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Streamflow Duration Assessment

Learn about Streamflow Duration Assessment Methods (SDAMs)

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Basic Information about SDAMs

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. Read more about the scientific basis for SDAMs.ExitRegulators 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. SDAMs have proven to be highly accurate; the Pacific Northwest SDAM, for example, correctly classified 84% of observations from a three-state study area and distinguished between ephemeral and intermittent/perennial streamflow with 94% accuracy. View the 2015 Streamflow Duration Assessment Method for the Pacific Northwest (Nadeau 2015) (PDF).

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SDAM Classifications

Different regulatory and scientific definitions have been assigned to the terms “perennial,” “intermittent,” and “ephemeral” streamflow, including definitions under the Navigable Waters Protection Rule. For purposes of the SDAM (see, e.g., Nadeau 2015), perennial streams flow continuously during a year of normal rainfall, often with a streambed located below the water table for most of the year, and intermittent streams flow for only part of the year, typically during a wet season when the streambed may be below the water table or when melt water from snow provides sustained flow. Nadeau (2015) also states that “perennial streams flow year-round during a typical year.” The SDAM defines ephemeral streams as those that flow only in direct response to precipitation including rainstorms, rain on snow events, or snowmelt.

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Regional Development of SDAMs

Existing SDAMs have been developed by state and federal agencies for a variety of purposes, for example in North Carolina to implement riparian buffer regulations, and in New Mexico to apply water quality standards. An SDAM for the Pacific Northwest, developed collaboratively by the U.S. EPA, the U.S. Army Corps of Engineers, and Oregon’s Department of State Lands, has been used since 2011 to inform state and federal regulatory decisions in the region. View examples of existing local and regional SDAMs. 

Regionally specific SDAMs account for important differences and geographical variation in climate, geology, biogeography, and topography that can influence the relationships between field indicators and streamflow duration. To be more reliable and accurate, it is necessary to tailor the methods to regional conditions. 

Currently, the EPA and partners are developing or modifying existing SDAMs for use across the U.S. Read more about the operational framework for developing SDAMs.ExitDeveloping a regionally specific SDAM generally includes the following steps:

  • Preparation: Literature review, identification, and evaluation of potential hydrologic data sources, and consultation with federal, state, and academic partners to identify potential indicators of flow duration and select sites for baseline data collection and validation studies.
  • Baseline Data Collection: Instrumentation of sites with conductivity loggers (which can detect the presence of water) for a minimum of one year to directly measure flow duration, with at least three site visits to collect streamflow indicator data during this time.
  • Validation Study: Collection of streamflow indicator data at sites with independently assigned flow duration classification (perennial, intermittent, or ephemeral) across the region. Validation study sites are in addition to, yet independent of, baseline sites in the same region. 
  • Method Development: Data analysis to develop a regionally specific method. This step also includes internal peer-review and interagency comment prior to release of the beta method.
  • Rollout: Engagement with stakeholders, as well as technical support and training for staff. A public comment period on the beta method, external peer-review, and any final revisions are also included in this step.
  • Continuous Baseline Sampling: Instrumentation is maintained at all baseline sites, and data collection continues on an annual basis to ensure that method development was not biased by interannual climatic and streamflow variation.

As SDAM development has expanded, the study regions for SDAM development have grown due to an increased understanding of SDAM indicators and the need for rapid SDAMs across the U.S. The Pacific Northwest method was developed first and covers the states of Idaho, Oregon, and Washington as defined in the SDAM for the Pacific Northwest. Arid West and Western Mountain method development began in 2018, with geographic scope defined based on the regions outlined in the U.S. Army Corps of Engineers regional wetland plant lists and wetland delineation manuals. View the National Wetland Plant List: 2016 Wetland Ratings (PDF).

Data collection efforts for the Northern Plains and Southern Plains (collectively referred to as the Great Plains) as well as the Northeast and Southeast regions are based on gradients in precipitation as outlined in the U.S. Army Corps of Engineers Ordinary High Water Mark Scientific Support Document. View the 2016 Technical Report, Synthesizing the Scientific Foundation for Ordinary High Water Mark Delineation in Fluvial Systems (PDF). Data collection and method development may be conducted separately for Alaska, Hawaii, and the U.S. Territories. To provide an accurate representation of the range of conditions found in larger regions, sub-regions may be developed during the data analysis and method development processes.

Please see the SDAM Status by Region webpage for more information about ongoing SDAM studies.  

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Examples of Existing Local and Regional SDAMs

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