The three areas in Illinois that were not covered by glaciers are the northwest corner, the far south, and the area where the Illinois River joins the Mississippi River. Glaciers did, however, influence these nonglaciated areas. In addition to rock and sand, meltwater deposited a finely ground silt which dried to a dust called loess. Windblown loess deposits along the lower Illinois River are among the thickest in the state. Several loess hill prairies occur in Big Rivers. A layer of loess, sometimes 150-feet thick, forms steep slopes and vertical cliffs.

SIZE: 626,795 acres; 979 square miles

LOCATION:Western Illinois, upriver of the confluence of the Illinois and Mississippi rivers; Adams, Calhoun, Greene, Jersey, Madison, Pike, and Scott counties.

Big Rivers is the largest of the RRAs. The area is characterized by a narrow band of bluffs and rugged topography that borders the floodplains of the Mississippi and Illinois Rivers. Forest is the predominant natural vegetation; hill prairies are common on western facing bluffs. Sinkholes and sinkhole ponds occur here.

LANDCOVER : Cropland is the predominant landcover type (42%) of the site, followed by upland woods (24%). Big Rivers has the highest bottomland woods acreage and the third highest upland woods acreage among the RRAs; it ranks seventh in percentages of both bottomland and upland woods. It has moderate nonforested wetlands resources.

NATURAL AREAS : Sixty-one Natural Areas, totaling 10,514 acres, are located in this RRA. The largest is Gardner Woods with 4,468 acres. Significant features include hill prairies, aquatic systems, woods, geological areas, glades, and caves.

BIOLOGICALLY SIGNIFICANT STREAMS : Twenty miles of Hadley Creek and nine miles of the Mississippi River are designated BSS.

HERITAGE SITES: There are 150 Heritage sites within this RRA representing seven significant community types, 11 plant species, 23 animal species, and eight rookeries.

STATE AND FEDERAL LAND: The state and federal governments own 47,448 acres. Most is federal land divided among Mark Twain National Wildlife Refuge and the pools of the rivers. Pere Marquette State Park and Calhoun County Conservation Area are state land located in this RRA.

NATURE PRESERVES : There are five Nature Preserves in this RRA. Communities protected in the preserves are bluffs, sink holes, ravines, bedrock outcrops, upland forest, and hill prairie.

NATURAL DIVISIONS : This site contains five natural divisions, predominantly the Middle Mississippi Border and Upper Mississippi and Illinois River Bottomlands.

SUMMARY OF SITE CHARACTERISTICS: SIZE, BIOLOGIC RESOURCES, AND PUBLIC LAND