The U.S. Environmental Protection Agency, Oregon Operations Office, and the U.S. Army Corps of Engineers, Portland District have developed a procedure to determine the duration of flow in Oregon’s streams, the Oregon Streamflow Duration Assessment Method Interim Version (B. J. D. Topping, T. L. Nadeau, and M. R. Turaski, 2009). The method will be further tested for a year before it is finalized, and the agencies encourage practitioners and the public to comment on their experiences in using it.

This assessment method distinguishes between ephemeral, intermittent and perennial streams by assessing and scoring over 20 different geomorphic, hydrological, and biological indicators. Examples of indicators include channel sinuousity, bed and bank structure, soil texture, debris piles, wetland plants, macroinvertebrates, amphibians, and fish. Users are trained in identifying and rating these indicators to ensure consistency of results among different practitioners. Stream type is determined by evaluating each individual indicator and assigning a corresponding score. These scores are then summed to generate a single total numeric score, with higher scores indicating intermittent and perennial streams. This method can be used to distinguish between the three types of streamflow duration, but is primarily designed to distinguish ephemeral streams from intermittent and perennial streams in a single site visit.

Following the U.S. Supreme Court’s recent decision in Rapanos, duration of flow is important in determining whether a stream falls under Clean Water Act regulatory jurisdiction of the U.S. EPA and U. S. Army Corps of Engineers under Section 404 (discharging dredge or fill material into waters of the United States). In Oregon, many projects requiring a Section 404 permit also require an Oregon Department of State Lands permit under Oregon’s Removal-Fill Law. Ephemeral streams are fed primarily by stormwater runoff and flow only for a short time in direct response to precipitation. Intermittent streams flow for a more sustained period of time than ephemeral streams, usually in winter and spring when the stream bed is below the water table, and may also be fed by snowmelt from surrounding uplands. Perennial streams contain water continuously during a year of normal rainfall, and are fed by groundwater supplemented by snowmelt and stormwater runoff. It is important to implement a standardized method to accurately determine the duration of streamflow at potential project sites.

As part of the development process for the Oregon Streamflow Duration Assessment Method Interim Version, Xerces staff was invited to participate in a field verification workshop of an earlier version of the method in September 2007. In 2008, the U.S. EPA and Army Corps of Engineers contracted with the Xerces Society to review the current literature on using macroinvertebrates as indicators of stream duration, and to provide a recommended aquatic invertebrate taxa list, field methodology, scoring protocol, and field guide that could be used to inform the macroinvertebrate scoring portion of the Oregon Streamflow Duration Assessment Method Interim Version. Read the full report.

Acknowledgements
The Xerces Society would like to thank the following individuals who contributed their expertise, including:
• Larry Eaton, North Carolina Division of Water Quality, Raleigh NC
• Ken Fritz, U. S. Environmental Protection Agency, Cincinnati OH
• Bill Gerth, Faculty Research Assistant, Oregon State University
• Jim Johnson, Odonatist, Vancouver WA
• Judith Li, Professor Emeritus, Oregon State University, Corvallis OR
• Dennis Paulson, Director Emeritus, Slater Museum of Natural History, Seattle WA
• Perianne Russell, North Carolina Division of Water Quality, Raleigh NC
• Robert Wisseman, Aquatic Biology Associates Inc., Corvallis OR

Additional information
Oregon Streamflow Duration Assessment Method
Clean Water Act Jurisdiction: Waters of the United States , U. S. Environmental Protection Agency
The Clean Water Act, U. S. Environmental Protection Agency
The Clean Water Act: An Owner’s Manual, 2nd edition, River Network
Field Operations Manual for Assessing the Hydrologic Permanence and Ecological Condition of Headwater Streams, Fritz, K.M., Johnson, B.R., and Walters, D.M., 2006. EPA/600/ R-06/126. U.S. Environmental Protection Agency, Office of Research and Development, Washington DC
Identification methods for the origins of intermittent and perennial streams, Version 3.1. North Carolina Department of Environment and Natural Resources, Division of Water Quality, Raleigh, NC, 2005