On September 9, 2026, the U.S. Environmental Protection Agency (EPA) and U.S. Army Corps of Engineers (Corps) (collectively, the Agencies) published a supplemental notice of proposed rulemaking (Supplemental Proposal) addressing the definition of “waters of the United States” (WOTUS). The Supplemental Proposal presents a perennial-water alternative to the “wet season” framework the Agencies announced in their November 2025 proposed rule, thereby further narrowing federal Clean Water Act (CWA) jurisdiction. Comments are due October 9, 2026.

The change responds to a key issue raised during the comment period for the 2025 Proposed Rule, which would treat a water feature as federally jurisdictional only if it held or conveyed water “year-round or at least during the wet season.” As discussed in our December 2025 legal alert, that proposal did not define “wet season;” the term did not appear in the Supreme Court’s Sackett v. EPA decision, and commentators criticized the approach from all sides.

Background

The 2025 Proposed Rule was the first comprehensive regulatory effort to reconcile Sackett with the Agencies’ prior definition of WOTUS. That 2025 Proposed Rule limited federal jurisdiction, in part, to waters that are “relatively permanent” (defined as present year-round or during the “wet season”) and wetlands that physically abut those waters with a “continuous surface connection.” It also narrowed jurisdictional categories, excluded groundwater, and clarified exclusions for certain ditches, prior converted cropland, and waste treatment systems.

Perennial Water Alternative

To address concerns about the poorly defined and difficult-to-implement “wet season” framework, the Supplemental Proposal would require that a water feature be “perennial” to qualify as a “relatively permanent” water under federal jurisdiction. The Agencies seek comment on defining “perennial” in one of two ways:

• “Having standing or continuously flowing water every day of the year during ordinary conditions.”
• “Surface water flowing continuously year-round,” consistent with the 2020 Navigable Waters Protection Rule.

The proposal distinguishes between anomalous and non-anomalous interruptions in flow:

• Anomalous (extraordinary) events, such as drought or an unusual dry spell, would not affect a water body’s jurisdictional status, regardless of duration.
• Non-anomalous (ordinary) events, such as low tides or a regularly occurring dry spell, would preserve perennial status only if the interruption occurs no more than once in a given year and lasts no more than 30 consecutive days. A water feature that experiences two or more ordinary dry periods in a single year would not qualify as perennial, even if each period is short.

For example, a stream that ordinarily goes dry for 20 consecutive days each August could remain perennial under the supplemental approach. By contrast, a stream that experiences two ordinary 10-day dry periods in the same year would likely not qualify.

Wetlands

The Supplemental Proposal would also narrow the proposed “continuous surface connection” definition for wetlands. A wetland would need to contain perennial surface water that is continuously connected to surface water in a jurisdictional water, such that it is difficult to distinguish where the jurisdictional water ends and the wetland begins. A separate alternative would go further by limiting jurisdictional wetlands to the portion that is “indistinguishably part of” an otherwise jurisdictional water. It would also remove “adjacent” from the definition of “continuous surface connection.”

The Agencies acknowledge that these supplemental definitions would substantially reduce CWA jurisdiction. The Supplemental Proposal states that the majority of wetlands currently jurisdictional under existing rules would not satisfy the proposed “continuous surface connection” standard, as most wetlands do not contain perennial surface water. The Agencies further anticipate that the arid West in particular would see pronounced reductions in federally regulated waters, and that the Section 404 program would experience the most programmatic changes, with projected reductions in the number of permits and required mitigation.

Continued Uncertainty

The Supplemental Proposal is not final. The Agencies have retained flexibility to pursue the 2025 approach, adopt a supplemental alternative, or combine elements in the two notices. The current comment period is limited to the supplemental alternatives; the comment period on the November 2025 proposal has closed.

Whichever approach the Agencies ultimately adopt, litigation risks remain, and implementation will likely remain challenging as jurisdictional determinations will require site-specific assessments that turn on the nature of the receiving water and complex factors such as ordinary hydrologic conditions, physical connections, and site features (e.g., roads, berms, culverts, ditches, or modified drainage).

Meanwhile, federal jurisdiction remains only part of the permitting analysis. As discussed in our June 2025 legal alert, a feature that falls outside federal CWA jurisdiction may still be protected under state or local law. In California, for example, waters that are not WOTUS may still be subject to the Porter-Cologne Water Quality Control Act and related State Water Board requirements.

Looking Ahead

Allen Matkins will continue to monitor these developments. Please contact our Environmental and Natural Resources teams with questions or for project-specific guidance.