7-Day 10-Year Low Flow Maps

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 every 10 years, it is usually an indicator of low flow conditions during drought.

The IEPA 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 to define permit limits for effluent standards and mixing zones under 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 as 11 regional maps. ISWS originally developed these Q7,10 maps in 1973 (see ISWS Bulletin 57), and they have been updated periodically to reflect changes in Q7,10 values over time.

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 ISWS, and
3. discharge data from wastewater treatment facilities and industries as reported to the IEPA.

The Q7,10 values are first estimated from a flow frequency analysis of the streamgaging data, and then adjusted based on stream withdrawal and effluents data 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 gaging data to estimate flow conditions at ungaged sites throughout Illinois, since it is not feasible to monitor more than a limited number of stream locations in the state.

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 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).

The Q7,10 values are affected by natural climatic variability and/or changes in water use and streamflow regulation. For example, a severe drought, such as that in 1988-1989 in central and western Illinois, can significantly affect the statistical analysis and expected frequency of low flow conditions in streams. Increases in water use can 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 due to the ongoing increase in 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.