Great Plains Prairie Pothole Wetland

The prairie pothole system is found primarily in the glaciated northern Great Plains of the United States and Canada, and is characterized by depressional wetlands formed by glaciers scraping the landscape during the Pleistocene era. This system is typified by several classes of wetlands distinguished by changes in topography, soils and hydrology. Many of the basins within this system are closed basins and receive irregular inputs of water from their surroundings (groundwater and precipitation), and some export water as groundwater. Hydrology of the potholes is complex. Precipitation and runoff from snowmelt are the principal water sources, with groundwater inflow secondary. Evapotranspiration is the major water loss, with seepage loss secondary. Most of the wetlands and lakes contain water that is alkaline (pH >7.4). The concentration of dissolved solids result in water that ranges from fresh to extremely saline. The flora and vegetation of this system are a function of the topography, water regime, and salinity. In addition, because of periodic droughts and wet periods, many wetlands within this system undergo vegetation cycles. This system includes elements of aquatic vegetation, emergent marshes, and wet meadows that develop into a pattern of concentric rings. This system is responsible for a significant percentage of the annual production of many economically important waterfowl in North America and houses more than 50% of North American's migratory waterfowl, with several species reliant on this system for breeding and feeding. Much of the original extent of this system has been converted to agriculture, and only approximately 40-50% of the system remains undrained.

The vegetation within this system is highly influenced by hydrology, salinity and dynamics. Potholes found within this system can vary in depth and duration, which will determine the local gradient of species. Likewise, plant species found within individual potholes of this system will be strongly influenced by periodic drought and wet periods. Deeper potholes with standing water throughout most of the year have a central zone of submersed aquatic vegetation. Potholes that dry during droughty times can have central zones dominated by either tall emergents or mid-height emergents depending on the depth of the marsh. Wet meadow species such as grasses, forbs and sedges can be found in potholes that are only flooded briefly in the spring. All of these types of potholes can be found within an example of this system. Grazing, draining, and mowing of this system can influence the distribution of these types of potholes and plant species within this system.

This system is characterized by closed basins, potholes, that receive irregular inputs of water from the surroundings and may export water as groundwater. The climate for the range of this system is characterized by mid-continental temperature and precipitation extremes. Across the range of this system, precipitation triples from 30-90 cm (west to east) and average annual temperature increases from 1-10°C (north to south). Snowmelt and spring rains typically fill many of the potholes in examples of this system. The region in the range of this system is distinguished by a thin mantle of glacial drift overlying stratified sedimentary rocks of the Mesozoic and Cenozoic ages; these form a glacial landscape of end moraines, stagnation moraines, outwash plains and lakeplains. The glacial drift ranges 30-120 m thick and forms steep to slight local relief with fine-grained, silty to clayey soils. Soils in outwash plains are coarser. Limestone, sandstone, and shales predominate as bedrock, and highly mineralized water can discharge from these rocks. The hydrology of this system is complex with salinity ranging from fresh to saline, and chemical characteristics varying seasonally and annually. Sites with substantial surface or groundwater outlet are typically fresh while sites with little or no outlet tend to accumulate salts. Rain and snowmelt are the primary water sources with evapotranspiration being the source of major water loss. Some potholes are connected to groundwater sources and can serve as groundwater recharge sources, some receive groundwater outflow, and some have both. Water depth in most potholes is shallow. Many have a maximum depth of <2 m and most are <1 m deep (Sloan 1970). Seasonal water level fluctuations mean that the depth during much of the growing season is less than these maximums.

A cycle of flooding and drying is the primary natural dynamic influencing this system. Snowmelt contributes substantially to the seasonal water input. In addition to runoff from snow melting within the watershed, snow tends to accumulate within the pothole due to the slightly more sheltered landscape position and the typically heavier and taller vegetation cover present in at least parts of the pothole. Spring rains contribute additional water, and potholes consistently have their yearly maximum water depth in late spring. Heavy rains in the summer can fill potholes, but the tendency is for water levels to fall as the growing season progresses. This fluctuation of water level during the year results in very different flooding regimes for different parts of the pothole. At the driest edge, the ground may be flooded or saturated for only a few weeks during the growing season, while the wettest parts of some potholes are flooded year-round. These different flooding regimes create environments favoring different types of plants and many potholes have strong zonation of vegetation (Johnson et al. 1987). From driest to wettest, these zones are wet meadow, shallow marsh, deep marsh, aquatic, and deep water. Many potholes do not have enough water to support the wetter vegetation zones so individual potholes may have shallow marsh at the center with a ring of wet meadow or deep marsh surrounded by shallow marsh which in turn is surrounded by wet meadow. The changes in water volume in a given pothole are also reflected in the salinity of the water. Prairie potholes are least saline in the spring when snowmelt and spring rains fill the wetland, and possibly flush water out of the basin through seasonal overflow, but salinity increases as evapotranspiration reduces the volume of water in the basin throughout the growing season (Stewart and Kantrud 1972).

