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Watershed Science

7-Day 10-Year Low Flow Maps

Download PDF copies of the paper maps that are contained in Illinois State Water Survey contract reports ISWS CR 440, ISWS CR 441, and ISWS CR 545 (or as updated since 2002). These maps provide information on low flow conditions that might be expected to occur only during droughts (once in 10 years), and associated effluent discharges, water withdrawals, and flow regulations for most Illinois rivers and streams.

These maps were developed by the Illinois State Water Survey with cooperative funding from the Illinois Environmental Protection Agency.

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Background Information

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.

Map PDF Links

  1. Rock River Region

    The low flow estimates presented here for the Rock River region represent an October 2002 update of previous published versions (Singh, 1973; Singh et al., 1988a). These new low flow values were estimated from the Illinois Streamflow Assessment Model for the Rock River watershed, the preparation and analysis of which is documented in Knapp (2002). The estimates produced by this model are based on hydrologic data collected up through 2001, which include USGS streamgage records, water-use records collected by the Illinois State Water Survey, and discharge data from wastewater treatment facilities and industries as reported to the Illinois Environmental Protection Agency. Selected water use and effluent discharge estimates have been updated so that the values represent the current (2001) hydrologic conditions in the region.

    Knapp, H.V. 2002. Streamflow Assessment Model for the Rock River Watershed: Hydrologic Analysis. Illinois State Water Survey Contract Report, in preparation.

  2. NE Illinois Streams

    The low flow estimates presented here for the Northeastern Illinois region represent a February 2003 update of previous published versions (Singh, 1973; Singh, 1983; Singh and Ramamurthy, 1993). For the Fox River watershed, low flow values were estimated from the Illinois Streamflow Assessment Model (ILSAM), the preparation and analysis of which is documented in Knapp (1988) and Knapp and Myers (1999). Flow estimates for this map reflect hydrologic conditions based on hydrologic data collected up through 2001, which include analysis of USGS streamgage records, water-use records collected by the Illinois State Water Survey, and discharge data from wastewater treatment facilities and industries as reported to the Illinois Environmental Protection Agency.

    Knapp, H.V. 1988. Fox River Basin Streamflow Assessment Model: Hydrologic Analysis. Illinois State Water Survey Contract Report 454.

    Knapp, H.V., and M.W. Myers. 1999. Fox River Basin Streamflow Assessment Model: 1999 Update to the Hydrologic Analysis. Illinois State Water Survey Contract Report 649.

    Map Updated 4/30/2003

  3. Kankakee Region

    The low flow estimates presented here for the Kankakee region are as published in a 1988 report by the Illinois State Water Survey (Singh et al., 1988b), and represent an analysis of streamflow conditions based on data collected prior to and including the 1984 water year. The low flows on some streams in the Kankakee region may have changed somewhat since 1984 as a result of changes in water use and natural climatic variability. However, it is expected that the low flows for most streams in this region, if estimated using present-day data, would remain similar to the values published on this map. There are no immediate plans to update the Q7,10 values for the Kankakee region.

  4. Spoon River Region

    The low flow estimates presented here for the Spoon River region are as published in a 1988 report by the Illinois State Water Survey (Singh et al., 1988a), and an analysis of streamflow conditions based on data collected prior to and including the 1984 water year. The low flows on some streams in the Spoon River region may have changed somewhat since 1984 as a result of changes in water use and natural climatic variability. However, it is expected that the low flows for most streams in this region, if estimated using present-day data, would remain similar to the values published on this map. There are no immediate plans to update the Q7,10 values for the Spoon River region.

  5. Sangamon Region

    The low flow estimates presented here for the Sangamon region represent an April 2002 update of previous published versions (Singh, 1973; Singh et al., 1988b). These new low flow values were estimated from the Illinois Streamflow Assessment Model for the Sangamon River watershed, the preparation and analysis of which is documented in Knapp (1999). The estimates produced by this model are based on hydrologic data collected up through 1997, which include USGS streamgage records, water-use records collected by the Illinois State Water Survey, and discharge data from wastewater treatment facilities and industries as reported to the Illinois Environmental Protection Agency. Selected water use and effluent discharge estimates have been updated so that the values represent the current (2000) hydrologic conditions in the region.

