Hydrogeology of Wetlands and Natural Communities
  

by:
Anthony H. Fleming

Wetlands are commonly defined as the interface between open water and the terrestrial environment. Hundreds of varieties of wetlands have been identified based on their position in the landscape, the duration of time their surfaces are wet (also known as the “hydroperiod”), their relation to surface-water bodies and groundwater processes ( hydrology ), water chemistry (for example, acidic or alkaline), the kind of soil beneath them, and the types of natural communities (namely, plants, insects, and animals) that inhabit them. All of these distinguishing features vary widely from region to region, and even among wetlands within the same region; the myriad combinations of wetland characteristics lead to a large amount of biodiversity, which is one reason wetlands are considered to be of great importance to the natural and human environment. Despite their great variety, however, all wetlands have one underlying feature in common: they are indelibly linked to and supported by water, either surface water, groundwater, direct precipitation, or some combination. This page gives a very brief overview of a few types of wetlands historically found in Marion County and their hydrogeologic settings .

A Plethora of Names

Considering the many factors that determine the classification of wetlands, it is not surprising that a confusing, and sometimes inconsistently applied, array of names has been used to describe the different types. No universally agreed-upon lexicon currently exists among the different scientific disciplines involved in characterizing wetlands (which probably explains why there is still fundamental debate in some quarters over whether a particular place really is a "wetland"), and as a result, many of the common terms applied to wetlands (for example, "fen," "wet meadow," "seepage swamp") mean different things to different people working in the field. The wetland names used in the following discussion are based primarily on observed hydrology, specifically whether the wetland is supported by groundwater, surface water, or precipitation, along with the nature of the wetland's hydroperiod, and secondarily on the structure of the wetland vegetation and water chemistry.

Figure 1.
Seasonal hydrology of wet woods: Water stands in a shallow swale in late winter. Snowmelt and runoff accumulate in low places in the landscape during late winter and early spring, and gradually dissipate as the growing season wears on. Photo by A. H. Fleming.

Undoubtedly the most common type of wetland in pre-settlement Marion County was a seasonally wet woods (fig. 1). Such wetlands are synonymous with the ecological term “till plain forest,” and consist of wet flats, depressions, and swales interspersed with slightly better drained hummocks, knobs, and low hills, in other words, the landscape associated with the poorly drained till plain regions of the county. The wet landscape of the till plain owes its existence to the low permeability of the underlying glacial till, which retards infiltration of precipitation and creates a seasonally perched water table . The hydrology of a wet woods is fed entirely by precipitation, thus the hydroperiod is relatively short in most cases, typically lasting from late winter (when the soil is still frozen and impervious) into late spring or early summer, when the higher air temperatures and rates of evapotranspiration associated with the growing season remove water from the soil at rates greater than precipitation. The original vegetation of the till plain consisted of a rich, mesic hardwood forest dominated by beech, sugar maple, tulip tree, ash, and some oaks and hickories, with an enormous variety of herbaceous plants, many of which are so-called “calciphiles” because they thrive in the calcareous soil formed on glacial till. The wettest parts of this landscape consisted of ephemeral depressional swamps occupied by hydric species such as pin oak, silver maple, black ash, and American elm. Good examples of wet woods can be seen in several Indianapolis Parks, such as Eagle Creek and Southeastway Parks, as well as several local nature preserves.

Figure 2.
These photographs exemplify the extreme hydrology of a riparian forested wetland along the White River in north-central Marion County. The late winter flood shown on the left contrasts with more normal mid-summer water levels shown on the right. The low water level also exposes a variety of floodplain features, including small meander scrolls, bars, and natural levees. Photos by A. H. Fleming and V. R. Ferguson.

