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Chapter 3. Wetland Soils of the Prairie Potholes
J. L. RICHARDSON ETAL.
water management, often conflicts with traditional uses of soil for dryland agriculture (Leitch, 1989). The societal importance of natural prairie wetlands for
ecosystem support and water management has been recognized in the controversial “swampbuster” provisions of the 1985 farm bill. Many important wetland
functions, however, have been poorly defined for the public. As an example, a
little appreciated function of prairie wetlands is flood abatement (Hubbard and
Linder, 1986; Richardson and Arndt, 1989). Drainage of large numbers of wetlands results in less water stored on the landscape. Wetland drainage increases
the catchment area of adjacent streams and drains (Moore and Larson, 1979) and
can aggravate downstream flooding (Novitzki, 1978; Brun er al., 1981). There
are considerable difficulties in defining exactly the benefits of such nonagricultural wetland uses. Because the benefits of wetland conversion for agriculture
are tangible and immediate, wetland drainage for agricultural use and to improve
cropping efficiency is still occurring at a rapid rate on the prairies. We feel an
understanding of wetland functions as reflected in hydric soils and hydric soil
development is necessary to manage the prairie wetland resource appropriately
for both societal and individual benefit. It is to this end that this discussion is
directed. Three reviews of prairie wetlands have been published (Adams, 1988;
van der Valk, 1989; Hubbard, 1989). Only Hubbard (1989) discussed wetland
soils; we are expanding his review considerably.
“Prairie potholes” are numerous water-filled depressions characteristic of the
glaciated portion of central North America that was once grassland. Although
prairie wetlands are occasionally found in Wisconsin, Texas, Illinois, Nebraska,
Oklahoma, and Missouri, we are confining our discussion to the wetlands of the
prairie pothole region (PPR) that extends from the prairie-forest line north of
Edmonton, Alberta, southward to the end of the Wisconsin-aged Des Moines
lobe in central Iowa (Fig. 1). Wetlands in the PPR are mostly kettle-type depressions formed on a till surface that has not yet developed an integrated network
of surface drainages. The depressions vary in size from less than 0.5 ha to several
hectares and usually contain surface water for some period of time during the
year. A few are permanent lakes.
Water accumulates in prairie wetlands as a function of complex interactions
between topography, vegetation, and climate as they influence the local hydrology. Hydrology, considered as the sum of all the factors influencing the chemistry, movement, and distribution of groundwater and surface water, is a unifying
principle of soil development that has been overlooked, although it is essential
in understanding wet soils (Richardson et al., 1992). Winter (1988; 1992) makes
a solid case for groundwater hydrology as a unifying concept for wetland ecology
WETLAND SOILS OF PRAIRIE POTHOLES
,' Utah ---- -l----L----L-_I
- _-- - '
Figure 1 The prairie pothole region as defined in this article (after van der Valk, 1989).
Zoltai (1988) points out that two important hydrologic factors, climate and
topography, really explain the existence of wetlands in any landscape. Depressions collect water; level areas do not have enough slope to create appreciable
runoff, and low areas such as floodplains have runon water from adjacent uplands
in addition to periodic flooding. In the PPR where integrated drainage networks
are lacking, topography is the main control on the movement of groundwater and
surface water in wetlands, and exerts a significant influence on hydric soil development. The glaciated landscape of the PPR is a mosaic of closed system
catchments that vary in size, topographic position, and relationship to the groundwater. Runoff as well as groundwater recharge and discharge are focused on the
wetland depressions occupying these catchments. Interdepressional uplands are
usually not involved in direct transfers of water to and from the water table
(Lissey, 1971) because low rainfall characteristic of the region confines recharge
to areas where the vadose zone is thin, e.g., in and around wetlands (Winter,
1983). PPR wetlands typically form relatively large, complex wetland systems
connected to each other by groundwater flow. Sediment stratigraphy of the area
around a wetland and the climatic factors of precipitation, evapotranspiration,
and freezing will in turn impact soil development (Arndt and Richardson, 1988;
1989a,b; 1992). We base PPR wetland soil development on hydrologic processes
and conditions created by a climatic gradient as impacted by topography, sediment lithology, and stratigraphy.
J. L. RICHARDSON ET AL.
The landscape of the PPR is very young, with most of the surface sediments consisting of Wisconsin-age drift that dates from 14,000 to 9000 years before present (YBP). Spruce forests followed the ice margin as continental glaciers gradually wasted northward during the close of the Pleistocene Epoch. As the climate
moderated and became drier, the forests were gradually replaced by prairie parkland, which was replaced by true prairie around 6000 YBP (Ritchie, 1976). Thus
the native vegetation in the PPR has been dominantly grass for the last 6000
years. Prairie grasses create dark-colored fertile soils that are high in organic
matter; nearly all the soils in the region are Mollisols (Soil Survey Staff, 1975).
The northern prairies were opened to settlement at the turn of the century.
Farming is currently the dominant land use, with wheat and sunflowers the typical cash crops in the north, and corn and soybeans dominant in the south. In the
hummocky landscape of the PPR, wetlands are often drained because naturally
wet soils are a hindrance to crop production (Leitch, 1989). Poor aeration in wet
soils restricts crop growth due to lack of sufficient oxygen for root respiration.
Only plants that are adapted to long periods of poor soil aeration can survive
(Bartlett, 1961, 1986; Gambrel1 and Patrick, 1978). Additionally, large equipment becomes mired when crossing wetlands except when the wetlands are quite
dry. However, temporarily and seasonally ponded wetlands are abundant in the
PPR and are usually flooded during spring planting. The necessity of continually
traveling around these wetlands with farm machinery increases production costs
and reduces the efficiency of tillage operations (Leitch, 1989).
Early studies of the soils in prairie wetlands focused on ephemeral, seasonal,
and temporary wetlands. Examinations of more permanent wetland soils were
not numerous. The soils in temporarily and seasonally ponded wetlands were
found to be calcareous on the pond periphery and leached in the pond interiors
(Redmond and McClelland, 1959). In Iowa, however, wetlands underlain by
upland loess deposits did not have distinct calcareous edge soils, but did have
leached centers exhibiting extremely well-developed argillic horizons (Ulrich,
II. CLIMATE, BASIC HYDROLOGIC CONCEPTS, AND
The PPR has a cool continental climate characterized by cold winters, hot
summers, and extreme variations in both temperature and precipitation. Temperatures may range from - 40 to 40°C annually. The precipitation regime
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