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Wetland Resources of Eastern South Dakota

Introduction and Methods


Wetlands are a prominent feature of the glaciated prairie pothole region of the U.S., including most of eastern South Dakota (Fig. 1). Wetlands have traditionally been viewed as economically unproductive refugia for native and exotic weeds and for small mammals and blackbirds that cause crop damage. Wetlands have been considered to be obstacles to planting, cultivating, and harvesting crops.

Perceptions of wetlands are beginning to change. The public is beginning to appreciate the biotic, hydrologic, and economic role of wetlands; however, this new appreciation is not universal, leading to controversy regarding appropriate uses for wetlands.

A detailed review of prairie wetland functions and values has been compiled by Hubbard (1988), and a community profile that describes the origin, hydrology, functions, and biota of prairie wetlands was developed by Kantrud et al. (1989).

JPG - Glaciated Prairie Pothole Region
Figure 1. The glaciated prairie pothole region of eastern South Dakota was delineated using the distribution of characteristic soils.

Previous attempts to summarize wetland acreage totals for South Dakota and some other states have underestimated actual wetland area. In some cases, wetland inventories focused on wetlands with specific functions and values (for example, waterfowl production), while other inventories ignored small wetlands (for example, inventories were based on soil surveys with relatively large minimum mapping units without consideration of hydric soil inclusions). Because wetlands play an important role in current debates about environmental protection, factual information about wetland abundance, characteristics, and distribution is important.

This publication identifies the extent, characteristics, and location of eastern South Dakota wetlands, mapped and classified by the U.S. Fish and Wildlife Service (USFWS) National Wetlands Inventory (NWI), and describes demographics of wetland basins (for example, potholes impoundments, and natural lakes). "Wetland" describes an area with a homogeneous water regime and plant community structure as delineated by the NWI (for example, an area with a temporary water regime and emergent vegetation vs. an area with a semipermanent water regime and submersed vegetation). A "basin" is defined as a depression on the landscape that contains at least one wetland but may contain more than one wetland with different water regimes or types of plant communities (Fig. 2) (Cowardin 1982).

JPG - Description of Wetland/Pothole
Figure 2. The term "wetland" describes an area with a homogenous water regime and plant community structure as delineated by the NWI. A "basin" is the depression that contains the wetlands extending upslope to the limit of the wetlands.

History of Wetland Drainage

Basins (for example, potholes) in the glaciated prairie pothole region sometimes number 100/mi² (40/km²) (Kantrud et al. 1989) and have been generally regarded as an impediment to agriculture and travel. Hewes and Frandson (1952) included a description of the prairie pothole region landscape from the early 1900s which read as follows:

... low knolls are separated by saucer-like [sic] depressions, in which empounded [sic] water often stands the year around ... in the main rainwater which falls upon the uplands has to escape by seepage or evaporation. Little ponds and marshes are found in almost innumerable places scattered all over the county.

Mitchell (1941) noted in recalling trips to town early in South Dakota's settlement period,

I used four horses and sometimes two. I would sometimes get stuck in sloughs, and if any of us saw anybody else stuck we always got out and helped them.

Much like today, the values of wetlands may not have been universally agreed upon even by early South Dakota settlers. Mitchell (1941) also recognized the value of wetlands and noted that the site of the first settlement in Kingsbury County was selected because of its proximity to emergent wetlands:

As we came from near New Ulm, Minn., and finding lots of sloughs lying between Lake Badger and Lake Thistad, and [as] these sloughs were covered with muskrat houses, [we] decided to locate here and build these dugouts and spent the winter here trapping.