Another widespread kind of wetland in Marion County is a riparian forest, which occupies the modern floodplains of the larger streams and rivers (fig. 2). Floodplains are characterized by complex microtopography-low-relief landforms such as natural levees, crevasse splays, bars, meander scrolls, and oxbow ponds shaped by moving water-creating a mosaic of different habitats within a compact area. The hydrology of riparian wetlands is almost entirely supported by surface water; although some groundwater discharge may occur locally along the edges of the floodplain, the vast majority of water found in these wetlands is a result of inundation, ponding, and soil saturation produced by overbank flooding. Incidentally, this ability to provide extended storage of floodwaters is one of the prime economic benefits of riparian wetlands, because it attenuates the flood peak and water velocity, thereby reducing the severity of flooding downstream and the associated damage to infrastructure. The water table also tends to be extremely shallow (corresponding to the elevation of the stream surface), resulting in prolonged waterlogging of soils and an anaerobic subsurface environment.

The floodplain environment is extremely rich in terms of fertility and soil quality, because annual floods regularly deposit a fresh layer of alluvium ( sand , silt, and clay) on top of the floodplain, thereby enriching the soil with fresh mineral s and creating a loose, loamy medium conducive to root growth. But it could also be described as “rich, but hard to live with, ” because the floodplain environment is one of frequent natural geologic disturbance resulting from scouring by moving water, regular periods of inundation, damage from ice jams, burial of the soil surface by sediment , and generally constant rearrangement of the landscape by moving water. As a result, the riparian forest is dominated by trees and shrubs, such as sycamore, green ash, American elm, and boxelder, which tolerate a large amount of disturbance and are capable of rooting from their trunks or resprouting when they are toppled by disturbance. Herbaceous plants likewise are adapted to rapid reproduction-they typically emerge, grow, flower, and produce seed in time spans considerably shorter than most of their terrestrial counterparts. A number of specialized wetlands occur within the riparian forest. Oxbow ponds and perennially inundated swales, for example, host unique aquatic ecosystems, while frequently scoured gravel bars inhibit the growth of woody plants and are frequently occupied by prairielike enclaves.

Figure 3.
Groundwater-fed wetlands at Holliday Park: Left-A long seepage face is visible to the left of the trail in this winter scene. Watercress growing around a spring orifice is the chief green plant visible at this time of year, but during the growing season, the entire surface of the seepage face is covered by specialized wetland plants adapted to waterlogged conditions and alkaline soil. Right-Groundwater laden with dissolved calcium and magnesium is depositing limestone around springs and seepage faces, including this ledge of cemented glacial gravel. Photos by A. H. Fleming.

Groundwater-fed wetlands are among the most complex types, and are well known for their biodiversity and for hosting many rare and specialized species (fig. 3). They typically occur near the bases of bluffs and long regional slopes, and in the bottoms of ravines-places where aquifer s are commonly truncated by erosion and groundwater discharges to the surface. Such wetlands are readily identified by prominent springs and seepage faces, where groundwater visibly issues from the soil. Springs are characterized by discrete orifices from which groundwater emerges, and in Marion County, they are frequently associated with outcrops of glacial gravel. Seepage faces, on the other hand, are more diffuse zones, often measured in tens, hundreds, or thousands of square feet, where the soil surface is constantly wet from the slow seepage of groundwater to the surface. Springs and seepage faces commonly occur together.

A few words need to be said about the two names most commonly applied to groundwater-fed wetlands in the Midwest-“fen” and “seepage swamp.” Both terms have been used inconsistently and sometimes interchangeably, resulting in considerable confusion over their meanings. By applying a few basic hydrogeologic and ecological concepts, however, it is possible to arrive at a simple and more hydrologically consistent definition of each: a “fen” is a wetland fed by calcium- or magnesium-enriched groundwater having an alkaline pH, and which is mostly devoid of a tree canopy; a “seepage swamp” is a forested seepage face with any type of soil or water chemistry. There are obviously ecological variations within each category.

