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II. Climate, Basic Hydrologic Concepts, and Wetland Classification
WETLAND SOILS OF PRAIRIE POTHOLES
varies from semiarid in the west to humid in the east. As an example of the
variation in average yearly precipitation in the PPR, Richardson et al. (1991)
had three sites representative of semiarid, subhumid, and humid regions. Mean
yearly precipitation (20-year norms) was 34 cm in semiarid regions, 50 cm in
subhumid regions, and 85 cm in humid regions. Yearly variations are also extreme. Droughts and pluvial cycles are the norm. The westerly winds that typically prevail in central North America provide little precipitation to the PPR,
because these air masses, which originate in the Pacific Ocean, lose most of their
moisture on the west side of the Rocky Mountains. Most of the precipitation in
the PPR occurs in the spring and summer, the result of weather systems occasionally bringing in moist air from the Gulf of Mexico. The frequency of weather
patterns that bring moist Gulf air decreases as one moves west in the PPR, explaining the west to east gradient in precipitation (personal communication, Dr.
John Enz, State Climatologist for North Dakota).
The interactions between precipitation, temperature, and evapotranspiration
(ET) are important factors in the water budget of wetlands, and can influence
wetland frequency on the landscape. Given the same landscape and landforms,
high precipitation coupled with low ET favors the development of wetlands because water inputs are maximized and ET losses minimized. Conversely, low
precipitation coupled with high ET inhibits the development of wetlands because
ET losses are maximized (Zoltai, 1988). In the PPR, potential yearly evapotranspiration (PET) generally exceeds mean yearly precipitation, with the ratio between PET and average precipitation being highest in the southern and western
portions of the region, and decreasing northward and eastward. The impacts of
high PET coupled with low precipitation on the water budget of prairie wetlands
are great. Shjeflo (1968) noted that on average usually more than 35% of the
water lost from wetlands in North Dakota is evapotranspired, but that the ET
loss as a percentage of the total water loss was greatly influenced by the dominance of seepage inflow over seepage outflow of groundwater. ET losses are a
smaller percentage of the total water loss in wetlands dominated by seepage
outflow, whereas ET losses can go up to 100% of total water losses in wetlands
dominated by groundwater seepage inflow. Millar (197 1) observed a positive
correlation between shoreline :wetland area ratios and wetland evapotranspiration, and noted that smaller and shallower ponds with a large shore1ine:pond
area ratio tended to have higher evapotranspiration rates and were only seasonally persistent. In summary, the high PET: precipitation ratio tends to mitigate
against high wetland density and permanence in the PPR. Permanent lakes are
few compared to the more humid glaciated regions north and east. Many wetlands of the PPR are only seasonally to semipermanently ponded, and wetland
density decreases from east to west.
A climatic factor that does favor the formation of wetlands in the PPR is the
timing and distribution of surface runoff. Prairie wetlands receive a significant
J. L. RICHARDSON ET AL.
portion of their water volume as surface runoff during spring snowmelt (Hubbard
and Linder, 1986), when frozen ground minimizes infiltration, and low temperatures and dormant plant communities minimize ET losses (Shjeflo, 1968; Lissey,
1971; Sloan, 1972). Shjeflo (1968) determined that snow accounts for at most
25% of total yearly precipitation, yet it accounts for at least 50% of the water
that reaches the wetland. Generally, a wet winter means a wet year for PPR
wetlands. Whereas intense summer thunderstorms may result in runoff, infiltration in the upland areas of the catchment after thaw, coupled with high evapotranspiration rates, minimizes runoff during the growing season.
Typical PPR wetlands lack integrated drainage networks. However, the presence of overflow or high-water outlets in some PPR wetlands indicates that partially closed systems are more common during unusually wet climatic cycles or
more pluvial periods, such as that which existed immediately after deglaciation.
