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Environmental Measurements and Modeling

Streamflow Duration Assessment Method for the Pacific Northwest

The Streamflow Duration Assessment Method (SDAM) for the Pacific Northwest is a scientific tool to provide a rapid assessment framework to distinguish between ephemeral, intermittent and perennial streams.

On this page:

Who should use this method?

  • Regulators
  • Consultants
  • Natural resource specialists

What is it used for?

This information helps determine whether a stream may be subject to jurisdiction under the Clean Water Act. 

In Oregon, Washington and Idaho, many projects that require a Clean Water Act Section 404 permit may also require a permit from the state - and all three states have a joint permit application process.

This method will help inform more timely and predictable jurisdictional determinations, and is also useful where knowledge of streamflow duration improves ecological assessment, management, and decision-making.

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.

This method does not imply or represent a change in the definition of Waters of the United States under the Clean Water Act.


How was it developed?

The method results from a validation study, conducted in two phases on 264 stream reaches across the range of hydrologic landscapes of Oregon, Idaho, and Washington (Nadeau et al. 2015 Exit). In the first phase, we undertook a two-year field validation study of a 2009 Interim SDAM for Oregon. We then constructed a new, more accurate method based on statistical analysis of the Oregon field data identifying the best indicators to discriminate streamflow duration classes.

In the second phase, we conducted a one-year field study in Washington and Idaho to evaluate the applicability of the statistically-based Final Method developed in Oregon to other areas of the Pacific Northwest. The results support application of the statistically-based method, initially developed in Oregon, across the diverse hydrologic landscapes of the Pacific Northwest.

The current method, substantively the same as the 2011 Final SDAM for Oregon, summarizes the three-state study which supports the application of the method across the Pacific Northwest, and thus replaces the 2011 Final SDAM for Oregon manual with a regionally consistent manual.

The associated field guide, which identifies macroinvertebrate indicators of streamflow, results from a review of the literature and current understanding of aquatic macroinvertebrates as indicators of streamflow duration in Pacific Northwest streams. For more information, see Xerces Society - Aquatic Macroinvertebrates as Indicators of Flow Duration in Pacific Northwest StreamsExit


Additional guidance on how to apply this method

The information below is intended to clarify how to apply certain elements of this method. If you have other questions, feel free to contact us at the numbers listed above. 

  1. How is the assessment reach defined for:
    1. Unconstrained settings:
      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.
    2. Constrained settings (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.
  2. 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. The method accurately (94%; Nadeau et al. 2015 Exit) and consistently distinguishes between ephemeral and at least intermittent (i.e. intermittent and perennial) streams.
  3. 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.”
  4. 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.
  5. 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.

Contacts

EPA: Tracie Nadeau (nadeau.tracie@epa.gov), 503-326-3685
Oregon Dept. of State Lands: Peter Ryan (peter.ryan@state.or.us), 503-986-5232
U.S. Army Corps of Engineers:
  • (Portland District) Brian Zabel (Brian.J.Zabel@usace.army.mil), 503-808-4379
  • (Seattle District) Jess Jordan (Dale.J.Jordan@usace.army.mil), 206-316-3967
  • (Walla Walla District) Robert Brochu (Robert.A.Brochu@usace.army.mil), 208-522-1645; or Christen Griffith (Christen.M.Griffith@usace.army.mil), 208-433-4470.

Documents

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