- 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.