What is the 7Q10? (Courtesy of Ma Riverways)

The 7Q10 refers to the lowest consecutive 7 day streamflow that is likely to occur in a ten year period. It is used by many states and the federal government in setting discharge limits in National Pollutant Discharge Elimination System (NPDES) water quality permits. A permit will only be granted if the proposed amount of pollutant that will be discharged into a river will not significantly impair the designated uses, such as drinking or swimming, when the streamflow falls to the 7Q10 level. In other words, NPDES permit holders are restricted from discharging pollutants that would cause pollutant concentrations in the receiving water to exceed permit limits, even at very low (i.e. 7Q10) streamflow levels. Although such a low streamflow value, roughly equivalent to a ten-year drought, is appropriately used in the context of limiting pollution discharges, the 7Q10 flow statistic is sometimes inappropriately claimed to represent an adequate streamflow for maintaining a healthy aquatic ecosystem, when in fact much higher streamflow levels are required.