Dahl (1990) estimated that approximately 35% of the area of natural wetlands in South Dakota prior to European settlement has been destroyed through human modifications. In South Dakota, most of this wetland area has been converted to agricultural production. Tiner (1984) estimated that 87% of wetland losses in the U.S. between the mid-1950s and mid-1970s was due to agriculture. Extent of drainage is correlated with time since European settlement, duration of agricultural land use, land value, and local community attitudes toward wetlands. Staunton (circa 1950) reported that:

Prior to World War II, potholes ... were considered by most farmers as an asset on his farm, as the drought years of the "thirties" were still fresh in his mind. He [the farmer] remembered well that the only hay he had harvested was on some class B pothole, that the meager income of the family was supplemented by cash revenue from muskrats harvested on his class A pothole. His class C pothole provided good grazing for his livestock, the class D pothole with a small crop of grain either as feed or to sell on the market. The general water table in this area [northeast South Dakota] was down as much as 33 feet.

Although wetland losses began with the development of agriculture in eastern South Dakota, pothole drainage began in earnest in the mid-1940s. The slogan "every acre to its best use" was the justification for draining an unknown number of basins across eastern South Dakota (Staunton circa 1950). Wetland drainage in eastern South Dakota was greatly accelerated by high post-World War II commodity prices (Evans and Black 1956). At the same time, transition from horse-drawn to mechanized farming and increased equipment size resulted in intensified farming practices and larger fields, encouraging even more drainage. As pastures were converted to row-crop and small-grain production, wetlands, which had been sources of forage and water for livestock, were drained because they hindered field operations or reduced crop yields during periods of normal to wet hydrologic conditions.

Acceleration of drainage in the 1940s and 1950s also was due to the creation of a favorable economic climate for drainage through provision of technical and financial assistance by the U.S. Department of Agriculture (USDA) Soil Conservation Service and Production and Marketing Administration (PMA) and to a barrage of propaganda from government agencies painting pothole drainage as responsible land use. Legislation promoting wetland drainage was often disguised under euphemisms such as "conservation," "flood prevention," or even "fish and wildlife enhancement." In official policy, pothole drainage was meant to provide cropland to replace highly erodible uplands, although in practice "worn-out" uplands were seldom retired (Schoenfeld 1949). In Day County, South Dakota, 2,500 basins comprising over 13,500 ac (5,400 ha) were drained during 1948-50 with PMA support (Schoenfeld 1949), and PMA data show that 188,000 ac (76,000 ha) were drained with federal assistance in the Dakotas and Minnesota during 1949 and 1950 (Tiner 1984). Staunton (circa 1950) reported that by 1950 some farmers had drained potholes at government expense, hoping to

... get paid for retiring these same drained lands, or receive payments for non-use in the production of basic crops when curtailment of crop production becomes necessary because of surpluses.

Federal agricultural programs increased the profitability of draining wetlands in South Dakota through cost sharing, below market-rate credit, and price and income supports to oversupplied agricultural markets (Goldstein et al. 1988). Furthermore, tax assessment strategies that valued wetlands based on their potential profitability following conversion, and increased land values following drainage, provided additional economic incentives to drain wetlands. However, Leitch and Danielson (1979) found that farmers were willing to drain wetlands to eliminate the nuisance of tilling around them even when it was uneconomical.

Reduction of direct federal assistance for wetland destruction began in 1978 with issuance of Executive Order 11990, which mandated that the effects of any federally funded or subsidized activity impacting wetlands be evaluated proactively and that steps be taken to minimize or mitigate impacts. In 1979, the U.S. Comptroller General reported that

...we may now be paying more to provide these benefits of wetlands through public work projects and environmental programs than we would have paid to preserve the wetlands ... (U.S. General Accounting Office 1979).

Even with curtailment of federal funding for wetland drainage, destruction of natural wetlands continues throughout the prairie pothole region. Surveys of wetland drainage in four major eastern South Dakota watersheds in 1983-84, repeated in 1989, indicated a 3% loss of basins during that 5-6 year period (Mack 1991). Road construction promotes drainage by creating convenient drainage outlets. Smith et al. (1989) estimated that 27,781 acres of wetlands (11,243 ha) were illegally drained into federal-aid constructed road rights-of-way (road ditches) in the prairie pothole region of Minnesota, North Dakota, and South Dakota.

