Provisional Monthly Streamflow Network - Background
Streamflow (discharge) is the volume of water that passes a specific location in a given period of time, typically expressed as
cubic feet per second (cfs). Monthly mean flow is the arithmetic average of the daily average 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, the Illinois State Water Survey, and the
U.S. Army Corps of Engineers. Provisional discharge data posted by WARM are obtained
from direct computer access to USGS. Provisional monthly mean flow values (for current and recent months) posted by USGS are
reported if available; otherwise, the provisional real-time discharge data posted by the USGS are used to estimate the daily
mean flows during periods of missing data, if reasonable, to calculate an estimated average flow for the month. Values reported
by WARM do not reflect final or official discharge values.
The monthly mean flows recorded during each calendar month in the history of a gaging station are published by the USGS. The long-term
median flow for the month is the 50 percent exceedence flow in the period of record, determined by ranking the monthly mean flow values
for the same calendar month from throughout the years of record; for the given month, the median flow value is such that half of the
previously recorded monthly mean flow values are greater than the median flow value and half are less than the median flow. The 90
percent exceedance and 10 percent exceedence probability flows are likewise determined by ranking the historical data. The 90 percent
exceedence flow value for the month has been exceeded in that same month in 90 percent of the years in the station's period of record.
The 90 percent exceedence value represents a low flow condition. Similarly, the discharge listed as the 10 percent exceedence flow for
a given month has been exceeded in only 10 percent of the years of record in the same calendar month. The 10 percent exceedence value
represents a high flow condition.
Tables and graphs at this website show monthly mean streamflows 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.