Under present climatic conditions, the lack of effective water removal by channels accentuates the importance of groundwater as a component of the PPR wetland water balance (Winter, 1988, 1992; Richardson et al., 1992). Surface runoff in closed systems or partly closed systems is depression focused-i.e., flow
converges on the depression occupying the lowest portion of the closed catchment. Convergent flow thus brings water (and the material it transports) to a
point on the landscape from the surrounding catchment. Once the water is collected in depressions, some equilibrium level in the wetland water balance is
reached where sediment saturation persists long enough to develop anaerobic
conditions in the soil zone favorable for the growth of hydrophytes (Cowardin
et al., 1979). This equilibrium level strikes a tenuous balance between the input
components of precipitation, overland flow, and seepage inflow, and the output
components of evapotranspiration and seepage outflow. Because precipitation
and temperature are so variable in the PPR, the exact elevation where saturation
persists long enough for hydrophytes to grow is extremely variable, and can
change several feet in horizontal distance from year to year. Thus, the presence
of hydrophytes represents a short-term indicator of wetness at best (Stewart and
Kantrud, I97 1). Several soil characteristics better indicate long-term saturation
status (the hydrologic regime) because they integrate the effects of anaerobic
conditions and water movement over time. Important morphological characteristics of the soil that are often-used indicators of wetness include ( 1 ) soil mottling, which reflects the distribution pattern of redox-sensitive soil constituents,
(2) high levels of organic matter, which build up due to slow rates of decomposition in anaerobic environments, and (3) the distribution of evaporites in the
WETLAND SOILS OF PRAIRIE POTHOLES
soil, which can indicate the intensity and direction of saturated and unsaturated
water flow. The spatial relationship of soil horizons within and between pedons
is also an often-used indicator of water movement in soils. We feel the hydrology
of PPR wetlands is reflected most clearly in the associated hydric soils. Because
groundwater fluxes in PPR wetlands are so important in explaining the development and morphology of hydric soils in the PPR, we will examine the characteristic groundwater hydrology in some detail.
1. Darcy’s Law
Groundwater movement can be analyzed and simulated by Darcy’s law and its
extensions (Freeze and Witherspoon, 1967). Darcy’s law (4 = Kdh/dl) indicates
that groundwater flow velocity ( 4 ) is the result of the presence of a hydraulic
gradient ( d h / d l ) in sediments with a characteristic hydraulic conductivity ( K ) ,
Darcy’s law applied to saturated conditions predicts that flow will increase if the
hydraulic gradient or hydraulic conductivity increases. We will first consider the
influence of topography and climate on the spatial distribution of hydraulic gradients in the PPR.
2. Distribution of Hydraulic Gradients:
Recharge and Discharge in the PPR
Seeps or hillslope wetlands often occur below steep slopes because the water
table gradient is also steep, favoring groundwater discharge (Fig. 2) (Winter,
1988). Many hydrology texts and mathematical simulations of groundwater
movement assume that groundwater recharge generally occurs in upland areas
adjacent to depressions and streams (e.g., Toth, 1963; Freeze and Witherspoon,
1967; Winter, 1983). Under these conditions it is universally assumed that the
water table will become a subdued replica of the surface topography: groundwater mounds will form under topographic highs and groundwater depressions
will be associated with topographic lows (wetlands, lakes, or streams). Using
this model all wetlands will be foci of groundwater discharge (Lissey, 1971).
The assumption of upland-focused recharge does explain many of the hydrologic
characteristics of wetlands in the glaciated humid regions near the PPR. Because
of an excess of precipitation relative to evapotranspiration, upland recharge often
occurs. Most streams in the humid areas are effluent (receive groundwater), and
it has been suggested that wetlands in humid regions typically receive water from
the groundwater flow system as well (Richardson et al., 1991, 1992). Many
wetlands in this situation fill and overflow, and form the “deranged” surface
drainages characteristic of glaciated humid areas north and east of the PPR.
