Prairie Pothole and Marsh Wetlands

Addicks and Barker Reservoirs

The term "pothole" is used up and down the coast to refer rather loosely to any freshwater depression.  The main difference between  a pothole and a marsh is mostly size.  Marshes occur in larger and generally less well-defined  depressions than potholes. 

Prairie potholes and marshes occur on the prairie from just west of Beaumont to the Rio Grande.  These wetlands once covered vast expanses of prairie before urbanization and agriculture destroyed most of them.  Approximately 30% of the prairies was once wetlands. 

The most extensive prairie potholes and marshes are found on the Lissie and Beaumont Geological Formations.  These potholes are remnants of the rivers that laid down the great floodplain and delta sediment deposits that make up most of the coastal plain.  The original morphology has been greatly modified by wind and other erosive agents.  The riverine characteristics of these scars is usually quite obvious, while in other places it is more obscure.  The wetlands with the most obvious riverine features are found on the Beaumont Formation and range in age from 15,000 to more than 30,000 years old, while the potholes on the Lissie Formation are for the most part more than 100,000 years old.  The Katy Prairie west of Houston is one of the more well known prairies on the Lissie Formation with abundant pothole wetlands.

(Cieno Soil Series, Prairie Pothole, Jackson County)

The surface soil to a depth of about 25 inches is a dark grayish brown sandy clay loam with brownish iron coatings along old root channels and cleavage faces.  The subsoil from 25 to 60 inches is a light brownish gray sandy clay loam with brown iron stains.  The soil is neutral in the upper part and moderately alkaline in the lower part.  This soil is ponded with water for periods ranging from a few weeks to several months in the winter and early spring.

Source: Jackson County Soil Survey,  Texas Coastal Wetlands Guidebook, by Daniel W. Moulton and John S. Jacob, Texas Sea Grant Publication TAMU-SG-00-605, p.15.

Prairie potholes and marshes are inundated by direct precipitation and by runoff from surrounding flats.  Groundwater may be a factor in many potholes.  Hydrology in pothole combinations is very diverse, with major changes occurring within just a few feet.  Deeper potholes can remain saturated for more than 6 months out of the year, wile nearby pimple mounds may be nearly semi-arid for most of the year.  Almost every intermediate hydrologic state will be found associated with potholes and marshes of varying depths and surrounding elevations. 

Hydrology varies greatly as you move down the coast.  Many potholes on the upper coast remain saturated for several months out of the year, with six months of wetness not uncommon in the deeper potholes.  Most Coastal Bend and lower coast potholes may remain saturated for only a few weeks to a month or so at a time.

Potholes and marshes typically have concentric zones of different habitat types.  These habitats are determined by elevation and hydrology.  The more permanent potholes may have floating and submerged plants like water lilies, pondweeds, southern naiad, and duckweed in t the open water zone.  The emergent zone might include cattails, bulrushes, burheads, arrowheads, and common reed.  A still higher woody zone may include trees and shrubs like black willow, buttonbush, rattlebush and coffee bean, baccharis, Chinese tallow-tree (an introduced invader) on the upper and mid-coast, and retama on the mid and lower coast.

The edges of less permanently flooded potholes and marshes might have bushy bluestem and various other grasses, spikerushes, rushes, and sedges as well as the shrubs and trees mentioned above.

Pothole/marsh wetlands are host to a very diverse fauna because of the great variety of habitats.  Reptiles and amphibians include alligators, Gulf Coast ribbon snakes, cottonmouth moccasins, red-eared sliders, southern leopard frogs, bullfrogs, and green tree frogs.  Birds include rails, cranes, all wading birds, dabbling ducks, cotts, common moorhens, snipe, blackbirds and grackles, shorebirds like killdeer, marsh and sedge wrens, swamp sparrows, and most all migrating songbirds. 

Agriculture was once the greatest cause of the loss of prairie potholes and marshes.  Urbanization is probably the greatest cause of loss today.  It is interesting to note that Federal wetland regulatory protection has not prevented the loss of these wetlands.  On the Texas coastal plain, freshwater marshes have decreased by 29% since the mid-1950s, a net loss of more than 235,000 acres!

Pothole/marsh wetlands are important to birds migrating across the western Gulf of Mexico.  These habitats are the first source of freshwater encountered by migrants and are heavily used by songbirds, shorebirds and waterfowl and other water birds.

These habitats are also important to resident wildlife, particularly in the semiarid coastal regions and in times of drought. 

Many of the prairie potholes and marshes have been modified by farming, mainly for rice from Victoria to Beaumont.  Even though rice farming has greatly modified the wetlands through land-leveling, many of the potholes remain.  Rice is farmed only 1 year out of 3, with the other two years fallow.  Because of the flooding that accompanies rice cultivation and the fallow period, these farmed potholes retain much of their wetland character. 

Rice fields can be considered a type of wetland because the fields remain flooded for significant periods.  Rice fields are by far, at about 1.5 million total acres, the most abundant type of wetlands on the Texas coast.  Rice fields provide many benefits to people and wildlife.  Wintering waterfowl populations, particularly geese, are dependent on rice farms.  Rice fields also provide habitats for large numbers of invertebrates such as insects and crayfish.  They attract herons, egrets and other wading birds, sandpipers, plovers and other shorebirds, gulls and terns, mammals, reptiles, and amphibians.

Urban sprawl proves to threaten rice fields too.  However, the most serious threat to rice farming now seems to be the changing markets that rice farmers have come to rely on.  Many of the acreage used to support rice farming will probably be converted to other crops that will not provide wetlands or support waterfowl.  Needless to say, this does not bode well for a large population of North American ducks and geese.

Source:  Texas Coastal Wetlands Guidebook, by Daniel W. Moulton and John S. Jacob, Texas Sea Grant Publication TAMU-SG-00-605, p.14-17.