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This action would update the Stormwater Phase II Rule, which automatically covers, on a nationwide basis, all small Municipal Separate Storm Sewer Systems (MS4s) located in “urbanized areas” as determined by the latest U.S. Decennial Census. In March 2022, the Census Bureau finalized its latest regulatory revisions based on the 2020 Decennial Census and no longer uses the term “urbanized areas.” This action would update the regulatory definition of small MS4 by replacing the retired term “urbanized area” with language consistent with the long-standing definition of “urbanized area.” This would eliminate potential uncertainty that would be caused by continuing to use a term that cannot be found in the latest Decennial Census. This action would consist of a clarification that permittees will continue to be automatically designated, based on the most recent census data, as Phase II MS4s in the same manner that they have been since the promulgation of the Phase II stormwater regulations. Consistent with the 1999 Stormwater Phase II Rulemaking, a small MS4 that is automatically designated into the NPDES program for stormwater under a past Census year will remain regulated regardless of the results of subsequent Census calculations (64 CFR 68751). This action would establish a clear, lasting, and nationally consistent way to ensure that discharges from MS4s are adequately controlled as needed “to protect water quality,” consistent with CWA 402(p)(6).