Click on “Download PDF” for the PDF version or on the title for the HTML version.

If you are not an ASABE member or if your employer has not arranged for access to the full-text, Click here for options.

Water resource managers must assess how variations in water quality impact designated uses. If streams are listed as water-quality impaired when they experience natural or very modest excursions from listed water quality criteria then there is a dilution of resources directed at streams that are legitimately listed. Here we discuss infrequent, brief, and small excursions to water quality criteria and how some states are trying to factor minor or natural water quality excursions into assessments. We further discuss water quality variability in time, space, and scale, and how this variability affects the biological response of organisms. This review shows that frequency, duration, and extent of water quality exceedances all combine to influence biological responses. Many aquatic organisms are adapted to avoid or tolerate infrequent, short-duration, and minor excursions. Recognizing when and where excursions from water quality criteria will have minor biological and/or designated use impacts allows water resource managers to focus on more serious water quality problems.