However, low precipitation and high evapotranspiration limit the development of
surface drainages in much of the PPR. Instead, PPR wetlands typically form
J. L. RICHARDSON ET AL.
Figure 2 Recharge, flowthrough, and discharge wetlands with mineralogical controls and soil
types fresh to saline wetlands in Nelson County, North Dakota. After Arndt and Richardson (1988,
1989b) and Richardson et al. (1992).
relatively large complex wetland systems connected to each other by groundwater flow. Whereas upland groundwater recharge may be characteristic of humid areas where precipitation exceeds potential evapotranspiration, several researchers have indicated that groundwater recharge is much more complex in
subhumid to semiarid areas.
Wetlands in the PPR have been described as surficial expressions of the water
table (Sloan, 1972), the free water surface at atmospheric pressure that can be
located at, below, or above the hydric soil surface. In the absence of integrated
surface drainages, it is apparent that groundwater recharge, groundwater movement, and groundwater discharge are intimately associated with PPR wetlands.
Lissey ( I 97 1) noted that groundwater recharge and discharge are focused on
depressions in hummocky areas of the PPR. He also observed that interdepressional uplands are relatively inactive regarding water transfers to and from the
water table. This depression-focused nature of groundwater and surface water
movement is a direct result of hummocky topography and a subhumid to semiarid climate. In the PPR, groundwater recharge occurs first where the vadose
zone is thinnest (Winter, 1983). Because the vadose zone in uplands is thick in
WETLAND SOILS OF PRAIRIE POTHOLES
the PPR and precipitation is meager, much of the summer infiltration occurring
on upland slopes never reaches the water table. Two additional factors act to
reduce recharge in the uplands for PPR wetlands: (1) hydraulic conductivity of
surface sediments is often ansiotropic, favoring lateral water movement over
vertical movement, and (2) spring snowmelt and runoff occur when the soil is
impermeable due to the presence of frost layers.
Groundwater movement in the PPR has necessitated the development of several hydrologic concepts and terms suited to the region. The groundwater system
interaction with the surface water in PPR wetlands is expressed as recharge, discharge, orjowthrough, depending on the dominant process at each site (Fig. 2).
Recharge wetlands recharge groundwater within the wetland basin. When rapid
overland flow is discharged to a recharge-type depression, infiltration into and
percolation through the pond bed eventually recharges the groundwater, producing a water table mound that slowly dissipates through lateral and downward
groundwater movement (Miller et al., 1985; Knuteson et al., 1989). In the PPR,
wetlands that are usually dry by midsummer receive the majority of their water as
spring snowmelt and recharge shallow groundwater aquifers (Richardson et al.,
1991). Discharge wetlands receive groundwater that is discharged into the wetland basin. Evaporative discharge is the upward capillary flow of water from a
near-surface water table in response to hydraulic gradients set up by higher evapotranspiration rates at the soil surface (Fig. 3). Flowthrough wetlands both recharge the groundwater system and receive groundwater as discharge. On a landscape scale, water can be thought of as moving laterally through flowthrough
wetlands (Fig. 2).
The groundwater recharge and discharge characteristics of PPR wetlands are
strongly influenced by climate, and reflect a climatic gradient from the humid east
to the semiarid west (Richardson et al., 1991). As examined above, semipermanently ponded wetlands in humid areas are typically discharge type (Fig. 4A).
However, in subhumid areas, semipermanent wetlands typically receive and yield
water to the water table (Fig. 4B). In semiarid regions, recharge to groundwater
is common in nearly any wetland basin (Fig. 4C) (Miller et al., 1985; Mills and
Zwarich, 1986; Richardson et al., 1991).
Flow reversals may occur with changes in hydraulic gradient. Recharge wetlands may briefly become discharge wetlands if the water table in adjacent uplands rises above usual levels, and discharge wetlands can become recharge wetlands if the water table is drawn down and a subsequent precipitation event floods
the basin. Flow reversals affecting entire wetlands, or parts of wetlands, are
frequent occurrences in the PPR. When considered on a small scale, such as that
involving the wetland edge, several flow reversals may occur in one season. In
Fig. 4A, the presence of the wetland on the downslope side of the water table
indicates a discharge condition. After a rain, edge-focused recharge as discussed
by Winter (1983) and Arndt and Richardson (1993) results in groundwater
J. L. RICHARDSON E T A .
Figure 3 (A) Wetland recharge and edge soils (after Knuteson et al., 1989). The solid arrows
are saturated flow and the open arrows represent unsaturated flow from the water table to the drier
soil surface. The Bk-horizon here is from a short flow distance and lacks gypsum and salinity.
