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Water: Module 4

Basic Course: Key Concepts (Module 4.g)

Parameters: Outstanding National Resource Waters (Tier 3)

Module 4. Antidegradation Requirements

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Tier 3 of the State or Tribe's antidegradation program protects outstanding national resource waters (ONRWs)—waters that have unique characteristics to be preserved (e.g., waters of exceptional recreational, environmental, or ecological significance).

Key Point. No degradation is allowed in an ONRW, except on a short-term basis—meaning weeks or months rather than years. An example of an allowable temporary degradation might be during repairs of a facility.

While States/Tribes are required to have provisions in their antidegradation policy that address ONRWs, it is left to the State/Tribe's discretion to identify waters as ONRWs.

Many States/Tribes have levels of protection that are similarly protective but may allow more flexibility when making water quality determinations. Examples of such designations include "Outstanding Natural Resource Waters," "Outstanding State Resource Waters," or "Exceptional Waters."

For informational purposes only–Not official statements of EPA policy.

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