Recent federal legislation and programs have reduced rates of natural wetland destruction in South Dakota. "Section 404" of the Clean Water Act (formerly the Federal Water Pollution Control Act, passed in 1972), administered jointly by the U.S. Army Corps of Engineers (COE) and the U.S. Environmental Protection Agency (EPA), prohibits discharging fill into waters of the U.S. "Waters of the U.S." have been variously interpreted to include or exclude prairie potholes over the history of the act.

However, the effectiveness of Section 404 has been diminished by authorizing the COE to issue permits, in consultation with the USFWS and subject to EPA review, which allow landowners to fill wetlands. Nationwide, of approximately 15,000 permit requests received annually, approximately 10,000 (67%) are approved, 4,500 (30%) are withdrawn by the applicant before action on the request is taken, and 500 (3%) are denied (Environmental Laboratory 1987). In addition, approximately 75,000 "minor activities" are authorized each year by the COE under regional and nationwide "general permits" that authorize activities without individual permit review, provided the activity causes only minimal adverse environmental impacts. Until 1997, nationwide, Permit 26 authorized the discharge of up to 10 ac (4 ha) of non-deleterious fill into wetlands of the United States, and notification of intent to fill was not required if the total wetland area to be filled was <1 ac (0.4 ha). Most eastern South Dakota wetlands could be filled under this general permit if not protected by other mechanisms.

Recognition that over 87% (Tiner 1984) of wetland losses are associated with agricultural activities prompted inclusion of the Wetland Conservation Subtitle ("Swampbuster") provisions in the 1985 Food Security Act, the Food and Agriculture Trade Act of 1990, and the federal Agriculture Improvement and Reform Act of 1996. USDA has authority to withhold a portion of federal commodity price supports and other agricultural subsidies from landowners who illegally convert wetlands.

In 1994, 86% of South Dakota farmers received federal assistance under USDA farm programs (N. Kappel, Consolidated Farm Service Agency, Huron, pers comm). Operators who do not participate in USDA commodity programs are not affected by Swampbuster regulations.

Reaction to Swampbuster regulations by most of the agricultural community is strongly negative. Conflicting philosophies and policies of federal agencies toward wetlands have reduced government efficiency, have fostered a perception of government waste, and have generated landowner animosity. Confusion over USDA jurisdictional wetland definitions and delineations, particularly for small, temporary basins, contributes to this reaction. In part, this is because benefits of wetland functions typically accrue to society at large and not to individual landowners with whom rest the costs of preserving wetlands and the decision to preserve or destroy them.

While USDA and other agencies historically encouraged drainage, the USFWS and private entities like Ducks Unlimited have worked to protect wetlands in South Dakota. The Migratory Bird Hunting Stamp Act of 1934 was amended in 1958 to enable the purchase of small tracts of uplands and wetlands in the prairie pothole region. Wetland acquisition was accelerated in 1961 with passage of the Wetlands Loan Act which authorized borrowing against future "duck stamp" revenues. Passage of the Small Wetlands Acquisition Act in the same year also authorized the USFWS to purchase small basins valuable to breeding waterfowl (Higgins 1981, Higgins et al. 1987). Approximately 700 USFWS Waterfowl Production Areas (WPAs) covering about 183,000 ac (74,000 ha) of uplands and wetlands were purchased in South Dakota by 1994 (USFWS Div. Refuges Realty Office, Denver, pers. comm.). The USFWS also has used funding from these acts to obtain easements on approximately 613,000 ac (248,000 ha) of eastern South Dakota wetlands through 1994 (USFWS Div Refuges Realty Office, Denver, pers. comm.). Landowners are constrained from burning, draining, filling, or leveling protected wetlands during the period of the easement, which is typically perpetual, unless a variance is issued by the USFWS.

