The goals of the National Water-Quality Assessment (NAWQA) program of the U.S. Geolog-
ical Survey (USGS) are to describe the status and trends in the quality of a representative part of
the Nation’s streams and ground water, and provide an understanding of factors affecting the water
quality. The lower Illinois River Basin (LIRB), which encompasses 18,000 mi2of central and
western Illinois, is one of 60 study units throughout the Nation selected for water-quality assess-
ment. Information about the environmental setting provides a framework of the basin characteris-
tics and includes natural and human factors that determine the similarities and differences in water-
quality conditions throughout the basin.
The LIRB includes the drainage area of theIllinois River south of Ottawa, Ill., to the conflu-
ence with the Mississippi. The altitude of land surfacegenerally is from 600 to 800 ft above sea
level. The flat topography of the basin is a result of the glacial tills that cover the basin. The major
surficial features are the moraines of the Wisconsinan glacial deposits in the northeastern half of
the basin. A population of1.3 million in 1990 has remained relatively constant since 1970 with the
major population center nearPeoria. Approximately 87 percent of the land use in the basin is agri-
cultural. Most of the agriculture is cropland for growing corn and soybean, and there is very little
remaining original prairie vegetation. The nativeprairie vegetation under which soils form con-
tributed to the high accumulation of organic materials, which is valuable to agriculture because of
the capacity to store water and nutrients. The climate includes cold, relatively dry winters and
warm, wet summers with normal precipitation from 35 to 38 inches per year.
The hydrology of the LIRB has been most affected by glacial processes and deposits that
cover the basin. Bedrock is mostly carbonate rocks that include some coal deposits. Five major tec-
tonic features(Glasford Structure, Peru Monocline, DownsAnticline, Pittsfield Anticline, and
Cap au Gres Faulted Flexure) and many minor features affect local geology and may affect water
chemistry. Erosion of thebedrock, prior to and during glaciation, cut largebedrock valleys that
have been filled with over 500 ftof glacial deposits. Two of the major buried bedrock valleys, the
Mahomet and Mackinaw, contain thick sand and gravel at the bottom of the valleys that are overlain
by hundreds of feet of till. The sand and gravel in these buried bedrock valleys are major aquifers,
and the overlying tills are thick confining units. The stratigraphic relation of glacial deposits is
complex because of the limited aerial extent of many deposits.
The Illinois River accounts for at least 22 percent of the flow in the Mississippi River just
below theconfluence of the Illinois and Mississippi Rivers. The Illinois River has a low gradient,
and the flow is regulated by five lock and dams from Grafton to Ottawaand on the Mississippi
River at Alton. Major tributaries to the Illinois River are the La Moine, Mackinaw,Sangamon,
Spoon, and Vermilion Rivers, but the upper Illinois River accounts for most of the inflow of water
to the LIRB. The Spoon River contributes the largest sediment load to the Illinois River. Lakes and
reservoirs line the banks of the Illinois River, and many backwater lakes and wetlands are in the
surrounding area. Presettlement wetlands covered as much as 40 percent of the land in counties
bordering the Illinois River, but most of these wetlands have been filled by sediment. The wetlands
provide an expanded aquatic habitat that is utilized by many birds, fish, and other aquatic life.
Glacial deposits from 200 to 500 ft below land surface include the major sand and gravel
aquifers. Aquifer properties vary greatly over short distancesand by depth. The nature of the gla-
cial deposits make it difficult to correlate and compare aquifers across the LIRB, but two aquifers
are composed of distinctive rock-stratigraphic units—the Sankoty and Mahomet Sands. The
Sankoty and Mahomet Sands are present in the Mackinaw and Mahomet buried bedrock valleys
