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Illinois/Indiana Prairies

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About the Illinois/Indiana Prairies

The Illinois/Indiana Prairies are the largest ecoregion in Illinois, extending through most of the north and central part of the state, into a small area in western Indiana.

This region ranges from nearly level to gently rolling. It is covered in loamy, calcium-rich glacial till, lake sediment, and mixed glacial drift, and loess in the west. The north also has some clayey glacial till. This region tends to have dark, highly fertile soils that formed under tallgrass prairie, and are ideally suited to Western agriculture. The climate here ranges from a humid continental climate to a humid subtropical climate in the south. Unlike areas farther east, precipitation is moderately seasonal, with a wet season peaking in May-June, and dry winters. Precipitation decreases and sunlight increases as one moves westward. Streams in this region, particularly in the west, tend to dry up during periods of drought.

Naturally this region was mostly covered in prairies, including wet, mesophytic, and dry, but also had significant areas of oak-hickory forest. This area used to be poorly-drained and had abundant ponds and swamps throughout, with many low-lying areas flooded seasonally.

This area is heavily utilized for agriculture. Lands have been drained by tiling and ditching, draining them through the existing natural drainage channels, and eliminating most of the marshes and pothole lakes. Corn and soybeans are dominant crops, and there is significant raising of hogs, and some cattle, sheep, and poultry, but overall, less livestock production than drier areas farther west. Agricultural runoff has increased stream turbidity and degraded aquatic habitats throughout. Unlike areas to the east, there are not even small woodlots interspersed throughout the land. What little forest remains is mostly along riparian areas, but even along rivers and streams, riparian buffers are inconsistent and broken; in places only a single row of trees lines the streams. There are a few protected areas of prairies, the largest and oldest of which is Midewin National Tallgrass Prairie, which is still severely altered from its natural state. There are a few small, scattered protected forests throughout, mostly along bodies of water.

There is some urbanization; Peoria is the largest city in this region, and its metro area is mostly, but not entirely, located here. The smaller cities of Bloomington, Champaign, Decatur, and Springfield are also located here. The rest of the region is rural and has only small towns throughout.

This region borders many different regions, mainly due to its large size. In the north, it borders the Rock River Drift Plain to the north, the Rock River Hills to the northwest, and the Valparaiso-Wheaton Morainal Complex to the northeast. The three pieces of the Sand Area are distributed throughout. To the west, it borders the Western Dissected Illinoian Till Plain, and in some places it borders and/or surrounds the River Hills, except in two small places where it directly borders the Upper Mississippi Alluvial Plain. It is bordered to the south by the Southern Illinoian Till Plain, except in the east where it is bordered to the south by the Wabash River Bluffs and Low Hills. In the east, it partly surrounds the Glaciated Wabash Lowlands, and in Indiana, is is bordered to the east by the Loamy High Lime Till Plains.


1. Woods, A.J, Omernik, J.M., Brockman, C.S., Gerber, T.D., Hosteter, W.D., Azevedo, S.H. "Ecoregions of Indiana and Ohio (Poster)", US Geological Survey (1998) Web.

2. 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.