Public and private efforts to both drain and protect wetlands in the prairie pothole region continue. Baseline data on the extent, characteristics, and distribution of wetlands are important to ensuring the future of wetland resources. An accurate assessment of wetland area, number of basins, and their distribution and characteristics is valuable because: (1) knowledge of remaining wetland habitat is important in predicting the impact of new state and federal wetland protection statutes and programs or the impact of modifications to existing statutes and programs; (2) previous status surveys of basins have relied on samples of a small percentage of the U.S.; (3) a wetland inventory may be used to identify regional differences in the characteristics of wetlands or basins and in their density and distribution; and (4) understanding the wetland habitat base enhances administration and management of wetlands biota.

The National Wetlands Inventory

Growing public awareness of the importance of wetlands prompted the USFWS in 1974 to instruct the Office of Biological Services to conduct an inventory of U.S. wetlands and to acquire scientific information on the characteristics and extent of wetlands. As the National Wetlands Inventory (NWI) was becoming fully operational, the USFWS officially adopted the Cowardin et al. (1979) classification system for wetlands and deepwater habitats. Cowardin et al. (1979) defined wetlands as:
... lands transitional between terrestrial and aquatic systems where the water table is usually at or near the surface or the land is covered by shallow water. For purposes of this classification wetlands must have one or more of the following three attributes: (1) at least periodically, the land supports predominantly hydrophytes, (2) the substrate is predominantly undrained hydric soil, and (3) the substrate is nonsoil and is saturated with water or covered by shallow water at some time during the growing season of each year.

Deepwater habitats are defined as "permanently flooded lands lying below the deepwater boundary of wetlands" (6.6 ft or 2.0 m) (Cowardin et al. 1979).

The NWI uses information on hydrology, hydrophytes, and hydric soils to delineate wetlands and deepwater habitats in accordance with national photographic, cartographic, and digitizing standards (USFWS 1994a, 1994b, 1995).

Hydrology refers to surface and subsurface water movements and accumulation. Wetlands usually are periodically saturated or ponded at some time during the growing season. Growing season is variously defined as the soil frost-free period, or the period when the soil temperature exceeds 4°C, which corresponds roughly to the period of biological activity in soil.

Hydrophytes are plants adapted to growth in water or in saturated soils. Wetland soils must be flooded or saturated long enough during the growing season to alter soil chemistry and to stress upland-adapted plants.

Hydric soils are soils formed under aquic or peraquic moisture regimes. This implies at least periodic inundation or saturation during the growing season. Because oxygen diffuses slowly through water, hydric soils are at least periodically anaerobic due to bacterial metabolic consumption of oxygen. Under anaerobic conditions, facultative and obligate anaerobic bacteria reduce oxidized forms of nitrogen, iron, manganese, and other compounds. Bacterial reduction of iron and manganese oxides produces the distinctive grey colors and mottles characteristic of hydric soils.

In the Cowardin et al. (1979) classification system, wetlands and deepwater habitats are relatively homogeneous with respect to hydrologic, edaphic, and biotic attributes. They are classified by hydrology, size, vegetation, and natural or anthropogenic origins and modifications. The Cowardin et al. (1979) system is hierarchical. The number of classification levels in the hierarchy is open-ended. The NWI has adopted conventions which generally use five levels of the classification hierarchy for wetlands and deepwater habitats in eastern South Dakota (Fig. 3). Cowardin et al. (1979) provided a detailed description of these classification taxa; a brief description is provided here.

GIF - Elements of Classification System
Figure 3. Elements of the Cowardin et al. (1979) classification system.

Systems, the highest level in the classification hierarchy, encompass wetlands and deepwater habitats with similar hydrologic, geomorphologic, biologic, and chemical characteristics. Wetlands in three of the five systems defined by Cowardin et al. (1979) occur in South Dakota: palustrine (lentic wetlands), lacustrine (deepwater lentic habitats or large lentic wetlands without trees or shrubs, persistent emergent vegetation, or emergent mosses or lichens), and riverine (lotic wetlands without trees or shrubs, or persistent emergent vegetation) (Table 1). Marine and estuarine system wetlands and deepwater habitats do not occur in South Dakota.

