For most pollutants, the measure of a river or stream’s low flow is the critical condition for receiving-water flow. (However, other critical flow conditions are possible, such as high flow for a river or stream where wet weather sources are a major problem.)

The critical-condition concept is that if an effluent is controlled such that it does not cause water quality criteria to be exceeded in the receiving water at the critical flow condition, then the effluent controls will likely be protective (i.e., ensuring that water quality criteria are attained at all flows).

• 1Q10 low flow, which is the once in 10 years low flow for acute aquatic life criteria.
• 7Q10 low flow, which is the 7-day average low flow for chronic aquatic life criteria.
• Harmonic mean flow for human health criteria for toxic organic pollutants.

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