The lower reach contained similar species, except bigmouth buffalo, smallmouth buffalo, and
green sunfish were not present in 90 percent of allyears sampled. White bass were present 90 per-
cent of all years sampled. In sharp contrast to the other sections of the river, goldfish and carp or
goldfish hybrids,nonnative pollution-tolerant fish, were present in only a few years.
Improvements in water quality of rivers and waterways in the Chicago area have allowed
nonnative species to invade the Illinois River by a manmade link to Lake Michigan. Nonnative spe-
cies from the Great Lakes include the white perch fish and European zebra mussel, which was first
reported in the Illinois River in 1991 (Sparks and Lerczak, 1993). Zebra mussels were carried from
Lake Michigan to St. Louis in the Illinois River. Nonnative species that have invaded the LIRB
from the Mississippi River include the Asian clam, Corbicula, which arrived in 1971 (Sparks and
Lerczak, 1993).
Deterioration of backwater habitat in the LIRB is mostly from heavy sediment loads, which
have notsignificantly changed in the last 15–20 years (Sparks and Lerczak, 1993). Suspended sed-
iment reduces visibility for sight predators, their ability to find food, and the amount of food. Cen-
trarchid fish have complex reproductive and social behavior that depend on visual cues, and their
eggs and larvae are susceptible to smothering with sediment or predation if the guardian male can-
not see and defend them (Sparks and Lerczak, 1993). The diversity in aquatic biology continues to
increase as initiatives help to improve the water quality of the Illinois River.
A particular environmental setting is defined by specific combinations of physiographic, geo-
logic, land-use, hydrologic, and water-use characteristics that may affect water quality (Huntzinger
and Ellis, 1993). Environmental setting is a broad categorization of an area that may be affected by
diverse local conditions and heterogeneity within a setting. Many variables that affect water quality
are not independent; therefore, the entire environmental framework must be considered when
assessing the water quality. For example, physiography is one variable that is often dependent on
geology, so both variables need to be studied.
Work by Panno and others (1994) have shown that the geology appreciably affects the water
qualityof aquifers in buried bedrock valleys in the LIRB. Structural features, such as monoclines,
synclines,and anticlines in the buried bedrock (fig. 14), affectthe water quality of the aquifers. In
the western part of the Mahomet Bedrock Valley, west of the Champaign and Piatt County border,
concentrations of sodium, chloride, carbonate, arsenic, and dissolved solids increase, probably
because of leakage of saline ground water from Pennsylvanian bedrock into the deepest bedrock
valley aquifers (Panno and others, 1994).
Land use and population have a major effecton water quality in the LIRB. Studies in the
Midwest have described the occurrence and distribution ofnitrogen and agricultural chemicals,
and concentrations in surface and ground water (Scribner and others, 1994; Kolpin and others,
1994; Schideman and Blanchard, 1995). Nitrogen and nitrate concentrations exceeding Maximum
Contaminant Levels (MCL’s) have been detected in public-supply and private wells in the LIRB
(Goetsch and others, 1992). Atrazine and alachlor have been detected in the Sangamon River Basin
and many other midwestern streams (Scribner and others, 1994). Many of the pesticides and nitro-
gen concentrations are associated with land use and anthropogenic effects. Volatile organic com-
pounds have been detected in ground water near the major metropolitan areas. Public-supply
facilities near Peoria have had detectable concentrations of bromoform and bromodichloromethane
