This method assesses a suite of indicators to determine whether a stream is ephemeral, intermittent or perennial.  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.

Xerces reviewed the current literature on using macroinvertebrates as indicators of stream duration in Oregon, and provided a recommended aquatic invertebrate taxa list, field methodology, scoring protocol, and field guide to inform the macroinvertebrate scoring portion of the interim version released in 2009.  Following extensive field testing, the initial 21 field indicators used in the interim version, which included a suite of macroinvertebrate indicators, were reduced to seven reliable indicators in the final version—three of which relate to aquatic macroinvertebrates.