A superb example of a large groundwater-fed wetland complex can be seen along the base of the bluff at Holliday Park, which is located along the west side of the gorgelike Rocky Ripple segment of the White River. The wetlands are concentrated along the outcrop of an Illinoian-age sand and gravel body, which has been truncated by erosion along the toe of the bluff. The sand and gravel body underlies a several-square-mile area beneath the upland to the west, where it receives recharge via percolation of water through the thick sequence of glacial till that overlies it. The relatively large extent of this aquifer results in a correspondingly large amount of groundwater discharge along the bluff. The line of seepage faces in Holliday Park extends for hundreds of feet along the base of the bluff, and probably produces at least several hundred gal per min of groundwater.

The sand and gravel aquifer that crops out at Holliday Park contains a large number of limestone and dolostone clasts (sand grains, pebbles) derived from the bedrock the glacier moved over on its way to Marion County. Over time, the limestone and dolostone particles progressively dissolve in the groundwater: the longer the groundwater resides in the aquifer, the greater the concentration of these minerals. As a result, the groundwater that emerges at Holliday Park is enriched in calcium and magnesium carbonate . However, calcium and magnesium carbonate are only soluble under the reducing (low-oxygen) conditions found deep in the saturated zone, and they quickly become insoluble when the groundwater encounters oxygen as it discharges to the surface. This process is actively depositing carbonate minerals along the surfaces of the springs and seepage faces, which is vividly illustrated by the ledges of cemented glacial gravel visible on the hillsides around the wetlands. The wetlands themselves are mostly forested seepage swamps, but also include several more open, fenlike areas populated by various species of sedges and herbaceous plants that indicate a generally alkaline environment. One of these is watercress, which typically grows right in the spring orifices. Watercress is not native to North America, but because of its edible, nutritious qualities, this green was widely planted around springs by settlers to provide an easily grown source of food, and has since become naturalized. Watercress grows only in circum-neutral to alkaline waters and derives nutrients directly from the water flowing past it.

Figure 4.

Figure 4: Left-The dimensions of this seepage face are defined almost exactly by the colony of skunk cabbage growing on its surface. Photo by A. H. Fleming. Right-The flowering organ of skunk cabbage consists of the fleshy hood, known as the spathe, which encloses the knoblike spadix, which hosts the sexual organs. Photo by G. P. Fleming. One of the more geologically interesting plant species found in the wetlands at Holliday Park is skunk cabbage, a member of a comparatively old family of plants known as Arums, which also include the familiar Jack-in-the-pulpit found in rich, moist woods throughout the county (fig. 4). Skunk cabbage is a hydrophile — an obligate wetland plant that grows only in places where the temperature near the soil surface is regulated by the constant discharge of groundwater, whose temperature is a constant 55o F year round. The hooded, red- and green-striped spathe is the first part of the plant to emerge, and encloses the knoblike flowering organ, called the spadix. The thick, fleshy spathe is uniquely adapted to trapping the latent heat released by the groundwater, which is why this plant is often seen flowering during the chilly days of late winter when everything else is still dormant. The ability to influence its own microclimate is further enhanced by the extraordinary fast metabolism of this plant, which produces sufficient heat from cellular respiration to melt snow and ice around the base of the plant. One might reasonably wonder how skunk cabbage manages to be pollinated at such an unlikely time of year. That's where the name comes in-Symplocarpus foetidus, the botanical name, refers to the fetid odor the plant gives off, a crucial part of the adaptation. The unwelcome stench is designed to recruit pollinators from some unusual candidates-carrion flies-attracted from far and wide by the dead meat undertones of the aroma (nature's janitors, it turns out, have many uses). Skunk cabbage emblemizes a unique evolutionary adaptation to a specific geologic environment, which gives the plant a distinct reproductive advantage. The affinity between this plant and groundwater discharge areas is so strong that it can reliably be used by geologists to map zones of shallow groundwater, even during dry periods when seepage is temporarily paused and the groundwater itself may not be visible.



 
 
 
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