    Knapp, H.V. 1999. Sangamon River Streamflow Assessment Model: 1999 Update to the Hydrologic Analysis. Illinois State Water Survey Contract Report 650.

    Map Updated 4/12/2002

  6. La Moine Region

    The low flow estimates presented here for the La Moine region are as published in a 1988 report by the Illinois State Water Survey (Singh et al., 1988a), and represent an analysis of streamflow conditions based on data collected prior to and including the 1984 water year. The low flows on some streams in the LaMoine region may have changed somewhat since 1984 as a result of changes in water use and natural climatic variability. However, it is expected that the low flows for most streams in this region, if estimated using present-day data, would remain similar to the values published on this map. There are no immediate plans to update the Q7,10 values for the LaMoine region.

  7. Kaskaskia Region

    The low flow estimates presented here for the Sangamon region represent an October 2002 update of previous published versions (Singh, 1973; Singh et al., 1988a). These new low flow values were estimated from the Illinois Streamflow Assessment Model for the Kaskaskia River watershed, which is currently being updated from the original version of the model, documented in Knapp (1990). The revised low flow estimates produced by this model are based on hydrologic data collected up through 2001, which include USGS streamgage records, water-use records collected by the Illinois State Water Survey, and discharge data from wastewater treatment facilities and industries as reported to the Illinois Environmental Protection Agency. Selected water use and effluent discharge estimates have been updated so that the values represent the current (2001) hydrologic conditions in the region.

    Knapp, H.V. 1990. Kaskaskia River Basin Streamflow Assessment Model: Hydrologic Analysis. Illinois State Water Survey Contract Report 499.

  8. Embarras Region

    The low flow estimates presented here for the Embarras region are as published in a 1988 report by the Illinois State Water Survey (Singh et al., 1988b), and represent an analysis of streamflow conditions based on data collected prior to and including the 1984 water year. The low flows on some streams in the Embarras region may have changed somewhat since 1984 as a result of changes in water use and natural climatic variability. However, it is expected that the low flows for most streams in this region, if estimated using present-day data, would remain similar to the values published on this map. There are no immediate plans to update the Q7,10 values for the Embarras region.

  9. Little Wabash Region

    The low flow estimates presented here for the Little Wabash region represent a March 2002 update of previous published versions (Singh, 1973; Singh et al., 1988b). These new low flow values were estimated from the Illinois Streamflow Assessment Model for the Little Wabash River region, the preparation and analysis of which is documented in Knapp and Myers (2001). The estimates produced by this model are based on hydrologic data collected up through 1999, which include USGS streamgage records, water-use records collected by the Illinois State Water Survey, and discharge data from wastewater treatment facilities and industries as reported to the Illinois Environmental Protection Agency. These data were analyzed and processed to represent the current (1999) hydrologic conditions in the region.

    Knapp, H.V. and M.W. Myers. 2001. Streamflow Assessment Model for the Little Wabash River Watershed: Hydrologic Analysis. Illinois State Water Survey Contract Report 2001-14.

    Map Updated 3/20/2002

  10. Southern Region

    The low flow estimates presented here for the southern region are as published in a 1988 report by the Illinois State Water Survey (Singh et al., 1988b), and represent an analysis of streamflow conditions based on data collected prior to and including the 1984 water year. The low flows on some streams in the southern region may have changed somewhat since 1984 as a result of changes in water use and natural climatic variability. However, it is expected that the low flows for most streams in this region, if estimated using present-day data, would remain similar to the values published on this map. There are no immediate plans to update the Q7,10 values for the southern region.

  11. Illinois & border rivers

    The low flow estimates presented here for the Illinois River and border rivers are as published in a 1988 report by the Illinois State Water Survey (Singh et al., 1988a), and represent an analysis of streamflow conditions based on data collected prior to and including the 1984 water year. The low flows on the border rivers may have changed somewhat since 1984 as a result of changes in water use and natural climatic variability. Low flows on the Upper Illinois River, for example, show a noticeable decrease in low flows since 1998 as a result of changes in the operation of the Lake Michigan diversion. The low flows on the Wabash River are expected to increase as flows have been partially regulated by upstream reservoirs. There are no immediate plans to update the Q7,10 values for this map.

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