Regulatory Developments, Special Topics in Environmental Management

A New WOTUS Definition of Tributary

Tributary is a key term in the EPA/Army Corps of Engineers’ recent Waters of the United States (WOTUS) rule, just as it was in the 2015 WOTUS rule the newer rule replaced.

Both rules provide definitions of a tributary for jurisdictional purposes—that is, which tributaries are WOTUS and therefore subject to Clean Water Act (CWA) permitting and which are not. The more recent action does not argue that most, if not all, tributaries do not contribute water to traditional navigable waters (TNWs), the fundamental CWA definition of WOTUS. But, they continue, the simple fact that a contribution to a TNW has been made in the past or occurs only during rainfall does not make such a tributary jurisdictional. Accordingly, in their new definition of a tributary, the EPA/Corps leave out several terms used in the older definition and include several new ones, which together serve to narrow the meaning of a jurisdictional tributary.

At least some of the EPA’s/Corps’ legal justification for the amended definition derives from what the U.S. Supreme Court plurality wrote in Rapanos v. United States (2006), one of the Court’s rulings addressing WOTUS.

“For instance, the plurality characterized an ‘expansive definition of “tributaries’” as including ‘dry arroyos connected to remote waters through the flow of groundwater over “centuries,”’ and described federal control over ‘irrigation ditches and drains that intermittently connect to covered waters’ as ‘sweeping assertions of jurisdiction,’” the EPA/Corp state.

Grasping the implications of the new definition requires study. The following should aid in understanding how the two definitions differ.

Water, River, or Stream

The 2015 rule states that a tributary is a “water that contributes flow, either directly or through another water” to a TNW. [emphasis added]

The 2020 tributary definition changes “water” to “a river, stream, or similar naturally occurring surface water channel.” In other words, the definition encompasses a more limited number of features through which water flows.

Flow Thresholds

The 2015 rule included no indication that a tributary is jurisdictional only if certain flow thresholds are met.

The 2020 rule states that a tributary is jurisdictional if it contributes surface water flow to a TNW in a typical year either directly or through another jurisdictional channel. A typical year means “when precipitation and other climatic variables are within the normal periodic range (e.g., seasonally, annually) for the geographic area of the applicable aquatic resource based on a rolling thirty-year period.” If a tributary is neither perennial nor intermittent in a typical year, it is not jurisdictional.

High-Water Mark

The 2015 rule said a surface feature can be identified as a tributary “by the presence of the physical indicators of a bed and banks and an ordinary high-water mark. These physical indicators demonstrate there is volume, frequency, and duration of flow sufficient to create a bed and banks and an ordinary high-water mark, and thus to qualify as a tributary.”

The 2020 definition of tributary does not include the above physical indicators of a tributary. For example, the EPA/Corps do not now hold that an ordinary high-water mark indicates a jurisdictional tributary. “Ephemeral streams in the arid West, for example, may have ordinary high-water marks that were incised years ago following a single large storm. It makes more practical sense for a feature to be first assessed as a tributary, after which the lateral extent of that tributary can be identified using the ordinary high-water mark,” the agencies state.

Nonjurisdictional Channels

The 2015 rule states: “A water that otherwise qualifies as a tributary under this definition does not lose its status as a tributary if it contributes flow through a [WOTUS] that does not meet the definition of tributary or through a nonjurisdictional water” to a TNW. [emphasis added] For example, if water from a tributary arrived at a TNW by way of a channel that is not jurisdictional, the tributary cannot be considered nonjurisdictional for that reason.

The 2020 definition of tributary does not entirely change this part of the 2015 definition, but it places conditions on it. Specifically, the 2020 definition states: “A tributary does not lose its jurisdictional status if it contributes surface water flow to a downstream jurisdictional water in a typical year through a channelized non-jurisdictional surface water feature…. The term tributary includes a ditch that either relocates a tributary, is constructed in a tributary, or is constructed in an adjacent wetland as long as the ditch satisfies the flow conditions of this definition.” [emphasis added] When compared with the 2015 definition, the addition of the terms “typical year” and “flow conditions” places new restrictions on which tributaries are jurisdictional if they pass through nonjurisdictional waters or do not flow directly into a TNW.

Unlike the 2015 rule, the new definition does not consider ephemeral tributaries jurisdictional, even if they have a high-water mark. Water flows in ephemeral features flow only in direct response to precipitation.