(B) Flownet illustrating a recharge wetland based on (A) and Knuteson et al. (1989) and Richardson
et al. (1992). Note that the equipotential lines are high in the surface and decrease with depth.
mounding at the edge (Fig. 4B) that produces a shallow, localized reversal of
flow landward of the groundwater mound. Because of the increase in hydraulic
gradient pondward of the mound, groundwater discharge to the pond is also
WETLAND SOILS OF PRAIRIE POTHOLES
Postprecipitationwet meadow mound
Phreatophyte wet meadow drawdown
..-.__..- .'/ '
Figure 4 (A) Expected flow. (B) A water table mound forms on a wetland edge shortly
after a precipitation event, creating a recharge point. (C) The wet meadow drawdown from evapotranspiration.
enhanced. Such reversals may be unimportant when considering large, deep flow
systems; however, reversals have been shown to be very important when examining soil morphology and salinity dynamics in wetland soils (Arndt and Richardson, 1993). Flow reversals induced by phreatophyte transpiration at the wetland edge were examined in detail by Meyboom (1966) in wetlands surrounded
by willow (Salix spp.). After the spring recharge, groundwater mounds typically
developed under these wetlands. However, transpiration by the phreatophytic
willow resulted in the formation of a water table depression around the wetland
periphery, which becomes a groundwater discharge site (Fig. 4C). Because high
rates of evapotranspiration are typical of the wetland periphery, the wetland edge
often becomes the location of focused discharge during significant drawdown
J. L. RICHARDSON ETAL.
periods (Fig. 4C). Soils formed under such complex hydrologic conditions are
discussed by Wilding et al. (1963), Mills and Zwarich (1986), Seelig el al.
(1990, 1991), and Richardson et a1.(1992).
3. The Influence of Sediment Hydraulic Conductivity
Changes in recharge and/or discharge produce flow reversals as discussed
above. The influences of sediment characteristics affecting hydraulic conductivity (K) are less obvious but have a strong influence on groundwater movement.
The important components of K that influence groundwater movement in the
PPR are the texture and the anisotropic nature of unconsolidated surface sediments. Anisotropy is a term that indicates that hydraulic conductivity within the
sediment is not the same in all directions. Preferential lateral flow (also called
interflow, throughflow, and stormflow) results from higher hydraulic conductivity in the horizontal when compared to the vertical dimension. Preferential lateral
flow is the result of several factors. Compaction resulting from the weight of
overlying materials progressively increases sediment bulk density and reduces
porosity with depth. Other factors include plant root macroporosity, textural
changes, and the presence of bedding planes, frozen or partially frozen soil, and
restrictive layers such as an argillic horizon (Kirkby and Chorley, 1967; Zaslavsky and Sinai, 1981). Preferential lateral flow in surface sediments reduces
groundwater recharge in sloping uplands and influences the distribution of evaporites and soil morphology of hydric soils (Steinwand and Richardson, 1989).
Texture and stratigraphy are features of surface sediments that directly influence K , PPR wetland groundwater topography, and the magnitude of groundwater flow. In lacustrine sediments and fine-textured tills, high hydraulic gradients are often observed because the very low hydraulic conductivity associated
with fine textures limits the quantity of flow. As an example, a groundwater
mound with an unusually steep gradient was found to be associated with a recharge wetland in a very low-relief landscape with fine-textured lacustrine sediments (Knuteson et al., 1989). The relief of the groundwater mound was several
times greater than the relief of the land surface. The persistence and magnitude
of the mounding under the recharge wetland were associated with depressionfocused recharge and fine-textured sediments. The mounding explained the major differences noted in soil type and leaching regime over short distances
Conversely, coarse-textured sediments have higher hydraulic conductivity than
do fine-textured sediments. The rapid movement of groundwater in these sediments prevents the development of steep hydraulic gradients (Winter, 1986).
