7-Day 10-Year Low Flow Maps - Map Comments and Information, Illinois State Water Survey

7-Day 10-Year Low Flow Maps


What is the Q7,10?

A 7-day low flow for a stream is the average flow measured during the 7 consecutive days of lowest flow during any given year. The 7-day 10-year low flow (Q7,10) is a statistical estimate of the lowest average flow that would be experienced during a consecutive 7-day period with an average recurrence interval of ten years. Because it is estimated to recur on average only once in 10 years it is usually an indicator of low flow conditions during drought.

The Illinois Environmental Protection Agency often uses estimates of the Q7,10 as the base flow condition in Illinois streams at which certain water quality standards apply. In particular, these flows are used for defining permit limits for effluent standards and mixing zones as defined by Illinois Pollution Control Board regulations. The Q7,10 is also used as a reference flow for several drought water resource management issues.

The Q7,10 values for Illinois streams are presented in the form of 11 regional maps. The Q7,10 maps were originally developed for Illinois streams in 1973 by the Illinois State Water Survey (Singh, 1973). These maps have been updated periodically to reflect changes in the Q7,10 values that occur over time.

How are the Q7,10 values estimated?

The data used to estimate the Q7,10 values come from three basic sources: 1) streamflow quantity records from stream gaging stations operated by the U.S. Geological Survey, 2) annual water use records collected by the Illinois State Water Survey, and 3) discharge data from wastewater treatment facilities and industries as reported to the Illinois Environmental Protection Agency. The Q7,10 values are first estimated from a flow frequency analysis of the streamgaging data, and are then adjusted based on the data on stream withdrawal and effluents and estimated impacts of other physical regulation of flows such as from dams and diversions. Regional studies using statistical hydrologic principles must also be conducted to translate the gaging data for estimating flow conditions at ungaged sites throughout the State, since it is not feasible to monitor more than a selected number of stream locations in the State.

The adjustments to account for water use and streamflow regulations are needed to update the Q7,10 estimates to reflect the present-day hydrologic conditions at the time that the evaluation is performed. Thus, the Q7,10 values that were estimated using data up through and including the 1984 water year reflect the expected low flow frequency in the streams as of 1984. (Note: The 1984 estimate is a statistical measure of frequency over the available period of record and does not reflect specific low flow conditions that occurred in 1984).

Why do the Q7,10 values change over time?

The Q7,10 values are affected by natural climatic variability and/or changes in water use and streamflow regulation.For example, the occurrence of a severe drought, such as occured in 1988-1989 in central and western Illinois, can have a significant impact on the statistical analysis and expected frequency of low flow conditions in streams. Increases in water use have the potential to either increase or decrease low flows in streams, depending on the stream location and the type of water use. For example, if a community obtains its water from wells and then discharges its treated wastewater to a nearby stream, an increase in water use can measurably increase the low flows in that nearby stream.To this extent, the low flows of many streams in Illinois are augmented above their "natural" conditions as a result of water use.

Because the Q7,10 values can change over time, they have been updated roughly every 15 years to account for possible changes in low flow frequency.The low flow conditions in northeastern Illinois streams have changed substantially over time because of the ongoing increase of water use in the Chicago metropolitan area (from both surface and groundwater sources). For this reason, the Q7,10 maps for northeastern Illinois have been updated more frequently - roughly every 10 years.

Map Updates (since 2002)

All map revisions since 2002 by Vern Knapp and Amy Russell.


7-Day 10-Year Low Flows of Streams in Northeastern Illinois. Illinois State Water Survey Contract Report 545, by K.P. Singh and G.S. Ramamurthy, 1993.

7-Day 10-Year Low Flows of Streams in the Kankakee, Sangamon, Embarras, Little Wabash, and Southern Regions. Illinois State Water Survey Contract Report 441, by K.P. Singh, G.S. Ramamurthy, and I.W. Seo, 1988b.

7-Day 10-Year Low Flows of Streams in the Rock, Spoon, LaMoine, and Kaskaskia Regions. Illinois State Water Survey Contract Report 440, by K.P. Singh, G.S. Ramamurthy, and I.W. Seo, 1988a.

The 7-Day 10-Year Low Flows of Illinois Streams. Illinois State Water Survey Bulletin 57, by Krishan P. Singh, 1973.

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