The palustrine system includes wetlands that contain trees, shrubs, and herbaceous vegetation, and wetlands without woody or herbaceous emergents. These wetlands are less than 6.6 ft deep at low water and less than 20 ac (8 ha) in size without a wave-formed or bedrock shoreline. Palustrine wetlands in South Dakota are generally small (for example, wetlands within prairie potholes). Palustrine wetlands may be larger than 20 ac if they support woody or persistent emergent vegetation. The palustrine system has no subsystems (Table 1).

Lacustrine system habitats include natural depressional wetlands and deepwater habitats, as well as artificial excavations or impoundments that are more than 6.6 ft deep, regardless of size, or that lack woody or persistent emergent vegetation and are greater than 20 ac in size, or which have a wave-formed or bedrock shoreline. Lacustrine habitats belong to one of two subsystems: (1) limnetic wetlands and deepwater habitats that are >6.6 ft deep, and (2) littoral wetlands that are <6.6 ft deep (Table 1).

Riverine wetlands are confined within a channel and lack persistent emergent or woody vegetation. Riverine wetlands in eastern South Dakota belong to one of three subsystems: (1) lower perennial wetlands that have low velocity flows and fine substrates, (2) upper perennial wetlands that have high gradient channels, fast flow, and coarse substrates of sand, gravel, or boulders, and (3) intermittent riverine wetlands for which the channel contains water during only part of the year (Table 1).

Table 1. Elements of the Cowardin et al. (1979) classification system used in eastern South Dakota and NWI codes for systems, subsystems, classes, and modifiers.
System L-Lacustrine
Subsystem 1-Limnetic 2-Littoral
Class UB-Unconsolidated Bottom UB-Unconsolidated Bottom AB-Aquatic Bed US-Unconsolidated Shore
System P-Palustrine
Class UB-Unconsolidated Bottom AB-Aquatic Bed US-Unconsolidated Shore EM-Emergent SS-Scrub Shrub FO-Forested
System R-Riverine
Subsystem 2-Lower Perennial 3-Upper Perennial 4 -Intermittent
Class   UB-Unconsolidated Bottom SB-Stream Bed US-Unconsolidated Shore  
Water Regime Special Modifiers
A Temporarily Flooded
B Saturated
C Seasonally Flooded
F Semipermanently Flooded
G Intermittently Exposed
H Permanently Flooded
b Beaver
d Partially Drained/Ditched
h Diked/Impounded
x Excavated

Classes, the next level in the classification hierarchy, relate to vegetation life form where vegetative cover is greater than 30% of the wetland area or to composition of the substrate in sites without vegetation. Classes are not unique to systems or subsystems, although not all classes occur in each (Table 1). Cowardin et al. (1979) described classes used by the NWI in detail. Cowardin et al. (1979) subclasses, which provide more detailed information on vegetation life form or substrate were not used by the NWI for classifying South Dakota wetlands.

Modifiers follow classes in the classification hierarchy. Modifiers may provide information on hydrology, water chemistry, pH, and soil needed to clearly describe the characteristics of wetlands (Table 1). Each NWI-delineated wetland is assigned a water regime modifier (Table 1). Special modifiers are the lowest level in the classification hierarchy used by the NWI and are not assigned to every wetland and deepwater habitat. In South Dakota, special modifiers were used to describe wetlands partially drained by artificial surface outlets, created by human excavation or impoundment, or created by beaver (Castor canadensis) (Table 1).

