commercial use.
Water-use data are collected by the Illinois State Water Survey (ISWS) for site specific infor-
mation. Aggregated information by county and hydrologic basin is available from the USGS
(Avery, 1995). Surface water is withdrawn from reservoirs and streams. There are 12 subbasins in
the LIRB that have been coded with an eight-digit hydrologic unit code (HUC) (Seaber and others,
1984). These include:Senachwine Lake (07130001), Vermilion River (07130002), Lake Chautau-
qua (07130003), Mackinaw River (07130004), Spoon River (07130005), upper Sangamon River
(07130006), South Fork of theSangamon River (07130007), lower Sangamon River (07130008),
Salt River (07130009), La Moine River (07130010), lower Illinois River (07130011), and
Macoupin Creek (07130012) (fig.
23). In the Lake Chautauqua Basin and South Fork Sangamon
River, substantially more water was used than in theother subbasins in 1988. Peoria is in the Lake
Chautauqua Basin, and Springfield is in the South Fork Sangamon Basin. The thermoelectric use
is substantial in these basins. Over 50 percent of the water was used for thermoelectric power in
the subbasins: South Fork Sangamon River, Salt River, Lake Chautauqua, lower Illinois River, and
Senachwine Lake. The majority of water used in the Vermilion, La Moine, upperSangamon,
Spoon, and Macoupin Basins is for public supply. The major use of water in the lower Sangamon
River and Mackinaw River is irrigation.
The largest freshwater withdrawals in Illinois, excluding the Chicago area, are in the LIRB.
The total freshwater withdrawals in 1988 for the
LIRB is4,050 Mgal/d
(table
4). Eighty-six per-
cent of this water was used for thermoelectric power. Agricultural use (for livestock and irrigation)
and public supply are the next largest water uses with 4–5 percent of the total, each. Industrial and
commercial use are 2 percent of the total, each. Mining accounts for 1 percent of the total water use.
The 1986 water use for Standard Metropolitan Statistical Areas (SMSA), which include the
largestcities in the LIRB, is summarized in Kirk (1987). Peoria and Normal use ground water for
most of their water supplies. Decatur, Bloomington, and Springfield withdraw mostly surface
water. Bloomington withdraws water from Lake Bloomington on Money Creek and Lake Ever-
green on the Mackinaw River. Because of the rapid population growth and recent droughts in these
areas, there is interest in developing a regional ground-water supply (Wilson and others, 1994).
Lake Decatur, which supplies the city of Decatur, is on the Big Creek, Friends Creek, Long Creek,
Sana Creek, and Sangamon River. Near Decatur, wells open to the sand and gravel deposits supply
ground water to Friends Creek when needed to replenish the lake. Lake Springfield is on Lake
Creek and Sugar Creek, but water is pumped from the South Fork of the Sangamon River when the
lake contains an insufficient volume of water for public supply.
Ground-water discharge to streams affects streamflow and water quality. Ground-water dis-
charge is precipitation that infiltrates into the soil and to the water table, and then flows into the
stream channel. The estimated average annual ground-water discharge in 30 subbasins of the LIRB
is 0.09, 0.24, and0.44 (ft3/s)/mi2for years with b
elow normal, nearnormal, and above normal
precipitation, respectively (Walton, 1965).
Ground-water discharge is least from basinswith surficial lakebed sediments and/or under-
lain by impermeable bedrock. Ground-water discharge is greatest from basins having considerable
surficialsand and gravel, and underlain by permeable bedrock. Ground-water discharge during a
year of near normal precipitation ranges from 0.09 to 0.36 (ft3/s)/mi2for basins in the LIRB (Wal-
ton, 1965). Maximum ground-water discharge occurs during spring and early summer, and mini-
