Key Concepts Module 4: Antidegradation

This module discusses the requirement that water quality standards include a framework and methodology for deciding if, when, and how water quality that exceeds the CWA 101(a) goal can be degraded by regulated activities and when that water quality must be maintained. Answers to the following questions are provided:

  • In what situations can a State or Tribe consider allowing degradation of water quality?
  • What is required when addressing antidegradation questions?
  • How do requirements address different types of uses or waters?
  • What does EPA look for when reviewing a State or Tribe's approach to antidegradation issues?

At the end of the module is a brief quiz intended to touch on some Core Modules regarding a State or Tribe's antidegradation program that are further examined in the classroom session of this module.

This module's main pages and brief quiz at the end take about 15 minutes to complete.

Weighing Uses Against Degradation of Quality

Under the Clean Water Act (CWA), once the existing uses of a water body have been established—by evaluating the water's quality relative to uses already attained—a State/Tribe must maintain the level of water quality that has been identified as being necessary to support those existing uses.

But what if the water quality is better than what is necessary to support the existing uses? That is, what if progress has been made toward attaining the CWA 101(a) goal uses?

Question. Can the State/Tribe then allow a degradation of water that is better than that necessary to protect CWA 101(a) uses? What if the degradation would provide economic or social development opportunities or other benefits to the State or Tribe?

The answer is yes. However, in order to effectively address the considerations involved in making such decisions, EPA's regulations require the State/Tribe to have established an antidegradation policy as part of its water quality standards.

A General Framework

In essence, a State or Tribe's antidegradation policy provides a framework for weighing the pros and cons of a proposed activity that could degrade water quality and for involving the public in the decision-making (40 CFR Section 131.12(a)).

Key Point. Within limitations established by EPA's regulations, the State or authorized Tribe has the discretion to permit activities that degrade water quality to a minimum level—that is, to the level of quality identified in the State or Tribe's water quality standards as being necessary to support the water body's existing uses.

Antidegradation policy can apply to both point and non-point source activities.

Key Point. Before permitting degradation for point sources, the State/Tribe must ensure that the most stringent technology-based controls required by statute and regulation will be implemented.

Because the Clean Water Act does not authorize regulatory non-point source control programs, EPA cannot require States/Tribes to implement such programs. States/Tribes, of course, are free to adopt requirements for non-point sources that exceed those of the CWA. Nonetheless, where States/Tribes have established an approach for evaluating non-point sources, the process must be followed before allowing degradation of water quality.

Key Point. Before allowing an increase in pollutant loads by non-point sources, the States/Tribes with a non-point source control program should ensure that reasonable and cost-effective best management practices for control will be implemented.

A Decision-Making Approach

Along with an antidegradation policy, States/Tribes also are required to identify their implementation method. In so doing, the State/Tribe establishes how and when the policy will be applied and what criteria will be used in its decision-making.

Key Point. An essential requirement of antidegradation policy implementation is to allow for public involvement in the decision-making. In all cases, the State/Tribe must pose the question to the community and other stakeholders: Do we want to allow the quality of this water body to degrade?

A State or Tribe's policy must be neutral regarding such questions, allowing the State/Tribe to decide about the degrading water quality based on implementation criteria.

Key Point. Taken together, the antidegradation policy and the policy's implementation method constitute the State or Tribe's antidegradation program.

Some EPA Regional Offices provide guidance on developing implementation methods.

A Three-Tiered Program

The general parameters of a State or Tribe’s antidegradation program must address three categories.

Tier 1.
Protection of water quality for existing uses.
Tier 2.
Protection of high quality waters.
Tier 3.
Outstanding National Resource Waters.

Key Point. For all tiers, a State or Tribe's review of activities that raise antidegradation considerations should be documented and subject to public review and comment.

At a minimum, States/Tribes must apply their antidegradation program to activities that are regulated under State, Tribal, or federal law, including:

  • Any activity that requires a permit or water quality certification.
  • Any activity subject to State/Tribal non-point source control requirements or regulations.
  • Any activity that is otherwise subject to State/Tribal regulations specifying that water quality standards are applicable.

Key Point. Federal antidegradation requirements do not create State/Tribal regulatory authority over otherwise unregulated activities.

Tier 1

Tier 1 of the State or Tribe’s antidegradation program must protect existing uses by maintaining the water quality necessary to support those uses. Tier 1 is applicable to all surface waters.

Existing uses are determined by considering data on both the use that has occurred and the water quality that has been achieved to support the use.

Tier 2

Tier 2 of the State or Tribe's antidegradation program protects "high quality" waters—that is, water bodies where existing water quality conditions are better than necessary to protect CWA 101(a) uses.

High quality waters must be addressed by the State or Tribe's antidegradation program because of the importance of such waters as a resource with economic, public health, and ecological value.

Key Point. Trade-offs must be carefully considered before a high quality water resource is allowed to be diminished in any way through degradation of water quality. The State or Tribe's antidegradation program must identify a procedure to be followed and questions to be addressed when confronting such issues.

Learn More. Two ways to identify high quality waters. Also, information about antidegradation review for high quality waters. Proceed to the Learn More Topic.

Tier 3

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."

EPA Review

EPA Regional Offices review new and revised antidegradation policies and antidegradation procedures developed by States and Tribes.

EPA’s key considerations for reviewing antidegradation programs:

Consideration A

Does the antidegradation policy protect existing uses, high quality waters, and ONRWs?

Consideration B

Are implementation methods identified?

Consideration C

Do protections for high quality waters apply to a broad spectrum of waters rather than just a narrow subset?

Consideration D

Is the wording used to describe the program consistent with CWA Section 131.12 (a)(1)-(4)?

Consideration E

the identified implementation methods consistent with EPA’s antidegradation provisions?


  • The existing uses of a water body must be protected.

  • Within limitations, a State/Tribe has the discretion to allow activities that could degrade water quality to a level that protects existing uses.

  • A State or Tribe's water quality standards must include an antidegradation policy and must specify both when the policy will be applied and what decision-making criteria will be used.

  • A State or Tribe's antidegradation program must include criteria specific to three categories: existing uses, high quality uses, and ONRWs.


  • Antidegradation policy can apply to both point and non-point source activities.

  • The community and other stakeholders must be given an opportunity to provide input as part of the decision-making process regarding a particular application of the policy to a water body.

  • EPA Regional Offices review both new and revised antidegradation programs developed by States and Tribes.


To complete your review of the topic in this module, please take the self-assesement quiz by reviewing each question and considering the possible responses.

A note about the quiz:

Your answers will NOT be scored or recorded. However, selecting the Submit button for each question will provide you with the correct answers on screen.

At the end of the quiz is a Get Password button. Select this after you have completed the quiz to obtain one of the six passwords you will need to obtain your Certificate of Completion at the end of the course.

Answer each of the questions below

1. Statement: A State/Tribe cannot allow an activity that will degrade current water quality once its water quality standards are established.

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2. Before deciding antidegradation questions, a State/Tribe must have an antidegradation program as part of its water quality standards. Such a program should include all but which one of the following?

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3. Statement: A State or Tribe's antidegradation policy must be identical to what EPA has outlined in the regulation (40 CFR 131.12).

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4. Along with existing uses and high quality uses, what other category of waters must a State or Tribe's antidegradation program address?

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5. Once a State/Tribe has established an antidegradation program and the program has been reviewed and approved by EPA, appropriate State or Tribal officials can decide degradation issues in closed-door sessions.

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