Wetland Delineation Techniques

The NWI delineated eastern South Dakota wetlands and deepwater habitats by analyzing high altitude, color infrared photography acquired by the National Aeronautics and Space Administration (NASA) and the National High Altitude Photography Program (NHAP). NASA (1:65,000 scale) and NHAP photography (1:58,000 scale) from April-June of 1979-1986 were used to delineate and classify eastern South Dakota wetlands (Fig. 4). Prior to photo acquisition, ground reconnaissance is conducted by USFWS personnel to determine when hydrologic conditions were appropriate for accurate wetland identification. Photography is acquired in wet years, when most basins are inundated, but not in excessively wet years. Water conditions at the time of photography usually reflect the normal distribution of wetlands. Collateral data for wetland delineation and classification includes USGS 7.5' topographic quadrangles, published county soil surveys, Water Resources Institute data, and hydrographic maps, when available.

GIF - Dates of NASA and NHAP Photography
Figure 4. Dates of NASA and NHAP photography used by the NWI to delineate wetlands and deepwater habitats in eastern South Dakota.

The production of NWI maps (Fig. 5, below) follows a rigid set of conventions for accurately identifying and classifying wetlands (USFWS 1995):

  1. Aerial photos are reviewed by an NWI contractor, and characteristic wetlands and problematic areas for wetland classification or delineation are identified.
  2. Specific sites with characteristic or problematic photosignatures are selected for field identification and a field trip route is planned.
  3. Field data, including information on plants, hydrology, and soils, are collected on selected sites and photosignatures are interpreted.
  4. Prior to photointerpretation, these sites are reviewed on aerial photos and data sheets to aid in identification of photosignatures characteristic of the work area.
  5. Photointerpretation is performed for the work area using a stereoscope of at least 4X magnification. Wetlands are delineated on acetate overlays placed on 9-in (22.5 cm) x 9-in aerial photos with black indelible ink using 6x0 pen points. Each delineated wetland is classified following Cowardin et al. (1979) and NWI Photointerpretation Conventions. Collateral data are employed when available.
  6. Subsequent ground truthing is conducted if necessary to resolve problems that arise during photointerpretation.
  7. Following photointerpretation, a 100% quality control check is performed by the contractor. Errors are corrected and the photos are sent to the NWI Regional Office where the NWI Assistant Regional Wetland Coordinator performs another 100% quality control check. Photos are subsequently sent to the NWI National Quality Control Team which reviews 30% of photos for national delineation and classification consistency.
  8. After completion of photointerpretation and quality control, delineated wetlands are transferred to mylar 1:24,000 scale USGS topographic base maps using a zoom transfer scope. Attribute information is transferred to a separate sheet of mylar. The completed product undergoes quality control by the contractor and subsequently by the NWI Cartographic Team. Draft maps are then produced and disseminated for state and federal interagency review and comment.
  9. Draft maps are reviewed in the field by NWI personnel to further increase the accuracy of wetland delineation and classification. Sites identified during photointerpretation, quality control, and interagency review are included in the draft map field review process.
  10. Draft maps are edited, corrections made on acetate overlays, and final maps are submitted for final production and distribution.
GIF - Enlarged Section of National Wetlands Map
Figure 5. Enlargement of a section of a National Wetlands Inventory final map.

Thorough descriptions of photointerpretation and cartographic conventions are distributed by the NWI (USFWS 1994a, 1995).

Digital wetland data are produced by private contractors under NWI supervision using digitizing conventions developed by the NWI (USFWS 1994b) that follow Federal Geographic Data Committee standards. Accuracy of digital data is checked by automatic and manual means. A description of digitizing conventions is distributed by the NWI (USFWS 1994b).

Wetland GIS Development

Individual 7.5' NWI digital wetland coverages were joined with ARC/INFO software to construct polygon, line, and point wetland coverages for each eastern South Dakota county. In the photointerpretation and cartographic processes, polygon wetlands were large enough to be encompassed by a line from a 6x0 pen tip. Linear wetlands were too narrow at the scale of aerial photography to be delineated as polygons and were represented as lines. Point wetlands were too small to be delineated as polygons and were represented as dots.

