ern Illinois. Historical and recent information from Federal, State, and local agencies describing
the physiography, population, land use, soils, climate, geology, streamflow, habitat, ground water,
water use, and aquatic biology is summarized to describe the environmental setting of the LIRB.
The LIRB is in the Till Plains Section of the Central Lowland physiographic province. The basin
is characterized by flat topography, which is dissected by the Illinois River. The drainage pattern
of the LIRB has been shaped by many bedrock and glacial geologic processes. Erosion prior to and
during Pleistocene time created wide and deep bedrock valleys. The thickest deposits and most
major aquifers are in buried bedrock valleys. The Wisconsinan glaciation, which bisects the north-
ern half of the LIRB, affects the distribution and characteristics of glacial deposits in the basin.
Agriculture is the largest land use and forested land is the second largest land use in the
LIRB. The major urban areas are near Peoria, Springfield, Decatur, and Bloomington-Normal. Soil
type and distribution affect the amount of soil erosion, which results in sedimentation of lakes and
reservoirs in the basin. Rates of soil erosion of up to 2 percent per year of farmland soil have been
measured. Many of the 300 reservoirs, lakes, and wetlands are disappearing because of sedimen-
tation resulting from agriculture activities, levee building, and urbanization. Sedimentation and the
destruction of habitat appreciably affect the ecosystem. The Illinois River is a large river-flood-
plain ecosystem where biological productivity is enhanced by annual flood pulses that advance and
retreat over the flood plain and temporarily expand backwater and flood-plain lakes.
Ground-water discharge to streams affects the flow and water quality of the streams. The
water budget of several subbasins show variability in ground-water contribution from runoff and
storage. More than half of the drinking water, including domestic and public-supply use, in the
LIRB is from ground water. Fifty-two percent of the public-supply water is from surface water.
Ground-water withdrawals mostly are from glacial sand and gravel aquifers. Structural features,
such as monoclines, synclines, and anticlines, in the buried bedrock affect the water quality of the
There are five natural environmental divisions in the LIRB. The Grand Prairie covers most
of the northeastern half of the basin, and the Western Forest-Prairie covers most of the southwest-
ern half. Implications of environmental setting for water quality in the LIRB are related primarily
to land use. The balanced fish community indicates that the lower Illinois River is affected less
from urban and industrial waste than the upper Illinois River. A decrease in dissolved oxygen con-
centrations and turbidity in the lower reaches of the basin in 1993 have resulted from the recent
influx of European zebra mussels to the LIRB. Many factors affect water quality in the LIRB. Bed-
rock and surface topography, type of glacial material, and land use most directly affect water qual-
ity in the basin.