CADDIS Volume 3: Examples & Applications

Identification of Most Probable Stressors to Aquatic Life in the Touchet River, Washington (Final)

EPA announced the availability of the final report, Identification of Most Probable Stressors to Aquatic Life in the Touchet River, Washington. This study includes the screening causal assessment of the Touchet River, a sub-watershed of the Walla Walla River in eastern Washington State, is the first application of the United States Environmental Protection Agency (USEPA) Stressor Identification (SI) process to a long stretch of river or to the Northwest.

The U.S. Environmental Protection Agency (U.S. EPA)’s Stressor Identification (SI) procedures were used to identify and prioritize factors causing biological impairment and to develop effective restoration plans for this river.  Six sites were sampled along the Touchet River over a two-year period; parameters measured included Washington State Department of Ecology (WSDE) benthic macroinvertebrate assemblage metrics and physical habitat measures; chemical analysis of pesticides and other pollutants; and in-situ temperature and pH measurements.  Nearly every measure of biological condition declined from upstream sites to downstream sites.

Seven candidate causes of biological impairment were considered: toxics, low dissolved oxygen (DO), alkaline pH, water temperature, sedimentation, reduced detritus, and reduced habitat complexity.  Toxic chemicals for which there were measurements did not exceed aquatic life criteria, and the low levels of pesticides were not considered a stressor by toxicologists.  Dissolved oxygen data rarely exceeded aquatic life criteria and were judged unlikely to cause the observed effects. The remaining candidate causes were evaluated based on multiple types of evidence from the case (spatial/temporal co-occurrence, stressor-response relationships from the field, and causal pathways) and from elsewhere (stressor-response from other field studies, mechanistically plausible cause) and consistency of evidence and consistency with other assessments.  Evidence was qualitatively scored and the body of evidence was weighed based on consistency of the evidence and best professional judgment of the ecology of the region.  Evidence corroborated temperature and sedimentation as highly probable causes of biological impairment.  Alkaline pH was also implicated for some areas but was judged to be less severe than temperature and sediment.  Reduced habitat complexity and cover were not directly causal, but could lead to sedimentation and warmer water. 

This screening causal assessment of the Touchet River is the first application of the SI process to a long stretch of river or to the Northwest.  To do this, several physiographically matched reference sites were used for comparisons and evaluation of natural and cumulative stressor gradients.  It is also the first USEPA report of a case using the SI method in an arid system or for endangered salmonids.  Thus, this case illustrates the range of applicability of the USEPA methodology. The assessment identified probable causes of impairments to macroinvertebrates and by association, certain salmonids. 


U.S. EPA. Identification of Most Probable Stressors to Aquatic Life in the Touchet River, Washington (Final). U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-08/145, 2010.


Oct 2007An external peer review was conducted by an independent contractor.
Jun 2010EPA released the final report.

This document has been reviewed in accordance with U.S. Environmental Protection Agency policy and approved for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.