available in time to be included in this report but is now available (Illinois Department of Natural
Resources, 1996).
Fish populations and diversity are key indicators of changes in water quality. The change
from theupper to the lower Illinois River to a more balancedfish community dominated by native
species indicates improvements in water quality in the LIRB (Sparks and Lerczak, 1993). Carp and
goldfish are more tolerant than most native species of low oxygen levels and toxic materials asso-
ciated with heavy sediment and pollution loads, and species populations often expand in the
absence of these pollution-tolerant predators (Lubinski and Sparks, 1981). The incidence of exter-
nal abnormalities (eroded fins, sores) in fish has declined from 1963 to 1992. However, abnormal-
ities are found more frequently in fishes that contact bottom sediments(catfish, carp) than in fish
that occupy the watercolumn, (bass, bluegill) indicating that there are pollutants or pathogens
associated with the sediments (Sparks and Lerczak,
1993) (fig.
26).
Nonnative species affect the water quality in the LIRB. A decrease in dissolved oxygen con-
centrations and turbidity have resulted from the recent influx of European zebra mussels to the
LIRB. The zebramussels are attacking rare native mussels andthreatening extinction within a few
years (Wendland and Dennison, 1993).
Other characteristics of the environmental setting of the LIRB affect the sources and sinks of
pollutants. The organic content of soils and rate of infiltration of soils are two soil properties that
directly affect the leaching of pesticides into surface or ground water. Some pesticides may be
sorbed to the organic material in the soils, thus, preventing transport ofpesticides to ground water.
A high infiltration rate will increase the leaching of pesticides and other constituents to the water
table. A low infiltration rate will restrict the transport of water and constituents to the water table
but will increase discharge to surface water. The ISGS has mapped the potential for contamination
of shallow aquifers from agricultural compounds (Keefer, 1995). Potential contamination is
strongly affected by soil type. Different types of soils are more easily eroded and, thus, transported
and deposited in aquatic environments.
Climate affects the amount of precipitation and evapotranspiration. Precipitation has a direct
effect on the hydrologic system—amount of streamflow, flooding and droughts, lake levels, wet-
land distribution, and ground-water recharge. High precipitation resulted in severe flooding in 1993
and low precipitation resulted in droughts in the late 1980’s. Precipitation accounts for 80 percent
of water inflow to the LIRB.
There are many factors affecting the environmental setting of the LIRB, such as physiogra-
phy and environmental divisions. Physiography is affected by geology and topography. Environ-
mental divisions are affected by bedrock, glacial history, topography, soils, and the distribution of
plants and animals. Thesefactors are interdependent. For example, the distribution of glacial
deposits is partially dependent on bedrock topography. Soils are dependent on the sourcemateri-
als, which are glacial deposits. Factors that most directly or indirectly affect water quality in the
LIRB are bedrock and surface topography, type of glacial material, and land use. Information on
factors affecting the environmental setting is important and considered when designing and inter-
preting data from the National Water-Quality Assessment of the lower Illinois River Basin.
