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Groundwater-Fed Wetlands in Arkansas

Arkansas Natural Heritage Commission - Friday, November 16, 2018
by Theo Witsell

One way wetlands are classified is by the source of their water. Wetlands that get their water from overbank flooding of rivers are different from wetlands that get their water from precipitation, and both are different from wetlands that are fed by groundwater. Biologically speaking, it is these groundwater-fed wetlands that are probably the most unique of Arkansas’s many wetland types. Certainly, they support the highest number of rare plant species.

Groundwater-fed wetlands can be broadly divided into three main types: 1) springs, 2) acid seeps, and 3) fens. Springs are point sources of groundwater emergence, though they often feed a linear spring run or spring branch, a stream fed primarily by groundwater. Springs can be small and seasonal, flowing only during the wet season, or can be perennial and never stop. Mammoth Spring in Fulton County is Arkansas’s largest spring, and the seventh largest in the world, with a flow of 9 million gallons per hour. Photo at left — Spring along Walnut Creek in Garland County. Photo by Craig Fraiser.

Without question, the most unique springs in Arkansas are the thermal springs in downtown Hot Springs which historically supported a highly unique plant community. Botanist George Engelmann visited the springs in 1835 and described the flora found there before the springs were capped and the native flora destroyed. His poetic account noted a number of southern species that thrived in the warm, moist environment surrounding the springs and he summed up the ecology of the area by noting “the breath of the South blows over this valley." Image at left — Historical illustration of Hot Springs showing steam issuing from the springs on Hot Springs Mountain.

Seeps, by contrast, are groundwater-fed wetlands in which water emerges over a broad area, seeping up through the soil, which can be sandy, gravelly, or mucky, depending on local conditions. They occur in highly specific geological settings where a porous geologic layer like sand, gravel, or sandstone lies over a non-porous layer like clay or shale. Water moves through the porous layer underground and emerges from the ground at the contact with the non-porous layer or at the top of the water table.

Seeps are classified based on their water chemistry, in particular their pH (acidity or alkalinity). The vegetation of these seepage wetlands can take several forms, including open herbaceous-dominated communities with orchids, sedges, and scattered overstory trees; and deeply shaded, forested communities dominated by ferns, sedges, and a diverse assemblage of shrubs. Groundwater can also feed and influence a variety of other wetland types, such as swamps, marshes, stream banks, and even lakes and ponds, which are often constructed by impounding seeps and springs. Photo at left — Wooded seep in the Ouachita Mountains, Saline County. Photo by Theo Witsell.

Acid seeps, sometimes called poor fens, have acidic groundwater, generally low in dissolved minerals, emerging from sand, gravel, or porous rock like sandstone or fractured chert or novaculite. Acid seeps can be found throughout the state in areas with these substrates but are most prevalent in the West Gulf Coastal Plain, in the Ouachita Mountains, and on the northern part of Crowley’s Ridge. Examples of plants found almost exclusively in seeps and groundwater-fed seepage swamps in Arkansas include cinnamon fern (Osmundastrum cinnamomeum), royal fern (Osmunda regalis), netted chain fern (Woodwardia areolata), winterberry (Ilex verticillata), possumhaw (Viburnum nudum), bamboo-vine (Smilax laurifolia), and coral greenbrier (S. walteri). Sweet-bay magnolia (Magnolia virginiana) is found in Arkansas only in acid seeps in the coastal plain, leading to forested seeps in this region sometimes being called bayheads or baygalls. Acidic seeps may have such nutrient poor water that they often support carnivorous plants such as sundews (Drosera spp.) and bladderworts (Utricularia spp.).

Fens are calcareous seeps formed where groundwater passes through limestone or dolomite, acquiring a high (alkaline) pH and often becoming rich in certain minerals. Fens occur in Arkansas primarily in the Ozark Plateaus, often along streams. They are among the few sites in Arkansas for a number of plant species typically found to the north. Examples include Riddell’s goldenrod (Solidago riddellii), Virginia mountain mint (Pycnanthemum virginianum), capillary beak-sedge (Rhynchospora capillacea), silky willow (Salix sericea), and Missouri willow (S. eriocephala). Photo at left — Streamside fen in the Ozark Plateau, Marion County. Photo by Theo Witsell.

A whopping 76 species on the ANHC’s rare plant list are found in seepage wetlands including three species of carnivorous plants: pink sundew (Drosera capillaris), horned bladderwort (Utricularia cornuta), and zigzag bladderwort (Utricularia subulata). Fifteen native orchids are known to occur in Arkansas’s seepage wetlands, many of them also of conservation concern. A few rare examples include Kentucky lady’s-slipper orchid (Cypripedium kentuckiense), rose-pogonia orchid (Pogonia ophioglossoides), crested fringed orchid (Platanthera cristata), and tuberous grass-pink orchid (Calopogon tuberosus). Photo at left — Crested fringed orchid (Platanthera cristata), a rare species found in open acid seeps in the West Gulf Coastal Plain and Ouachita Mountains. Photo by John Pelton.

 

 

 



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