major aquifers found in the northern and western parts of the LIRB.
The Illinois River is part of the upper Mississippi River drainage basin. The Illinois River
accounts for at least 22 percent of the flow in the Mississippi River just below the confluence of the
Illinois and Mississippi Rivers. A low gradient is characteristic of the lowerIllinois River and is
the principal difference between the lower and
upper Illinois River (fig. 17). Less
than10 mi down-
stream from the beginning of the lowerIllinois River near Ottawa, the Starved Rock Lock and Dam
regulate the flow from the Starved Rock Pool to the Peoria Pool of the Illinois River. Downstream
from the dam, the gradient of the river is low (0.2 ft/mi)relative to the gradient upstream from the
dam(1.1 ft/mi).
Locks and dams regulate the flow of the Illinois River. The Alton, La Grange, Peoria, and
Starved Rock Pools are the reaches of stream (navigation pools) between the lock and dams from
Grafton to Ottawa. The general change in stream elevation between lock and dams is 20 ft, and each
pool is named for the dam immediately downstream. The Alton Lock and Dam is on the Mississippi
River and may regulate flow on the Illinois River. During the floods in 1993, backwater from the
Mississippi River could be measured on the Illinois River as far north as Valley City.
The U.S. Geological Survey (USGS) has operated 17 surface-water-monitoring stations on
theIllinois River from Ottawa to Hardin, Ill., since 1903, and only three of these stations on the
Illinois River were active in 1996. There were 26 active surface-water-monitoring stations across
the basin in 1995(fig. 18). Measurements
include:
•discharge at 26 stations,
•sediment at 7 stations, and
•gage height at 1 station.
Surface-water flow had been measured for more than 20 years at 22 of the historical stations.
The mean annual flow from the upper Illinois and Fox Rivers to the lower Illinois River is
12,600 ft3/s near Ottawa, Ill. The flow in the Illinois River increases approximately 10,000 ft3/s
acrossthe basin from Ottawa to Valley City. The meanannual flow of the Illinois River at Valley
City (61 mi upstream from the river mouth) is 22,600 ft3/s.
Precipitation (80 percent) and discharge from the upper Illinois River Basin (20 percent)
account for most of the inflow of water to the LIRB, but many changes occur across the basin. Dis-
charge to the lower Illinois River across the basin consists of return flow, surface runoff, and
ground-water discharge. Return flow is water that has been released from some facility. Typical
examples of return flow are discharges from industrial and municipal wastewater-treatmentfacil-
ities. The combined return flow, which is from sewage-treatment facilities and overland runoff, has
an appreciable effect on the water budget of the basin. The combined return flow, based on average
annual discharge for all facilities, was 4,400 Mgal/d in 1991. The return flow has increased over
100 percent from 1986 to 1991. The median of the average annual flow per facility was 0.26 Mgal/
d in 1991, but there is a large standard deviation of 123 Mgal/d. The 1991 average per facility was
17 Mgal/d. An annual average of over 600 Mgal/d was discharged by three facilities in 1991.
