The Grand Prairie of Illinois is part of the Prairie Peninsula, a wedge of grassland projecting eastward from the Great Plains into the Midwest. It stretches from western Iowa to Indiana, with outlying extensions as far east as Ohio. Deciduous forest occurs both north and south of the Prairie Peninsula. Overall precipitation is equal between the Prairie Peninsula and deciduous forests to the east. 

How did Native Americans help maintain the prairies?
Drought in the Prairie Peninsula is more severe than in other places in Illinois. Tallgrass prairie produces large amounts of fuel that become highly flammable in the fall or during drought. In the past, Native Americans burned huge areas annually to drive animals when hunting. These fires prevented most trees from growing on the prairie. Annual fires also maintained open forest types such as savannas and barrens by killing saplings of tree species that are not adapted to fire.

An account of a fire during a bison hunt comes from the translated journal of a Jesuit priest in 1680:

When they see a herd, the Indians assemble in great numbers. They set fire to the grass all around these animals except for one passage left on purpose. There the Indians station themselves with their bows and arrows. The buffaloes, wanting to avoid the fire, are thus forced to pass by the Indians, who at times kill as many as a hundred and twenty of them in one day. The buffaloes are distributed according to the needs of the families.
- Father Louis Hennepin (1938) on the use of fire for hunting bison in the Kankakee region in 1680.

Photograph courtesy of Vern LaGesse

Later groups of Native Americans cleared bottomlands for cultivation. They girdled trees (killed them by stripping off a band of bark around the trunk) and used fire to clear land for garden plots. They also harvested trees from the upland forests growing by streams and used the lumber for fuel and construction of houses, tools, and defensive palisades (log walls).