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Summary: The Streamflow Duration Assessment Method for Oregon is a scientific tool developed by EPA and the U.S. Army Corps of Engineers to provide a rapid assessment framework to distinguish between ephemeral, intermittent and perennial streams.
This information helps determine whether a stream may be subject to jurisdiction under Section 404 of the Clean Water Act. In Oregon, many projects that require a Section 404 permit also require an Oregon Department of State Lands permit under Oregon’s Removal-Fill Law and there is a joint permit application process. The method does not imply or represent a change in the definition of "Waters of the United States."
Who should use the method? Regulators, consultants and natural resource specialists are encouraged to use the method. It provides information the Corps and Oregon Department of State Lands need to make more timely and predictable jurisdictional determinations. The method may also provide an initial framework to address other stream-related issues such as stream function and compensatory mitigation, and to incorporate relevant tools and procedures as they develop. While use of this method may inform a more robust stream assessment, it was specifically developed for the purpose of determining streamflow duration and does not provide a stand-alone assessment of stream function or condition.
Background: An interim version of the method was released on March 6, 2009 for a trial implementation period. The method has been field tested by interagency teams of state and federal scientists, and was also the subject of a two-year validation study, in cooperation with EPA’s Office of Research and Development, Western Ecology Division, testing the method at more than 170 sites in western and eastern Oregon in both the wet and dry seasons.
As a result of the validation study, the initial 21 field indicators used in the interim version were reduced to just seven reliable indicators in the final version. Results of the validation study are being published separately.
Frequently Asked Questions
- How is the assessment reach defined if I am in an unconstrained setting?
Assessments should be made over the length of a stream reach, rather than at one point. The assessment reach chosen should be representative of the stream, and should have a length equivalent to 35 - 40 channel widths. For narrow streams, the length of the assessment reach should be a minimum of 30 meters. We recommend walking the stream prior to choosing an assessment reach.
- How is the assessment reach defined if I am in a constrained setting (e.g. within highway right-of-way or a single tax lot)?
If it is not possible to assess a reach that is 35 to 40 channel widths in length, then apply the method for the entire length of stream that you can access. Make a note of your reach length and constraints on the data sheet. If possible, inspect upstream and downstream sections of the stream and make a note of your observations.
- Are the results from using this method the same as a jurisdictional determination?
No. Jurisdictional determinations are made by the regulatory agencies based on current guidance and policy, and the method is not meant to be a jurisdictional determination. Rather, the method provides information necessary for the agencies to make timely decisions because it helps determine the duration of streamflow. We anticipate the method will be most useful in distinguishing between ephemeral and intermittent streams. For example, the Corps and EPA assert Clean Water Act § 404 jurisdiction over ephemeral streams on a case-by-case basis, after assessing whether there is a “significant nexus” to downstream waters. Thus, the method (1) provides substantive evidence to support categorizing a stream as ephemeral, intermittent, or perennial, (2) informs the determination as to whether the “significant nexus” test is necessary, and (3) provides information relevant to the functions performed by the stream. For further information on the Rapanos decision and “waters of the United States” covered by the Clean Water Act.
- Can this method be applied to ditches?
Yes, the method can be used, in combination with best professional judgment, to assess the flow duration of ditches. Our observations are that some hydrologic and biologic indicators tend to persist (wetland plants, for instance, may occur on the margins of the ditch) even with frequent maintenance of ditches. When applying the method in ditches or modified streams be sure to note that the site is a “disturbed site/difficult situation” and describe it in the “notes.”
- What part of the stream do I assess when working in a braided stream system?
Identify the extent of the channels, based on the outer limits of ordinary high water marks, and apply the method to that area as a whole. Some indicators may be present or more apparent in the main channel versus the side channels; make a note of those differences on the field assessment form.
- Is the method applicable when a stream goes subsurface in a portion of the assessment reach?
Streams observed flowing subsurface during the assessment visit may flow on the surface during wetter times of the year; therefore, it is important to check the entire reach for indicators of streamflow. In addition, the accuracy of an assessment can be improved by conducting a follow-up visit during a wetter time of the year.
Clean Water Act jurisdiction:
Links to partner information:
For more information, contact:
- EPA: Tracie Nadeau (email@example.com), 503-326-3685
- U.S. Army Corps of Engineers: Tina Teed (Tina.J.Teed@usace.army.mil), 503-808-4384
- Oregon Department of State Lands: Peter Ryan (firstname.lastname@example.org), 503-986-5232