over half of the lakes.
Thirty-four percent of the lakes (23) evaluatedby IEPA in the LIRB in 1994 were primary
or alternate public-water supplies. The general water quality of public-supply lakes is better than
for other lakes in the basin. Forty-three percent of the public-water supplies (10) indicated poorer
water quality from 1984 to 1994, and 26 percent (6) indicated improved water quality. The median
changes in water quality were negligible from 1984 to 1994.
Many of the lakes and wetlands are disappearing because of sedimentation resulting from
agriculture activities, levee building, and urbanization. Seventy-six percent of the lakes in the LIRB
assessed by the IEPA are substantially or moderately affected by sediment input. The median depth
for lakes in the LIRB is 5.1 ft (Gregg Good, Illinois Environmental Protection Agency, written
from retreating continental glaciers, were left buried in hills of gravel that the glaciers had pushed
forward. Warmer temperatures melted the blocks of ice to form lakes. Plants grew around the lakes
and eventually filled in the lakes, transforming open water to wet soil and marsh (Bell, 1981). Wet-
lands are important as natural filters for excess nutrients, such as nitrogen and phosphorus, sus-
pended solids, and other constituents (Bell, 1981).
Wetlands provide a sink for nutrients and sediment that filters water. Wetlands also reduce the
biological oxygen demand (BOD) of the water without serious water-quality degradation (Bell,
1981). Presettlement wetlands were widespread throughout theIllinois River Valley and counties
bordering the Illinois River. Wetlands covered as much as 20–40 percent of the land in counties
bordering the Illinois River(Suloway and Hubbel, 1994). By 1987, the total areain most counties
was less than 2 percent wetlands(fig. 20).
Three of the five nationally recognized types of wetland systems are present in Illinois—
palustrine (marshes, bogs, swamps, bottomland forests, and small ponds), lacustrine (lakes, rivers,
and streams), and riverine (free-flowing rivers and streams) (Suloway and Hubbell, 1994). Palus-
trine wetlands are found mostly in southern Illinois but are present from Peoria to Beardstown
along the Illinois River. Lacustrinewetlands are found in small areas along the Illinois River from
Peoria to Beardstown. The riverinewetlands are the most abundant wetland system in the LIRB,
especially west of the lower Illinois River. The riverine wetlands are abundant but are much smaller
individually than palustrine or lacustrine wetlands. The National Wetland Inventory and Illinois
Wetland Inventory contains one area in Cass County that was classified as swamp (Suloway and
Hubbell, 1994). The natural wetland distribution, by river basin, showsthe Illinois River Basin to
contain the most wetlands followed by the La Moine and Sangamon River Basins. The Illinois
River south of Beardstown contains the greatest number of natural wetlands—wetlands that are not
diked, impounded, or excavated. The Spoon River has more open-water palustrine wetlands than
other river basins in the LIRB because of dikes, impoundments, or excavations of abandoned min-
ing pits and associated haulage roads (Zuehls and others, 1981).
Little study has been completed on the hydrology of wetlands in central Illinois. The Illinois
State Geological and Water Surveys are presently (1996)collecting data for the Illinois Depart-
ment ofTransportation to characterize the hydrology of selected wetlands (Mike Miller, Illinois
StateGeological Survey, written commun., 1995). Mostwetlands in central Illinois are surface-
waterdepressions recharged from overland flow. The organic-rich material blanketing the wet-