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Chapter 3. Wetland Soils of the Prairie Potholes

Chapter 3. Wetland Soils of the Prairie Potholes

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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

in general.




















I c







,' Utah ---- -l----L----L-_I




'<_ _Iowa

- _-- - '


7 )



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.




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,

1949; 1950).




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




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

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Chapter 3. Wetland Soils of the Prairie Potholes

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