Federal antideg reg at CFR 131.12. States must have both an “antidegradation policy” and “methods for implementing” the policy.  Tier I: “Existing instream water uses and the level of water quality necessary to protect the existing uses shall be maintained and protected.” Tier II: Where “quality of the waters exceed levels necessary,” degradation allowed only after meeting certain criteria. Tier III: No degradation of ONRWs allowed. Tiers will be explained in following sections.

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Antidegradation Regulation

Although the regulations don’t use the “tier” terminology, they do describe the three levels of protection noted previously. Tier 1—protecting existing uses—is fairly straightforward, as is Tier 3—preventing degradation in “outstanding national resource waters,” or ONRWs. Tier 2 is a bit more complex . . . . Let’s start with Tier 3.

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