Wetland Resources of Eastern South Dakota
Appendix A
Formation of Eastern South Dakota Basins
Almost all of South Dakota east and north of the Missouri River is covered by glacial drift (Fig. 42). Most natural basins in the glaciated prairie pothole region (Fig. 1) of South Dakota formed from the melting of ice deposited with lithic debris during glacial stagnation or retreat. Wetland acreage and the density of basins on the landscape is a function of the nature of glaciation, time since glaciation, and post-glacial landscape changes that typically reduced wetland acreage and basin density.
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| Figure 42. Limits of the drift sheets of Late-Wisconsin glaciers in eastern South Dakota. |
The most recent episode of glaciation in eastern South Dakota, the Late Wisconsin glaciation, began approximately 25,000 (Bluemle 1991) to 20,000 (Hallberg and Kemmis 1986) years before present (ybp). We use the term pre-Late Wisconsin glaciation to include all earlier glacial advances.
Pre-Late Wisconsin glaciers advanced from the northeast, and pre-Late Wisconsin glacial topography of eastern South Dakota influenced the path of Late Wisconsin glacial advances. The Prairie Coteau and Missouri Coteau (Fig. 7) have cores of bedrock left by ancient streams eroding Cretaceous shales and limestones. As pre-Late Wisconsin glaciers advanced southwestward, they deposited extensive moraines that formed most of the Prairie Coteau. Then, as Late Wisconsin glaciers advanced southward, they encountered the Prairie Coteau and split into two lobes, the James Lobe, which flowed down the James River Lowland, and the Des Moines Lobe, which advanced through the Minnesota-Red River Lowland into southern Minnesota and Iowa.
Wisconsin glaciation was less extensive in eastern South Dakota than pre-Late Wisconsin glaciation (Fig. 42); however, loess deposition and erosion probably eliminated many wetlands and basins as modern expressions of pre-Late Wisconsin drift. Glaciers retreated from eastern South Dakota most recently about 10-12,000 ybp (Hallberg and Kemmis 1986, Bluemle 1991), although detached, stagnant ice probably persisted until about 9,000 ybp.
Because the last episode of glaciation in eastern South Dakota ended only 10-12,000 ybp, eastern South Dakota has a topographically young landscape compared to western South Dakota where erosional forces have been at work longer. Consequently, most of the landscape of eastern South Dakota has unintegrated drainage (that is, runoff from snowmelt or precipitation travels by overland flow into isolated basins rather than into tributary streams).
The thickness of the ice mass, amount of englacial and superglacial debris, and rate of glacier movement determined the amount of debris deposited and the modern topography and basin characteristics. Basin morphometry, watershed area, and basin-groundwater interactions determine each basin's water regime. High relief, "knob-and-kettle" terrain developed where thick ice flowed over escarpments and deposited large volumes of englacial and superglacial debris when the ice stagnated. This type of topography tends to include basins with steep sides and semipermanent water regimes. Many of these basins formed upon the melting of ice blocks deposited near the surface of till during the wasting process (Flint 1971).
Landscapes with less relief, like glacial lake deposits or areas of moraine overlain by stratified drift flowing out of melting glaciers, tend to include shallow basins with gradually sloping perimeters. Most basins in these landscapes have temporary or seasonal water regimes. Ice blocks that melted early in the wasting process or that were deeply buried in till also formed shallow depressions. Small basins are abundant on recessional moraines overlain by stratified drift in the James River Lowland and Minnesota-Red River Lowland. Small basins are also abundant near the position of active ice margins, usually near coteau slopes, where stagnant ice underwent greater compression and shear and was broken up into small, numerous blocks. Large basins tend to occur on the interior Prairie Coteau and Missouri Coteau in areas away from active ice margins and where the terrain is less radical and watersheds are larger.
The distribution of highest basin density in eastern South Dakota is correlated with the limits of the last Late Wisconsin glaciers in the James River Lowland and Minnesota-Red River Lowland (Fig. 28). Local relief in these physiographic regions rarely exceeds 9 m. Over most of the region, streams and integrated drainage networks are poorly developed. Visher (1917) estimated that over 80% of the James River Lowland has surface drainage into closed basins. Topography is characterized by smoothly rolling, broad, subparallel ridges formed as recessional moraines with abundant small, shallow basins. Inter-ridge areas between recessional moraines, which mark zones over which glaciers retreated more quickly, contain large basins or riverine wetlands that drain toward the James River.
Basin density is also high at the northern end and along the eastern margin of the Prairie Coteau and along the northeastern margin of the Missouri Coteau. High relief knob-and-kettle topography is characteristic of these regions.
Advancing Late Wisconsin glaciers transported englacial and superglacial debris to the top of the coteaus and stagnated. As imbedded blocks of ice melted, basins formed in the till surface, and dead-ice moraine topography developed. The density of basins along the margins of the Prairie Coteau and Missouri Coteau appears to be positively correlated with coteau slope inclination; that is, the highest density of basins occurs adjacent to the steepest coteau slopes, which determined the position of the active ice margin and the compressive forces to which the ice mass was subjected. Compressive forces fractured the ice mass into small blocks which in turn melted to form numerous small basins.
Basins are generally least abundant where glaciation has been less recent and where post-glacial landscape changes have reduced basin density. Most post-glacial landscape changes have been erosional, resulting in the evolution of integrated surface drainage networks. Where sufficient time has elapsed since glaciation, basins may be relatively uncommon. These regions include areas east of the Big Sioux River in the interior of the Prairie Coteau, an ice-free zone during Late Wisconsin glaciation; southeastern South Dakota; and the western Missouri Coteau which slopes downward toward the Missouri River. These areas were covered by glacial drift from pre-Late Wisconsin glaciation and may have had high densities of basins at one time.
Depositional processes also may have reduced basin density. Aeolian sediment deposits may have covered basins east of the Big Sioux River and in extreme southeastern South Dakota. Furthermore, basins occur at low density over most of the Lake Dakota Plain because lacustrine sediments were deposited over the post-glacial landscape under glacial Lake Dakota.
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