Background
Flow (discharge) is the volume of water that passes a specific location in a given period of time, typically measured in cubic feet
per second, cfs. Mean monthly flow is the arithmetic average of the daily flows recorded at a streamgaging station during a given
month. Flow data for rivers and streams are obtained from gaging stations operated by the U.S.
Geological Survey (USGS). The USGS gaging station network in Illinois is supported in part by the
Illinois Department of Natural Resources,
Office of Water Resources and Illinois State Water Survey,
and the U.S. Army Corps of Engineers. Provisional discharge data posted at this site are
obtained from direct computer access to the USGS. Provisional monthly mean flows posted by USGS are listed if available; otherwise,
daily mean discharge data posted by the USGS are used to estimate the mean flow for the month. Values reported do not reflect final
or official discharges.
Statistical parameters provide a basis of comparison of the current flow conditions to conditions that have been observed in the
past. Long-term mean flows for each calendar month at each gaging station are published by the USGS. These values are computed using
the period of record available. The median flow (50 percent exceedence flow) is determined by ranking from low to high the mean
monthly flows, for a specific calendar month, for each year of record. For the given calendar month, half of the recorded mean monthly
flows are higher and half of the recorded flows are lower than the median flow. Other similar statistics are the 90 percent and 10
percent exceedence probability flows. These values are likewise determined by ranking the historical data. The the 90 percent
exceedence flow is a flow that has been exceeded 90 percent of the time in that month for the station's period of record
(a low flow condition). Similarly, the discharge listed as the 10 percent exceedence flow that has been exceeded in only 10 percent
of the years (a high flow condition).
Tables and graphs available at this site show monthly mean flows and related statistics for 26 gaging stations. Graphs and tables of
real-time and historical flow at all USGS gaging stations are posted by the USGS and may be accessed for each station in the table
using the link given in the first column.
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