Significant groundwater flows often occur in sand-dominated landscapes, although very steep hydraulic gradients will be absent. We speculate, based on our
field observations, that the presence of coarse textures enhances upland-focused
recharge, and discharge-type wetlands are common.
WETLAND SOILS OF PRAIRIE POTHOLES
4. Flownet Examples of Recharge, Flowthrough,
and Discharge Wetlands
A flownet is a mesh of equipotential lines and associated streamlines that indicates the direction and magnitude of groundwater flow. By convention, equipotential lines indicate constant values in head, and adjacent lines indicate equal
head drops. Streamlines indicate the linear path of water flow. Streamlines always meet equipotential lines at right angles, and groundwater flow is always
from higher to lower magnitude equipotential lines. All streamtubes formed by
adjacent streamlines have equal amounts of flow per unit time. Thus fast groundwater flow is indicated where streamlines converge. Conversely, where streamlines diverge, groundwater flow slows. Closely spaced equipotential lines indicate the presence of large hydraulic gradients. Conversely, where equipotential
lines are spread far apart, low gradients are found.
Figure 2 illustrates the groundwater-surface water interactions of recharge,
flowthrough, and discharge wetlands using flownet analysis. Recharge occurs as
water collects in a depression and infiltrates into the soil. The magnitude of
equipotential lines associated with recharge wetlands decreases from the surface
maximum (Fig. 2a), indicating the downward flow (Fig. 2b). Because the water
is snowmelt and runoff derived, intermittent ponding with fresh water leaches
the soil and promotes the development of an argillic horizon (Amdt and Richardson, 1988). In Fig. 3b we also illustrate a recharge wetland condition. The equipotential lines in the flowthrough wetland are perpendicular to the surface and
decrease in magnitude from left to right (Fig. 2a). Lateral water flow from left
to right is indicated (Fig. 2b). Leaching of minerals will not play an important
role in soil development in flowthrough wetlands because the presence of continuously saturated conditions and brackish water limits the development of an
argillic horizon. The magnitude of the equipotential lines steadily decreases as
the discharge wetland (20) is approached (Fig. 2a), indicating groundwater seepage (discharge) to the wetland (Fig. 2b). These discharge wetlands concentrate
salts in many cases.
The evaporites tend to accumulate in soils at the pond margin. Evapotranspiration on the edge of wetlands or any other landscape position with a shallow
water table creates a condition of abundant soil water loss. This evaporative
discharge allows evaporite minerals such as calcite and gypsum to accumulate in
the periphery of flowthrough and discharge wetlands (Arndt and Richardson,
1988; Steinwand and Richardson, 1989).
Several systems are currently used to classify wetlands in the PPR. The most
important classification systems include the Canadian system (Zoltai, 1988), the
J. L. RICHARDSON ETAL.
United States Fish and Wildlife system (Cowardin et al., 1979), the Stewart and
Kantrud (1971, 1972) system, and an extension of the Stewart and Kantrud
(1971, 1972) system that incorporates hydrology and landscape position (Arndt
and Richardson, 1988). We will examine the last three systems in some detail
1. United States Fish and Wildlife Service System
The Cowardin et al. (1979) wetland classification was developed for use nationwide by the United States Fish and Wildlife Service. It has been adopted as
the standard system of wetland classification in the United States. The Cowardin
et al. (1979) classification is hierarchical in nature. The highest level of classification consists of five systems based on major ecological type: they are marine,
estuarine, lacustrine, riverine, and palustrine. Within each system, wetlands are
further differentiated into subsystems, classes, and dominance types based on
criteria that include ponding permanence, dominant vegetation, water chemistry
and pH, and soil type. In essence, the Cowardin et al. (1979) system is designed
to discriminate ecologically important zones within a wetland; however, basins
can often be classified at the highest levels. In the PPR virtually all “pothole”type wetlands would be classified into the Palustrine System. Basins would be
classified into the Palustrine Emergent class, based on the presence of emergent
vegetation. Zones within the wetland are classified at the more detailed subclass
and dominance-type levels. Specifically, Cowardin et al. (1979) define wetlands
and palustrine systems as follows:
Wetlands are lands transitional between terrestrial and aquatic systems where
the water table is usually at or near the surface or the land is covered by
shallow water. For purposes of this classification wetlands must have one or
more of the following three attributes: (1) at least periodically, the land supports predominantly hydrophytes; (2) the substrate is predominantly undrained
hydric soil; and (3) the substrate is nonsoil and is saturated with water or covered by shallow water at some time during the growing season of each year.
