Northern Prairie Wildlife Research Center
A total 932,829 basins occur in eastern South Dakota, comprising 2,128,674 ac (861,463 ha). This number and acreage includes only naturally or artificially closed depressions (for example, potholes, lakes, and impoundments) and does not include riverine wetlands. Of the total acreage and number of basins, 55.7% of the basins and 18.3% of the basin acreage are temporary basins, 35.9% of the basins and 26.0% of the basin acreage are seasonal basins, 8.1% of the basins and 34.0% of the basin acreage are semipermanent basins, and 0.2% of the basins and 21.7% of the basin acreage are permanent basins (Fig. 18).
|Figure 18. Number and acres of eastern South Dakota basins by water regime.|
Size Structure of Basins
Most eastern South Dakota basins are small potholes. The median size of South Dakota basins is 0.4 ac (0.16 ha). Of the temporary, seasonal, semipermanent, and permanent basins delineated from NWI data in eastern South Dakota, 58.8% are ≤0.5 ac (0.2 ha) in size, 72.9% are ≤1.0 ac (0.4 ha), 83.4% are ≤2.0 ac (0.8 ha), and 92.1% are ≤5.0 ac (2.0 ha). Only 2.6% are >10.0 ac in size. These percentages are consistent with estimates from the prairie pothole region of North Dakota (Cowardin et al. 1981; D. Cohan, USFWS, Bismarck, pers. comm.).
Basins ≤0.5 ac in size comprise 6.8% of the total basin area in eastern South Dakota, basins ≤1.0 ac comprise 12.1% of the total acreage, basins ≤2.0 ac comprise 20.1% of the total acreage, and basins ≤5.0 ac comprise 34.6% of the total acreage. Basins >5.0 ac in size comprise 65.4% of the total acreage of eastern South Dakota basins, and basins >10.0 ac comprise 53.8% of the total acreage of basins. The <1.0% of basins >100.0 ac in size comprise 25.7% of eastern South Dakota basin acreage (Table 4, Table 5, Table 6, Table 7).
Deeper basins tend to be larger than shallower basins. The median sizes of natural permanent, semipermanent, seasonal, and temporary basins in eastern South Dakota are 98.54 ac (39.88 ha), 2.27 ac (0.92 ha), 0.49 ac (0.20 ha), and 0.30 ac (0.12 ha), respectively (Figs 19-27). Although the 75.7% of basins ≤1.0 ac in size comprise 12.1% of total basin acreage because they are small, nearly all have temporary or seasonal water regimes and provide different functions and values than larger, deeper basins that comprise a larger percentage of the total acreage of eastern South Dakota basins.
Basins cover from 4.6% to 15.6% of eastern South Dakota counties (Table 8) and display clumped distributions. That is, high densities of temporary, seasonal, and natural semipermanent basins tend to occur in discrete regions of eastern South Dakota, in part reflecting the manner in which they were formed during and following glacial retreat. Most of eastern South Dakota was covered by Late Wisconsin glaciers that entered the state approximately 25,000 years ago and retreated from the state for the last time about 10,000 years ago (Appendix A). As Late Wisconsin glaciers advanced southward they encountered the Prairie Coteau, a wedge-shaped highland comprised of a bedrock core covered by pre-Late Wisconsin glacial till. The advancing glaciers split into two lobes, the James, that flowed down the James River Lowland, and the Des Moines, that flowed through northeastern South Dakota into southern Minnesota and Iowa.
Because eastern South Dakota has a recent glacial history, much of the landscape has unintegrated drainage. That is, surface runoff flows into insular depressions or basins that formed from the melting of ice blocks deposited with the glacial till. The thickness of the ice mass and the amount and manner of deposition of glacial debris determined the modern topography and characteristics of basins. High-relief, knob-and-kettle terrain developed when glaciers deposited large volumes of englacial or superglacial debris and commonly includes numerous deep basins. These basins, which often have semipermanent water regimes, tend to be small where the ice mass was subjected to considerable shear and compression and large where the ice mass was less fractured. Conversely, landscapes with less relief like glacial lake plains, or ground moraine overlain by stratified drift flowing out of melting glaciers, often contain shallow basins with temporary or seasonal water regimes.
Basins tend to be least abundant in areas that were not covered by Late Wisconsin glaciers. Basins in these areas, like the western slope of the Missouri Coteau or the Big Sioux River Valley, have disappeared as integrated drainage networks of streams developed, or they have been filled by aeolian sediment deposition. Basins are most dense along the paths of glacial advance and retreat in the James River Lowland and Minnesota-Red River Lowland (Central Lowland), as well as on the northern and eastern Prairie Coteau and the northeastern Missouri Coteau (Fig. 28).
|Figure 28. Distribution of basins expressed as number of basins/10 mi2.||Figure 29. Distribution of basins expressed as acres of basins/10 mi2.|
The area of the landscape covered by basins is a function of basin abundance and basin size but reflects the same general patterns as basin density (Fig. 29). The most notable differences occur on the Prairie Coteau, the Missouri Coteau, and in James River Lowland. Basin acreage is relatively high on the Prairie Coteau farther south than where the highest basin density occurs. Large semipermanent basins and lakes are common on most of the northern half of the Prairie Coteau west of the Big Sioux River. The acreage of basins is lower over most of the James River Lowland and northeastern Missouri Coteau where basins are abundant but most are small.