low permeability, which increases the overland flow. Water tables may be a few feet
below wetland elevations, but the impermeable surface retains water rather than recharging aqui-
fers. Areas of ground-water seeps in east-central Illinois may contain ground-waterdepression
wetlands recharged from overland flowand ground-water inflow.
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 lakes (Sparks and Lerczak, 1993). The expanded aquatic habitats are utilized as
feeding areas by migratory birds and as breeding areas and nurseries by fish and other aquatic life
(Sparks and Lerczak, 1993).
Wetland areas are habitats that provide nesting, food, and cover for fish, waterfowl, and wild-
life. The Illinois River was once one of the most productivefishing and duck hunting areas in the
country, but the wetland habitat upon which fish and wildlife depend for food and breeding was
either converted to agricultural land or covered in silt (Bell, 1981). By 1937,50 percent of the Illi-
nois River flood plain had been converted from wetland to agricultural land bydrainage and levee
districts (Bell, 1981). Small-fish species, such as shiners and minnows, were less consistently col-
lected from the La Grange to Alton Pools than upstream, which could indicate differences in habitat
conditions or other undetermined factors (Sparks and Lercsak, 1993).
Turbidity and siltation have further degraded the wetland habitat of the Illinois River. Aquatic
wetland plants are more sensitive to fluctuation in water levels than plants rooted in moist soil. Tur-
bidity reduces light penetration through water, thus, inhibiting growth of aquatic wetland plants.
The presence and extent of aquifers varies across Illinois from areas such as Mason County,
where the aquifer is present within a few feet of the land surface, to areas such as Champaign
County, where aquifers are more than 100 ft below land surface and are protected by overlying
materials of low permeability. Most shallow aquifers, less than 200 ft below land surface, are com-
posed of unconsolidated glacial material, andbedrock aquifers commonly are the Mississippian-
Pennsylvanian-aged formations.
Aquifer properties of unconsolidated glacial material vary greatly over short distances and by
depth (Kempton and others, 1982). Variability of aquifer properties is greater for younger forma-
tions and less variable for older formations
(table
2)
(Kempton and Visocky, 1992). The till is
almost impervious where it is composed of primarily clay and yields very little water to wells; a
sandy till is relatively more porous and permeable.
The nature of unconsolidated glacial deposits makes it difficult to correlate or compare these
aquifer systems across the LIRB. Lithic discontinuities, genetic associations, and stratigraphic
position of the deposits often have been the basis for aquifer nomenclature of the Quaternary
deposits. An understandingof these characteristics is needed to interpret the hydrostratigraphy and
the relation of aquifer properties to water quality. The processes of deposition ofmaterials derived
from glaciers generally are local, such as lacustrine deposits or end moraine deposits. Some Qua-
ternary deposits form sheets over an areaor fill an older bedrock valley. The basal units in the bur-
ied bedrock valleys are often continuous orspatially connected. Various references in Illinois
literature use genetic terms to define aquifers, such asglacial drift aquifers or glacial till aquifers.
These aquifer names define the depositional origin of materialcomposing the aquifer.
