Home » Regions » North America » Eastern Temperate Forests » Southeastern USA Plains » Interior River Valleys and Hills » Upper Mississippi Alluvial Plain

Upper Mississippi Alluvial Plain

Page contents

About the Upper Mississippi Alluvial Plain

The Upper Mississippi Alluvial Plain is a region stretching along the Mississippi River and its major tributary the Illinois river, in far southeastern Iowa, northeastern Missouri, and western Illinois. This region extends upstream from where the Missouri river flows into the Mississippi. Along the Mississippi, it extends to where the Mississippi river cuts through the Driftless Area. This region has been severely altered by humans over the past 100 years, with nearly all the forests cleared for agriculture, the river channelized, and numerous dams constructed.

This region is a broad alluvial plain, including the floodplain of the Mississippi and Illinois rivers themselves as well as those of major tributaries. Although the landscape is flat overall, it is dotted by alluvial features including natural levees, oxbow lakes, islands, and scattered sand sheets and dunes. Soils here are formed exclusively on alluvial deposits, and tend to be deep and silty to clayey, and mostly poorly drained, but there are areas of sandy soils that originated as glacial outwash.

The climate here spans the transition between the warmer end of humid continental in the north and the colder end of humid subtropical in the south. There are four well-defined seasons, and winters can be severe, especially in the north. Precipitation is weakly seasonal, peaking in May, slightly lower in late summer to fall, and is lower in winter due to cold temperatures. Due to the location in the interior of the continent, day-to-day weather can be unpredictable, and major heat and cold waves are possible at any time of year.

Original vegetation here is not well-known, but probably consisted mostly of northern floodplain forests, with some oak-hickory forest on the locally driest sites, and also some wet and mesic tallgrass prairies, dry prairies on the sand sheets, and some marshes. Floodplain forests probably included eastern cottonwood (Populus deltoides), silver maple (Acer saccharinum), American elm (Ulmus americana), green ash (Fraxinus pennsylvanica), pin oak (Quercus palustris), and willow. Wet prairies were dominated by prairie cordgrass (Spartina pectinata).

Nowadays this region is almost entirely cleared for cropland. It produces corn, wheat, soybeans, feed grain, and hay for livestock. This region is sparsely populated: although it gets close to some urban areas, its propensity for severe flooding makes it less attractive for building than adjacent high-elevation areas; it does however have a number of small towns throughout.

Downstream, this region is bordered by the Middle Mississippi Alluvial Plain; this border is well-defined, where the Missouri river flowing into the Mississippi, and is marked by a dramatic increase in turbidity and a change in the aquatic biota, although the change is less evident on land. In Iowa, this region is surrounded at its northern end by the Rolling Loess Prairies. In Illinois, it is bordered at its ends by the Illinois/Indiana Prairies. Along much of its length, in Illinois and Missouri, it is bordered on one or both sides by the higher, more rugged River Hills. In Illinois there are two areas where it borders sections of the Sand Area. There is also a border to the west in southeast Iowa and northeast Missouri with the Loess Flats and Till Plains. At its far northern end in Illinois, it is bordered to the east by the Rock River Hills and to the north by the Blufflands and Coulees of the Driftless Area.

References

1. Woods, A.J., Omernik, J.M., Pederson, C.L., Moran, B.C. "Level III and Level IV Ecoregions of Illinois", U.S. Environmental Protection Agency, Washington, DC (2006) Web.