The lower Illinois River Basin (LIRB) encompasses 18,000 mi2of central and
western Illi-
nois(fig. 1). The basin extends from
the confluence of the Illinois and Fox Rivers near Ottawa, Ill.,
down to the confluence of the Illinois and Mississippi Rivers at Grafton, Ill. Major rivers in the
basin include theSangamon (5,420 mi2), Spoon (1,860 mi2), La Moine (1,350 mi
2), Vermilion
(1,330 mi2), and Mackinaw (1,140 mi2) Rivers. The LIRB is 1 of 59 study units in the U.S. Geo-
logical Survey’s National Water-Quality Assessment (NAWQA) program, which began full-scale
implementation in 1991 with 20 studies (Warner and Schmidt, 1994). A second set of 15 studies
began in 1994, which includes the study of the LIRB. The NAWQA studies are intended to assess
long-term water-quality changes. Each set of 15 to 20 studies will be active for 7 years and then a
3-year period of minimal data collection before the study becomes active again. The goals of the
NAWQA program are to(1) describe the status and trends in the quality of arepresentative part
of the Nation’s streams and ground water and (2) provide a sound scientific understanding of the
natural and human factors affecting water quality (Leahy and Wilber, 1991). Design of a water-
quality assessment generally considers the characteristics of the hydrologic system because com-
ponents of thesystem and their interaction determine the similarities and differences in water-qual-
ity conditions throughout a basin. An effective regional water-quality assessment strategy is based
on environmental setting, which incorporates many interrelated features (for example, physiogra-
phy, geology, land use, climate, andhydrology).
This report describes the environmental setting for the LIRB. The environmental setting will
serve as the basis for the sampling design and strategy of the water-quality assessment of the basin.
Historical and recent information from Federal, State, and localagencies describing the physiog-
raphy, geology, soils, population, land use, climate, streamflow, habitat, ground water, water use,
and aquatic biology is included in this report, and the effect of these characteristics on water quality
is summarized.
Three previous studies in the basin include the hydrology of the western region interior coal
province (Zuehls, 1987; Zuehls and others, 1981 and 1984) and a report on the waters of the upper
and lower Illinois River (Talkington, 1991). The seven-volume report by the Illinois Department of
Energy and Natural Resources and The Nature of Illinois Foundation (1994) describes the chang-
ing environment and critical trends for the State of Illinois. Data and publications from the Long
Term Resource Monitoring Program (LTRMP) as elements of the U.S. Army Corps ofEngineers
Environmental Management Plan provide insight to habitat, flow, and sediment transport in the Illi-
nois River Basin.
The assistance provided to the author bymembers of the lower Illinois River NAWQA Liai-
sonCommittee and agencies they represent is recognized. Liaison Committee members represent