In addition to seasonal water level fluctuations, there are longer-term changes in water levels that affect prairie potholes (Kantrud et al. 1989a). Multi-year patterns of above or below average precipitation result in shifting vegetation zones within a single site. A multi-year dry period will cause a pothole to shrink, and the environments suitable for each vegetation zone will move towards the center, possibly eliminating the wettest zones altogether. A multi-year wet period will fill potholes, moving the environments conducive to each vegetation zone away from the center and possibly creating habitats for new, wetter zones in the middle. Changes in water depth of several feet are possible over a few to several years (Stewart and Kantrud 1972). These multi-year changes in the location of vegetation zones promote floristic diversity by creating shifting environments at any one place on the landscape. During the wetter seasonal or multi-year periods, temporary connections may be formed among otherwise discontinuous wetlands, allowing the spread of species and possibly affecting water chemistry through flushing of salts or other dissolved chemicals into or out of basins (Leibowitz and Vining 2003).

Fire and grazing can affect this system. Fire could spread from adjacent upland prairie, especially in the fall when water levels tended to be low and vegetation was driest. The wet prairie/wet meadow zone burned most frequently, but in the fall, dense, dry tall emergent vegetation in shallow or deep marshes could carry fire, as well. These fires could remove standing dead vegetation, allowing more light to reach the ground and returning nutrients to the soil, but they did not result in a conversion to a different system. In the eastern portion of this system's range, fire was more important in keeping woody species from invading. Native ungulates grazed the margins of potholes and used them as water sources. Muskrats live in larger, wetter potholes and, when populations get high, can have significant effects on the vegetation by eating Typha spp. and substantially reducing its cover (Kantrud et al. 1989b).

The main threat to prairie potholes is changes to the hydrologic regime. Other important threats are overuse of sites for agricultural purposes, runoff from surrounding cropland, and introduction of exotic species. More than one of these may result from the same disturbance. Negative changes to the hydrologic regime often take the form of partial or complete drainage of potholes. This is typically done to allow the land to be used more consistently for crop production. Conversion to cropland completely destroys a prairie pothole, but even if the drainage is incomplete or done on nearby wetlands, it can harm the site. Partial drainage reduces the amount of water in a pothole, effectively shrinking it. Drainage of nearby wetlands can affect groundwater flow in a wider area, increasing or decreasing the amount of water flowing into nearby wetlands. Changes in landscape-level water drainage patterns can result in a more consistent water supply to a pothole and may reduce the multi-year water level and associated vegetation changes. Without these longer-term changes in water level, dense emergent vegetation dominated by a few species tends to take over the margins of these wetlands (Kantrud et al. 1989b).

The drier margins of prairie potholes are most susceptible to agricultural use. The wet meadow zone can be farmed in drier years and can be mowed for hay or used for grazing later in the growing season in most years. Moderate grazing or mowing is unlikely to have serious impacts, and may even increase diversity, but consistent heavy grazing or mowing usually reduces species diversity and vegetation cover and opens up sites for colonization by weedy species. Sites that are not directly used for any agricultural purpose can be negatively impacted by runoff from nearby fields. Row crops are more prone to erosion than pasture or small grains, but all can cause significant sedimentation of potholes, affecting water chemistry and turbidity and even filling them in over time (Preston et al. 2013). Runoff from agricultural fields can introduce pesticides, herbicides, and increased nutrient levels. Road construction through a pothole can affect water movement, effectively creating two new wetlands, and can cause increased sedimentation and chemical runoff from road maintenance.

The climate in the prairie pothole region became warmer and wetter during the 20th century but the effects were not uniform. The western edge became effectively drier while the eastern edge became effectively wetter (Millett et al. 2009). This is due to the greater increase in average temperature creating more evaporation compared to lesser increases in precipitation in the west and greater increases in the east. If this trend continues, it will shrink the potential range of this system on its western margins as those wetlands dry out, and convert some prairie potholes to permanent lakes or ponds on the eastern margin as increased precipitation eliminates the wet-dry cycle in some wetlands. This potential shift in the range of prairie potholes would also move more of the range out of the relatively less intensively farmed and drained landscape of the central and eastern Dakotas and southern Canadian Prairie Provinces and into the more intensively farmed areas of western Minnesota and northern Iowa where wetland draining and filling has been much more extensive (Johnson et al. 2005).

Prairie potholes are naturally dynamic and the native species in them respond quickly to changing environments so they have the potential to recover if the natural processes are returned and the seed bank is sufficiently intact.

This system can be found throughout the northern Great Plains ranging from central Iowa northwest to southern Saskatchewan and Alberta, and extending west into north-central Montana. It encompasses approximately 870,000 square km with approximately 80% of its range in southern Canada. It is also prevalent in North Dakota, South Dakota, and northern Minnesota.