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 stream flow at the reach scale.

• What is the method used for?
• How was the method developed?
• Additional information about on how to apply the method
• Contacts
• Documents, Tools, and Data

What is the Method Used For?

Long-term hydrologic data to assess streamflow duration is often limited, especially for streams that do not flow year-round. SDAMs are rapid field assessment methods that use hydrological, geomorphological, and/or biological indicators, observable in a single site visit, to classify streamflow duration as perennial, intermittent, or ephemeral at the reach scale. Regulators and water resource managers can use rapid, reach-scale methods to determine streamflow duration classifications (i.e., perennial, intermittent, ephemeral) and to help implement many federal, state, and local programs.

For instance, this information could help determine whether a stream may be subject to jurisdiction under the Clean Water Act, although use of the method is not a requirement for determining jurisdiction.

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 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 class 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 the Method Developed?

The SDAM for the Pacific Northwest 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). 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 2015 SDAM for the Pacific Northwest, 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 (Nadeau 2018), 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 - Macroinvertebrate Indicators of Streamflow Duration.

Additional Information on How to Apply the 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 below.

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 federal law and current guidance and policy, and the method outputs are not a jurisdictional determination. Rather, the method provides information that may be useful for the agencies to make timely decisions because it helps determine the duration of streamflow. The method accurately (94%; Nadeau et al. 2015) 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. The agencies have observed 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

• (Portland District) Andrea Seager (andrea.r.seager@usace.army.mil), 541-465-6882.
• (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, Tools, and Data

• User Manual for the SDAM for the Pacific Northwest (pdf) (2.39 MB)
• Streamflow Duration Field Assessment Form (pdf) (189.68 KB)
• Field Guide to Macroinvertebrate Indicators of Streamflow Duration: Oregon, Washington, and Idaho (pdf) (21.6 MB, June 2012)
• Journal Article Describing the Development of the SDAM for the Pacific Northwest. 
• Data Used to Develop the SDAM for the Pacific Northwest. United States Environmental Protection Agency (2022). HydroShare.  
• 2011 Final Streamflow Duration Assessment Method for Oregon (pdf) (3.17 MB, November 2011, EPA-910-R-11-002)
  (for archival reference only)
• 2009 Interim Streamflow Duration Assessment Method for Oregon (pdf) (5.37 MB, March 2009)
  (for archival reference only)