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Anoka Sand Plain and Mississippi Valley Outwash

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About the Anoka Sand Plain and Mississippi Valley Outwash

The Anoka Sand Plain and Mississippi Valley Outwash is a region in Minnesota extending from near the center of the state, toward the southeast, and broadening towards its southeastern end. It encompassess the Mississippi River valley upstream from the Twin Cities. This region was formed when the flow of the Mississippi River was blocked near what is now St. Cloud, by the Grantsburg Lobe of the Des Moines Lobe. This region formed as a combination of a sandy lake plain, and outwash from the diverted river. As the glaciers melted, Glacial Lake Ann was formed temporarily.

The topography here is flat on a large scale but there are numerous local hilly features, some quite steep, including dunes, tunnel valleys, and moraines. There are also kettle ponds and depressions. Soils throughout most of the region are fine sands that are poor at holding moisture, except in some of the lowest areas where peaty, organic muck soils accumulated; these poorly-drained soils tend to be found in kettles and tunnel valleys. There are also poorly-drained prairie soils along the Mississippi river.

Prior to European settlement, this region was mostly covered with savanna and oak barrens on the drier, sandier sites. These sites mostly supported stunted growth of bur oak (Quercus macrocarpa) and jack oak (Quercus ellipsoidalis). In the very northernmost reaches of this region there was also jack pine (Pinus banksiana), which reaches its southernmost range limit here. Much of the sand plain was so dry it supported only brush, with few or no trees. There was also wet prairie in the east of the region. Along the Mississippi was a small band of floodplain forest and upland prairie on the terraces farther from the river. Away from the river there were probably only small areas of forest, mostly towards the center of the region, on regions where topography and/or local soil conditions provided protection from fire; these consisted mostly of sugar maple (Acer saccharum) and American basswood (Tilia americana).

On the sand plain, drought was the most important limiter of plant growth, and the main source of mortality of trees; both dominant oak species here are highly resistant to fires. But fire was important at excluding other species from the ecosystems here. During prolonged periods of drought, loss of vegetation resulted in extensive wind erosion and dune movement.

Nowadays there is substantial agriculture here, but much of the region is too wet and poorly-drained to be cultivated, so is left as natural wetlands. There is also more forest nowadays than previously, covering about a quarter of the area. A full 20% of this area is dedicated just to corn and soybean production, with smaller amounts of pasture, hay, and other crops. Areas with peaty or mucky soils are used for sod or vegetable production, after these areas were drained. The southernmost part of this region is part of the Twin Cities metropolitan area, adjacent to Minneapolis in particular, and as such has experienced extensive suburban development. The smaller city of St. Cloud, which also has its own metro area, is entirely contained in this region, and there are also numerous larger towns throughout. Overall this area is quite populous and developed relative to how far north and west it is.

Although this area has been heavily altered, there are still many local areas where the original vegetation cover is relatively intact. High-quality natural areas here include dry oak savanna, dry prairie, and dry oak forest, all of which were originally common here, and there are also intact, rarer communities including fens, wet meadows, floodplain forest, marshes, hardwood swamps, tamarack swamps, shrub swamps, and sugar maple-basswood forests. There is significant public protected land here, mostly state-owned, as well as privately conserved land. The largest threat to this region's ecosystems lies in development pressure along the Twin Cities-St. Cloud corridor, which tends to center along the Mississippi River, an area that is also a focal point for biodiversity.

This region is bordered to the southeast by the St. Croix Stagnation Moraines, a region originally with hummocky terrain, but heavily developed as it contains most of the Twin Cities metro area. To the northeast over most of this region, and also surrounding this region to the west in the northern half, are the McGrath Till Plain and Drumlins. The southern half of this region borders the Big Woods to the southeast, a historically-forested region with numerous lakes and finer-textured soils that are better at holding water. At the very northernmost end of this region, there is a small border with the Minnesota/Wisconsin Upland Till Plain.

References

1. Albert, Dennis A. "Regional landscape ecosystems of Michigan, Minnesota, and Wisconsin: a working map and classification.", General Technical Report NC-178, U.S. Department of Agriculture, Forest Service, North Central Forest Experiment Station, St. Paul, MN (1995) Web.

2. Denis White "Ecological Regions of Minnesota: Level III and IV maps and descriptions", (2020) Web.