Northern Prairie Wildlife Research Center
Natural Semipermanent Basins
Natural semipermanent basins (basins containing at least one natural semipermanent wetland) total 23,997, cover 377,660 ac (152,837 ha), and comprise from 0.1% to 6.5% of the total area of eastern South Dakota counties (Table 8). These basins are abundant on the northern Prairie Coteau and Altamount Moraine and in the northern Minnesota-Red River Lowland (Fig. 34). As the James and Des Moines lobes of Late Wisconsin glaciers advanced southward, the thick mass of ice and lithic debris flowed over the northern end and eastern margin of the Prairie Coteau (Appendix A). The steep escarpment in these regions was eroded by the glacial advance, and the ice mass was heavily fractured by shear and compression. Large volumes of till were deposited on the top of the Coteau as the glacier stagnated and underwent wasting. Semipermanent basins on the Prairie Coteau are analogous in formation to seasonal basins on the northeastern Missouri Coteau; however, the volume of lithic debris deposited on the Prairie Coteau was greater, and modern topographic relief is greater. Deep basins with angular profiles developed when thick blocks of ice near the surface of glacial debris melted (Flint 1971). Deep basins in a high-relief landscape with a relatively high precipitation:evaporation ratio result in semipermanent water regimes for many basins in this area.
|Figure 34. Distribution of natural semipermanent basins expressed as number of basins/10 mi2.||Figure 35. Distribution of natural semipermanent basins expressed as acres of basins/10 mi2.|
The acreage of natural semipermanent basins is relatively great throughout the Prairie Coteau, except east of the Big Sioux River (Fig. 35). In the interior of the Prairie Coteau, semipermanent basins tend to be larger but less numerous than near the Coteau slope, because the ice mass was less compressed and fractured in this area. Consequently, although the density of semipermanent basins is lower in this area, the acreage of semipermanent basins is great. The Prairie Coteau slopes downward toward the west (Flint 1955); and because the glacial advance encountered a more gradual coteau escarpment in the west, the James lobe of Late Wisconsin glaciation advanced farther over the Prairie Coteau than the Des Moines lobe. Consequently, most natural semipermanent basins on the Prairie Coteau are found west of the Big Sioux River, which marks the limit of Late Wisconsin glacial advance in most locations (Flint 1955).
Semipermanent Basins due to Dugouts or Other Excavations
Dugouts are commonly excavated in natural basins and may impart a semipermanent water regime to basins where no natural semipermanent wetland exists (for example, a 0.1 ac (0.04 ha) dugout excavated in a 1 ac [0.4 ha] seasonal wetland delineated by the NWI results in a 1.1 ac [0.44 ha] basin with a semipermanent water regime). In other cases, dugouts may appear as isolated basins or they may be excavated in the channels of linear wetlands. The 38,663 basins with semipermanent water regimes due to dugouts or other excavations cover 237,069 ac (99,148 ha) and comprise from 0.2% to 2.7% of the total area of eastern South Dakota counties (Table 8). These basins are scattered throughout eastern South Dakota. Their density is low throughout the Lake Dakota Plain physiographic region and in extreme southeastern South Dakota (Fig. 36) because land use in these areas is primarily tillage agriculture while dugout excavation is commonly associated with areas of livestock production.
The acreage of semipermanent basins due to the presence of dugouts is inflated in regions where dugouts are commonly excavated in large, natural temporary and seasonal basins, imparting semipermanent water regimes to these basins. These regions include the central James River Lowland lying west of the James River and the northern Missouri Coteau (Fig. 37).
|Figure 36. Distribution of semipermanent basins due to dugout excavation expressed as number of basins/10 mi2.||Figure 37. Distribution of semipermanent basins due to dugout excavation basins expressed as acres of basins/10 mi2.|
Impoundments with Semipermanent Water Regimes
The 11,527 impoundments with semipermanent water regimes cover 99,411 ac (40,231 ha). They comprise from 0.1% to 1.5% of the total area of eastern South Dakota counties (Table 8). Impoundments are generally associated with wetlands flowing in channels. Consequently, most impoundments are found in areas that were not covered by Late Wisconsin glaciers, where integrated drainage networks have developed sufficiently to focus runoff in channels and where livestock grazing is the primary land use. Most impoundments with semipermanent water regimes in eastern South Dakota occur on the west slope of the northern and central Missouri Coteau (Figs 38, 39).
|Figure 38. Distribution of semipermanent and permanent impoundments expressed as number of basins/10 mi2.||Figure 39. Distribution of semipermanent and permanent impoundments expressed as acres of basins/10 mi2.|