. Comprehensive plan for the Illinois Nature Preserves System. Nature conservation; Wilderness areas. the division was originally forested but prairie and marsh also occurred. The more sluggish nature of s the Illinois River and its distinctive backwater lakes distinguishes the Illinois River Section from the Upper Mis- sissippi River Section. BEDROCK The bedrock of the two river valleys is deeply covered by alluvial deposits TOPOGRAPHY The bottomlands of the upper Mississippi River and the Illinois River are characterized by broad floodplains and gravel terraces formed by glacial flood waters
. Comprehensive plan for the Illinois Nature Preserves System. Nature conservation; Wilderness areas. the division was originally forested but prairie and marsh also occurred. The more sluggish nature of s the Illinois River and its distinctive backwater lakes distinguishes the Illinois River Section from the Upper Mis- sissippi River Section. BEDROCK The bedrock of the two river valleys is deeply covered by alluvial deposits TOPOGRAPHY The bottomlands of the upper Mississippi River and the Illinois River are characterized by broad floodplains and gravel terraces formed by glacial flood waters. SOILS The soils are from recent alluvium and gla- cial outwash. They are poorly drained, alkaline to slightly acidic, and vary from sandy to clayey. In general they are lighter than the alluvial soils of the Lower Mississippi River Bottomlands Divi- sion. PLANT COMMUNITIES FOREST: The bottomland forests are gener- ally dominated by silver maple, American elm, and green ash. Pin oak is the most important oak; and pecan, bur oak, sycamore, honey locust, hickories, and black walnut are frequent. Black willow and river birch are common in the Missis- sippi River Section. A few southern lowland spe- cies, including water locust, overcup oak, sugar- berry, deciduous holly, and swamp privet, range into the southern part of this disivision. PRAIRIE: In presettlement times mesic prai- rie and wet prairie occurred in the broad bottom- lands. The species composition of these prairies was similar to that of the prairies of the Grand Prairie Division. MARSH: Marshes containing the typical marsh species are important features throughout both sections. SPRING BOGS: Springfed bogs with peat de- posits are found on terraces along the Illinois Riv- er. These are unique to the Illinois River and the species composition differs from that of the bogs of the Northeastern Morainal Division. Distinctive plants include black ash, willows, poison sumac, and skunk cabbage. AQUATIC HABITATS Oxbow lakes
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