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Hydrologic Functions

The hydrology of prairie wetlands is known only superficially, even though it has been obvious for decades that this knowledge is crucial if the conflicts between agricultural water users and those concerned with flood protection, water quality, ground-water recharge, and fish and wildlife production are to be resolved (Winter 1988). The basin wetlands that are the subject of this community profile form the core of these conflicting interests.

The value of prairie basin wetlands as retainers of surface water flows has recently been reviewed by Winter (1989). He states that, before drainage, the numerous small depressions that characterize the topography made much of the region non-contributary to surface runoff until depressions were full of water and began to overflow from one to the next. In addition, when overflow did occur, the extremely low regional gradient of the land surface generally made runoff velocities small.

Although the number of runoffretaining basins in the Prairie Pothole Region of the Dakotas was large and much drainage has been completed, controversy as to the proven effects of wetland drainage as a cause of increased flooding continues. Several completed studies have been criticized, largely because of problems of study design and impacts of conditions antecedent to the studies (Winter 1988). Even so, the results of some of these studies will be reported here.

Ludden et al. (1983) in the Devils Lake basin of North Dakota concluded that depressions can store as much as 72% of total runoff in a 2-year frequency runoff event and 41% of total runoff from a 100-year-frequency runoff event, with a maximum storage capacity of nearly 8 x 10 m available. In an undrained control block in North Dakota's J. Clark Salyer National Wildlife Refuge, all local runoff plus 58% of the inflow was retained in wetlands (Malcolm 1979).

Increased streamflows in the southern Red River Valley in North Dakota were strongly correlated to the increase in area artificially drained in each drainage basin (Brun et al. 1981), and wetlands in the Red River system significantly reduced flood levels downstream in major metropolitan areas (Jahn 1981).

A little-understood function of prairie wetlands is their role in regional ground-water recharge and discharge (Winter 1989). Considerably more work needs to be done on this subject, especially on waterflow rates through strata of various glacial deposits. Of eight hydrological studies of palustrine and lacustrine wetland systems conducted in the glaciated prairies of North America, seven showed that these wetlands contributed to ground-water recharge (Adamus and Stockwell 1983). The eighth study showed that wetlands with permanently flooded water regimes contributed groundwater discharge year-round. Preliminary studies indicate that basin wetlands in North Dakota supply water to discharge sites up to 20 km distant (Swanson et al. 1988).

Water Quality Functions

It was shown in Section 2.4 that prairie wetlands naturally vary greatly in the chemical composition of their surface waters. This is only one aspect of "quality." The quality of water depends on what the water is to be used for. The human users of water found in prairie wetlands include cities, households, ranches, and crop irrigators, who usually desire water low in dissolved minerals or organic compounds, as well as commercial chemical companies, who extract usable minerals most effectively from some of the most saline surface waters found in the region. Other users desire water of low temperature or low in suspended material. For purposes of this section, water of highest quality is considered that unchanged from its original chemical composition under pristine conditions.

Of the 11 biological and nonbiological functions of wetlands listed by Adamus and Stockwell (1983), sediment trapping and nutrient removal have the greatest positive impact on water quality. These two functions have recently been reviewed for prairie wetlands by Neely and Baker (1989), and this section will draw heavily from their findings.

Most prairie wetlands that have escaped drainage now lie in watersheds devoted primarily to agricultural crop production. Hence, these wetlands frequently experience large inputs of sediment, fertilizer, and other agricultural chemicals transported in sediment, surface runoff water, and subsurface drainage. These contaminants are often removed from the water in passing through wetland basins. Prairie wetlands can thus be important in preservation of local water quality.

Contaminants transported mainly with sediment include chlorinated hydrocarbon insecticides; herbicides such as trifluralin, profluralin, and paraquat: and total organic N and total P including phosphate. Ammonium nitrogen and a majority of the pesticides used in the midwestern United States are carried to wetlands mostly with surface water runoff. Nitrate nitrogen is the most important chemical carried by subsurface drainage, and can be transported mostly by surface drainage in relatively impervious or wet soils. Grue et al. (1988) demonstrated that aerial application of methyl and ethyl parathion caused mortality on aquatic invertebrates for up to 18 days after spraying.

Only the role of prairie wetlands as sinks or traps for N and P has been studied to any great extent, and most of the information comes from studies of a single Iowa wetland (Davis and van der Valk 1978; Davis et al. 1981; Neely 1982). These nutrients can be transferred from the water column of wetlands to the atmosphere, to the interstitial water of sediments, or to biomass within the wetland. Each of these compartments can also return nutrients to the water column.

The Iowa wetland mentioned above removed 86% of the NO₃^- nitrogen, 78% of the NH₄⁺ nitrogen, and 20% of the PO₄³⁺ phosphorus inputs from agricultural runoff in a single year, but the wetland showed such high efficiency as a nutrient trap only when no outflow occurred. Nevertheless, sequential processes of nitrification and denitrification can result in large N losses to the atmosphere, to surface outflow, and to the ground-water system in wetlands experiencing frequent drying and inundation. Thus it would seem that temporarily and seasonally flooded prairie wetlands would be especially efficient in removal of excess N that entered their basins.

Investigators of the Iowa wetland referred to above also reported high N and P concentrations in tissues of the two most common emergent plants, Typha glauca and Sparganium eurycarpum. If it is assumed that other emergents also accumulate these nutrients, then the removal of hay and forage from prairie wetlands by humans and their livestock would be a means by which their function as nutrient traps could be maintained for longer periods.

Many prairie wetlands likely retain sediment, nutrients, and other anthropogenic substances simply because the basins have no outlet. No studies have addressed the ability of- these wetlands to withstand sustained inputs of these contaminants. In addition, no study has addressed the question of the fate of nutrients and pesticides that enter highly saline prairie wetlands.