7Q10

7Q10 low flowseven-day ten-year low flow

The lowest 7-consecutive-day average streamflow expected on average once every ten years — the design low flow for sizing effluent dilution.

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hydrology
permitting
water-quality

The 7Q10 is a low-flow statistic: the lowest average streamflow over any seven consecutive days that, statistically, recurs about once every ten years. It is the conventional design low flow in NPDES permitting — the worst-case dilution condition a discharge must meet water-quality criteria under.

Because it represents the stream at its weakest, the 7Q10 is the denominator in an assimilative capacity screen: the lower the design flow, the less clean water is available to dilute a pollutant load or to absorb a reduction in flow from consumptive cooling. In the analysis the receiving water's 7Q10 is read from the NPDES permit fact sheet stream-flows table, not estimated.