oration, subsurface outflow, and a thorough understanding of the connection between shallow and
deep aquifers. The sand and gravel glacial aquifers are not spatially continuous, and aquifer prop-
erties vary over short distances. A water budget of the shallow aquifer would need to be done for
several small areas that are representative of the major aquifers in the basin. Recent work by Berg
and others (1997) summarizes areas of the State where ground- and surface-water interactions are
important. This
work can be used to target areas where ground wateris significant in a water-bud-
get analysis.
Many approaches are available for determining natural environmental divisions in the LIRB.
Omernick (1987) includes two divisions or ecoregions in the LIRB—Central Cornbelt Plains and
Interior River Lowlands. A detailed approach is the natural divisions defined by Schwegman
(1973). The natural divisionsin the LIRB are distinguished by bedrock, glacialhistory, topogra-
phy, soils, and the distribution of plants and
animals (fig. 25). The five divisions are the Grand Prai-
rie, Western Forest-Prairie, Illinois River Bottomlands, Illinois River Sand Area, and Middle
Mississippi Border. The Grand Prairie had the largest percentageof original undisturbed prairie
area. The Illinois River Bottomlands encompass rivers, bottomlands, andassociated backwater
lakes along the main stem ofthe Illinois River that were originally forested andcontained some
prairie and marsh. The Illinois River Sands Area contains sand and dunes in the bottomlands of the
Illinois River, which are naturally vegetated by scrub oak forest and dry sand prairie. The Illinois
chorus frog is a threatened species and restricted tothis area along the Illinois River. The Western
Forest-Prairie is a dissected till plain with forests and prairie on the uplands. The Middle Missis-
sippi Border is anarrow band of river bluffs, limestone cliffs, andrugged terrain bordering the
floodplain. The dark-sided salamander, western worm snake, andwintering bald eagles are found
in the part of theMiddle Mississippi Border without overlying glacial deposits.
The Illinois Natural History Survey (INHS)collects data at 21 long-term electrofishing sites
inthe LIRB where fish populations have been surveyed annually (except for a few years of no
funding or high flood water) from 1957 to 1992 (Sparks and Lerczak, 1993). The Illinois River is
divided into upper, middle and lower river for evaluation by the INHS. Twelve species of fish were
consistently collected in 90 percent or more of all years (over 30 years of data collection) in the
middle Illinois River, which includes a number of pollution-sensitive centrarchid species, as fol-
lows:
Gizzard shadLargemouth bass
Common carpBluegill
Emerald shinerBlack crappie
Bigmouth buffaloWhite crappie
Smallmouth buffaloGreen sunfish
Channel catfishFreshwater drum
The middle Illinois River, as defined by the INHS, is between Starved Rock Lock and Dam,
and La Grange Lock and Dam. The consistent collection of bigmouth buffalo, a native fish that
requires backwaters for spawning, is related to the large backwater habitatareas along the middle
Illinois River.