The Palustrine System includes all nontidal wetlands dominated by trees,
shrubs, persistent emergents, and emergent mosses or lichens, and all such wetlands that occur in tidal areas where salinity due to ocean-derived salts is low. It
also includes wetlands lacking such vegetation, but with all of the following four
characteristics: (1) area less than 8 ha (20 acres); (2) active wave-formed or
bedrock shoreline features lacking; (3) water depth in the deepest part of basin
less than 2 m at low water; and (4) salinity due to ocean-derived salts is low.
The Palustrine System was developed to include vegetated wetlands traditionally called by names such as marsh, swamp, bog, fen, and prairie (Cowardin
et al., 1979). It also includes the small, shallow, permanent, or intermittent
WETLAND SOILS OF PRAIRIE POTHOLES
water bodies often called ponds. Palustrine wetlands may be situated shoreward
of lakes, river channels, or estuaries; on river floodplains; in isolated catchments;
or on slopes. They may also occur as islands in lakes or rivers. The erosive forces
of wind and water are of minor importance except during severe floods.
2. The Stewart and Kantrud System
The Stewart and Kantrud (1971, 1972) system was developed specifically as a
research tool for the PPR. It uses plant community composition criteria to describe and define distinctive wetland basins. The Stewart and Kantrud system is
based on zonal plant communities that, in addition to hydrologic variables, reflect salinity and pond permanence. Wetlands are assigned class numbers from I
to V depending on the plant species and zonation present. In classes I through
V, higher numbers indicate the presence of central vegetation zones that reflect
increasingly wet conditions: from central low prairie communities through central wet-meadow, shallow-marsh, and deep-marsh communities, to central open
water (Figs. 5 and 6 ) . Subgroupings based on species composition reflect salinity
tolerances: from fresh, through brackish, to saline subgroups (Table I).
3. Hydrologic Classification of Wetlands
We believe that a wetland basin classification based on pond permanence and
salinity is the best method to make ecologically meaningful delineations for most
wetlands in the PPR. The separations are easier, more accurate, and require less
time than other systems. Arndt and Richardson (1988) outlined a field-oriented
basin-delineation system that inferred recharge-flowthrough-discharge hydrology by relating soil morphology to the observations of Stewart and Kantrud
(1971, 1972), Sloan (1972), and Lissey (1971). Stewart and Kantrud (1971)
identified pond permanence and salinity as defined by wetland plant communities, and Sloan (1972) and Lissey (1971) related salinity, topographic position,
and pond in duration to groundwater recharge-discharge relationships. They
found that, although wetlands can be defined or classified by hydrologic function
as recharge, flowthrough, and discharge wetlands, these wetlands actually form
a continuum on the landscape. The emphasis is on dominant flow conditions,
because seasonal and climatic variation creates intermittent reversals. Recharge
and discharge wetlands are relatively easy to distinguish. Recharge wetlands are
temporarily to seasonally ponded with fresh water and have leached soil profiles
consistent with their hydrologic function. Discharge wetlands are semipermanent
to permanently ponded, and often have saline soils. Efflorescent salt crusts are
often evident around the pond edge. Even though they are permanently ponded,
the catchment area for the pond is small relative to the pond itself, indicating a
large amount of groundwater discharge. Flowthrough wetlands “bridge the gap”