County boundaries (digitized from 1:100,000 scale maps) (Fig. 6) were extracted from U.S. Census Bureau Topologically Integrated Geographic Encoding and Reference System (TIGER) digital data files. Wetland coverages also were created for eastern South Dakota physiographic regions and hydrologic units. Physiographic regions (Fig. 7) were delineated from the Natural Resource Conservation Service STATSGO soil GIS (1:250,000 scale) (USDA-SCS 1993; Johnson et al. 1995). USGS 8-digit hydrologic units (watersheds) (Appendix C, Fig. 43) were digitized from a 1:500,000 scale paper map (USGS 1978). County polygon, line, and point coverages were joined to cover each physiographic region and hydrologic unit. Physiographic regions and hydrologic units were clipped out of the composite coverages. County boundaries passing through wetlands within physiographic regions and hydrologic units were deleted.

GIF - Eastern SD Counties GIF - Eastern SD Physiographic Regions
    Figure 6. Eastern South Dakota counties.          Figure 7. Eastern South Dakota physiographic
                   regions.

Converting NWI-Delineated Wetlands to Basins

Most data on prairie pothole region wetlands and wetland biota are collected for basins. We converted wetlands delineated and classified by the NWI to "basins" to complement these data. The protocol for converting wetlands to basins and for classifying basins was developed by USFWS personnel to support mapping of potential duck breeding-pair distribution (Cowardin et al. 1995). Protocol for different applications could be developed. It is inappropriate to make direct comparisons between NWI wetland classifications and basins (for example, the area of wetlands and basins by water regime) because "basins" are composite features composed of one to many wetlands.

NWI 7.5' wetland coverages were converted to basin coverages with a series of ARC Macro Language programs (AMLs) and INFO programs in ARC/INFO. Point wetlands were buffered with a radius of 25 ft (7.62 m, area = 0.045 ac or 0.0182 ha), and linear wetlands were buffered by a distance of 24 ft (7.32 m) (total width of a buffered linear wetland was 48 ft) to convert them to polygons. Buffer distances were selected based on mean widths of point and linear wetlands determined from aerial photographs in the prairie pothole region and to maintain consistency with basin coverages created by USFWS personnel for North Dakota (Cowardin et al. 1995). Buffered point and linear coverages were overlaid on the polygon coverage. Arcs separating wetlands were deleted to produce composite, contiguous basin features classified by water regime using the following guidelines:

  1. Within a basin, arcs separating contiguous wetlands were deleted and the composite polygon classified by the most permanent water regime regardless of size (Fig. 8a).
  2. Buffered linear wetlands (natural or excavated) entering a basin were combined with the basin (Fig. 8b).
  3. Excavated wetlands connecting two natural basins were maintained as separate features (Fig. 8c).
  4. Natural temporary wetlands separating wetlands with more permanent water regimes were maintained as separate features (Fig. 8d).
GIF - Wetland Conversion to Basins
Figure 8. Wetlands delineated by the NWI were converted to basins by dissolving arcs and classifying the basin's water regime by the most permanent wetland within it. 3=Semipermanant; 2=Seasonal 22=Seasonal ditches; 21=Temporary.

Approximately 1% of basins were manually evaluated to assure accuracy in the basin creation process. Neatlines from 7.5' basin coverages were deleted where they passed through a basin in county coverages. County basin coverages were combined, and physiographic regions and hydrologic units were clipped out as described above for wetland coverages. County boundaries were deleted where they passed through a basin within a physiographic region or hydrologic unit.

In the data summaries that follow, wetlands, deepwater habitats, and basins that fell on state boundaries were partitioned along the boundary, and only the area within South Dakota is reported. Only the area of the Missouri River and its reservoirs that fell within the boundaries of eastern South Dakota counties was included in the analyses. Summary statistics were generated in ARC/INFO.

Grid Creation for Spatial Analysis

A grid of 10 mi2 (25.60 km2) cells was generated in ARC/INFO and was overlaid on basin physiographic region coverages. Area and number of basins by water regime in a cell were assigned as attributes to that cell. Any basin within a cell or on the boundary of a cell was counted within that cell. Only the area of the basin within a cell was assigned to that cell.


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