The principal source of surface runoff is precipitation that enters the stream as overland flow,
which is rainwater or snowmelt that flows over the land surface toward stream channels. In urban
areas there is less infiltration and much more overland flow.
Ground-water discharge, which is ground water that discharges into a stream channel from a
spring or as seepage, varies across the basin (Berg and others, 1997). Berg and others (1997) char-
acterize low-flow conditions by basin for the State. Basins in the LIRB less than 36 mi2appear to
have the most ground-water contribution as a percentage of the total inflow.
Although the largest discharge from a tributary to the lower Illinois River is from the Sanga-
mon River, the largest tributary sediment load is
from the Spoon River (fig. 19). The discharge of
the Illinois River increases gradually downstream, but the sedimentload increases dramatically in
the La Grange Pool (Demissie and others, 1992). The Spoon River andSangamon River are the
largest tributary sources of sediment to the La Grange Pool. The sediment load is notably greater
for the lower compared to the upperIllinois River. The major sources of sediment are watershed,
streambank, and bluff erosion, which result, in part, from agricultural land-use practices (Demissie
and others, 1992). In backwater lakes, sedimentation causes infilling and loss of habitat.
Precipitation has the greatest effect on floodsor droughts in central and western Illinois. The
floods of 1927, 1943, 1970, 1974, 1982, and 1993 are thelargest recorded based on 30-day and
183-day high mean value for discharge measured at Illinois River at Valley City and Spoon River
at Seville (Zuehls, 1991). The severe flooding of the Mississippi River in 1993 resulted from unusu-
ally intense and persistent rainsand high soil moisture. Backwater from the Mississippi River
affected the Illinois River over 60 mi upstream from the confluence. Some tributaries of the Illinois
River (Big Bureau, La Moine, Macoupin, Sangamon, and Vermilion) had annual mean discharge
exceeding 200 percent of the normal, whereas annual meandischarge for the Mackinaw and Spoon
Rivers exceeded 300 percent (Zuehls and others, 1994). The 1993 flooding along the Illinois River
inundated large areas of farmland and many urban areas in Peoria and other cities.
The flood plain of the Illinois River is linedwith dozens of backwater lakes that are remnants
of marsh that covered the Illinois River Valley afterglaciation. Sediment filled much of the marsh
area, but there are still more than 300 backwater or bottomland lakes, in the LIRB (Talkington,
1991). The Illinois Environmental Protection Agency (IEPA) investigated the need for protection,
restoration, and management of 67 lakes in the LIRB in 1984 and 1994. The following discussion
is based on data from the 1984 and 1994 IEPA study (Sefton and Little, 1984). The lakes are inland
lakes classified as State or public with surface areas of 20 acres or more. Two lakes in the LIRB are
valuable public natural resources according to the IEPA—Lake of the Woods, Champaign County,
and Siloam Springs, Adams County.
Thirty-nine percent of the lakes (26) evaluated in the LIRB had a decrease in overall water
quality, and18 percent of the lakes (12) had improved water quality from 1984 to 1994 (Gregg
Good, Illinois Environmental Protection Agency, written commun., 1994). In 1994, 76 percent of
the lakes in the LIRB were moderately or substantially affected by sediment, which is a 5-percent
decrease since 1984. Twenty-two percent of the lakes (22) were moderately or substantially
affected by macrophytes in 1994. The trophic state index (TSI) increased for 56 percent of the lakes
