandy, but clayey soils dominate most marshes.  These young soils are  alluvium (carried by moving water) deposited mostly during the Holocene during the last 10,000 years.

Salt marsh soils have the most organic matter of any Texas wetland soils.  They are still considered mineral soils because the organic matter is never more than 20% in the surface horizon, and usually much less.  In contrast, the Mississippi Delta in Louisiana has highly organic soils.

(Tracosa Soil Series, near Bastrop and Christmas Bays, Brazoria County)

The surface soil to a depth of 4 inches is a dark gray mucky (organic rich) clay that flows easily between the fingers, leaving a slight residue.  It is strongly saline with a  pH of 7.5.  The subsoil to a depth of 62 inches is a very sticky and plastic (moldable) gray clay.  It is strongly saline with a pH of about 8.0.  This soil is inundated with 2-12 inches of water during the daily high tides.  One or two inches of water usually remain on the surface during low tides.

Source:  Soil Survey of Brazoria County

Salt marshes are flooded by tides and their salinity and plant communities depend upon how much freshwater is delivered to the wetlands by the rivers that flow into the bays.  The high marsh is only irregularly flooded by tides, and may go for extended periods without flooding.  The low marsh, on the other hand, is subject to regular flooding, at least once a day.  Areas that are continuously flooded dare outside the salt marsh zone, but may still have some submerged vegetation near the shore.

Gulf Coast tides do not vary much in elevation, usually only about 1 foot in vertical difference between low and high tide.  We generally see larger tidal ranges due to the wind, particularly if the wind is in the same direction as the tide.  Some of our lowest tides occur at low tide in the winter with a  strong northwest wind.  There are many tidal flats that are exposed only at this time of year.  The highest tides often occur at high tide with a southeasterly storm.

Cordgrasses of the Spartina genus are the most prominent salt marsh vegetation.  Flooding frequency, duration and the salinity level are the most important variables that control the kinds of plants that occur in the salt marsh.  In the high marsh, salt meadow cordgrass might be the most common grass, whereas in the lower marsh, saltmarsh cordgrass is more common.  Additional vegetation includes saltgrass, saltmarsh bulrush, and needlegrass rush among others.

Typical animals include herons, egrets, ibises and other wading birds that feed on the fish, shrimp, crabs and other invertebrates found in the wetlands.  Shorebirds and waterfowl are abundant.  The American alligator feeds on fish, snakes, turtles, frogs, muskrats, nutria, swamp rabbits, rats and anything else it can catch.  Bobcats, coyotes, raccoons, skunks, mink and river otters also hunt in the marshes.

Estuarine wetlands are threatened by land subsidence and relative sea level rise that causes marshes to drown and be converted to open water.  Subsidence and dredged channels and canals allow saltwater to intrude  inland into fresher areas causing vegetation changes.  Direct loss of marshes occur as a result of dredging, filling, and spoil deposition caused by various kinds of development.  Dredging activities cause increased water turbidity that threatens seagrasses.  Dams on the rivers that supply freshwater and sediment to the bays have altered the fresh water inflow processes that are important in maintaining the estuaries.  Runoff pollution from urban and agricultural sources, and oil and chemical spills damage vegetation and animals.  Since the mid-1950s, the area of salt and brackish mashes on the Texas coast has decreased by more than eight percent; a net loss of more than 31,000 acres.

Along the entire coast, unvegetated tidal mud/sand flats have decreased in area by about 13 % since the micd-1950s; a net loss of more than 30,000 acres.  Much of the loss was due to the construction of dredge-spoil compartments along the Intracoastal Waterway and other ship channels.

A more permanent threat to estuarine wetlands is development in the intertidal zone for resorts, marinas, beach houses and boat docks, roads and causeways, canals and ship channels.

Valuable economic resources such as finfish and shellfish use estuarine wetlands during at least part of their life cycles.  The wetland habitats are used for spawning, nursery areas, and food.  Shrimp, crabs and oysters all depend upon estuarine habitats.  Predatory fish like red and black drum, sea trout, and southern flounder are supported by estuarine invertebrates and forage fishes like bay anchovy, striped mullet, bay silversides, killifishes and gobies. 

Seagrass beds provide feeding habitat for game fish, migratory waterfowl and sea turtles.  they serve as nursery areas for fish, crabs and shrimp.  These fish and wildlife habitats help support economically important commercial and recreational fishing, hunting and bird watching industries.  Estuarine wetlands perform other functions such a protecting shorelines from erosion and dampening storm surges.

 Source:  Texas Coastal Wetlands Guidebook, by Daniel W. Moulton and John S. Jacob, Texas Sea Grant Publication TAMU-SG-00-605, p.8-12.