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Prairie Basin Wetlands of the Dakotas:
A Community Profile

Chapter 2 -- Abiotic Environment

2.3 -- Hydrology

Wetlands exist because specific geologic settings and hydrologic processes favor ponding of water or soil saturation. A unique combination of glaciation and climatic conditions in the Prairie Pothole Region has produced a large number of dynamic aquatic ecosystems that have a tendency to be nonintegrated (not receiving or contributing to channelized surface flow). Nonintegrated basins have the potential to impound large volumes of water and undergo long-term, rather extreme changes in water depth and biotic conditions in response to climatic trends. Waterlevel fluctuations in typical seasonal and semipermanent North Dakota wetlands are shown in Figure 5. Note the greater rate of water loss and greater frequency of dryness in the seasonal wetland. The low-grade shorelines of prairie wetlands combine with semiarid climate to produce dynamic wetlands; e.g., small increases in water level cause great increases in the proportion of a basin inundated, and conversely, hot, dry conditions often remove surface water from large areas of a basin in a relatively short time.

gif -- water level changes

gif -- water level changes

Figure 5. Water level changes during the ice-free season over a 6-year period in a seasonal (lower) and semipermanent (upper) basin wetland in North Dakota (from Kantrud et al. 1989).
Hydrologic regimes are dictated by climate and geology (Figure 6) that establish the environment for hydrologic processes (Winter 1989). Atmospheric, surface, and ground water interact with basin topographic setting and the hydraulic characteristics of glacial tills to establish wetland hydrologic functions. Precipitation and a combination of evaporation and transpiration are, respectively, the major components of water gain and loss in prairie wetlands (Winter and Woo, in press). Evaporation and transpiration are controlled largely by wind and vapor fluxes above the wetland. Areas like the Prairie Pothole Region with persistent winds, high summer temperatures, and dry overlying air masses can be expected to have high evaporation and transpiration rates during the summer months.

gif -- environmental factors

Figure 6. Environmental factors influencing prairie wetland ecology.
Lissey (1971) found that most of the water available for ground water recharge is derived from spring snowmelt. Snow collects in topographic depressions and 50% to 90% of the meltwater from topographic highs collects in depressions because snow melts before the ground thaws. Lissey concluded that except for occasional intense storms, summer rainfall is not a major contributor to recharge in the uplands.

Glacial topography and geologically governed permeability variations have a significant effect on surface and ground-water hydrology. Tills in moraines are generally silty and clayey materials that are not very permeable. Outwash deposits, on the other hand, generally consist of stratified sand and gravel, materials that are very permeable. The tills in the eastern Dakotas tend to be much higher in shale-derived material. These tills are more clayey and less permeable to water than those to the west and south, which have a larger proportion of limestone, sandstone, and siltstone derivatives (Winter 1989). Because prairie wetlands are characteristically nonintegrated, ground-water flow systems play a dominant role in their hydrologies. Lissey (1971) described the principles of depression-focused ground-water recharge and discharge in the glaciated prairie of southwestern Manitoba. LaBaugh et al. (1987) studied the hydrology of a wetland complex in the Cottonwood Lake area of Stutsman County, ND, that contained wetlands situated at different altitudes along a topographic slope and hydraulic gradient. This area has been described in detail by Winter and Carr (1980). LaBaugh et al. (1987) confirmed the concept of depression-focused ground-water recharge and discharge as proposed by Lissey (1971).

Wetland basins in the Cottonwood Lake area perform three basic functions with respect to ground water (Lissey 1971). These functions are reflected in the water chemistry of these wetlands (Swanson et al. 1988). Some basins function as ground-water recharge areas; such basins tend to be temporarily or seasonally flooded; they hold water for only a few months each year, and the water is generally low in dissolved solids. Some basins are through-flow systems with respect to ground water; that is, ground water flows in through parts of their bed while other parts recharge ground water. Through-flow basins hold water over longer periods and the water tends to have higher concentrations of dissolved solids. Some basins serve only as discharge areas for ground water. Lakes that receive discharge from both regional and local ground-water flow systems and do not lose water to seepage or surface outflow are highly saline, having specific conductance as high as 70 mS/cm.

Wetland hydrologic functions control the chemical characteristics of prairie lakes, and as a result, plant and invertebrate communities.

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