The Clean Water Act: Flow Standards

Narrative standards are qualitative, broad statements that establish desired goals and outcomes.  Numeric standards logically follow the narrative and establish quantitative, measurable criteria through which goals and outcomes will be met. Numerous states have recognized the need for a narrative or numeric water quality standard for flow protection. 

Four of six states in US Environmental Protection Agency’s Region 1, the New England states, have developed narrative flow standards.  Vermont’s water quality standards provide a good example of narrative and numeric flow standards.  In EPA Region 4, the Southeast, Tennessee developed narrative standards for flows needed to support recreation and aquatic life in 2008.

Both high and low stream flow conditions should be addressed in standards because both are essential for shaping the physical, chemical and biological processes of a river.  Broadly accepted scientific studies conducted over the past 30 years clearly demonstrate how various flow regimes affect physical conditions for aquatic life; for example, water depth and velocity, and access to certain substrate types and cover. 

Stream flows necessary for physical habitat are also essential for biological processes.  In many cases, water chemistry parameters such as dissolved oxygen and temperature are directly affected by stream flow.  

Designated Uses and Performance Standards

The following are examples of designated uses and water quality standards that depend on water flow and can be impaired when flow alteration affects the magnitude, frequency, duration, timing and rate of change of water quantity. Again, the Clean Water Act does not allow the impairment of existing and classified designated uses of streams and rivers in favor of off-stream uses.  This means flow for uses that withdrawal water, such as industrial, agricultural and drinking water uses, must be in addition to that required for aquatic life and recreation uses. We recommend developing narrative and numeric flow standards for each of these uses.

Aquatic Life

The conditions necessary for the myriad species of flora and fauna included in the “aquatic life” criterion are directly tied to water quantity and the magnitude, frequency, duration, timing and rate of change of flow events.  For example, floodplains and wetlands are critical nursery, feeding, and breeding grounds for aquatic species.  These functions cannot be maintained without the right amount of water at the right time. 

If a discharge from a dam, for example, meets all water quality standards for chemistry, but fails to address physical aspects such as volume and timing, the discharge fails at protecting aquatic life. Additionally, aquatic communities require high volume flushing flows critical for sediment transport, clean substrates and species lifecycles.

Primary and Secondary Recreational Contact

A key component of water-based recreation is water quantity.  Swimming and boating take a certain amount of water to be possible, while sudden large increases in water flow can make both of those activities unsafe for the public. Fishing requires healthy aquatic communities. To focus only on the chemical or pathogen concentrations of water without considering the physical and biological integrity, can fail to protect primary and secondary recreational uses.

Industrial, Agricultural and Drinking Water Uses

Clean drinking water is critical to our economy and society, and is given paramount priority in the Clean Water Act.  Water quantity is critical to drinking water and industrial uses because without enough water to assimilate nutrients and pollutants, the water can become unfit for human or animal consumption, or industrial processes.  Likewise, pollutants carried by stormwater flows can impair water quality for off-stream uses.