Assessment of Probable Causes of Reduced Aquatic Life in the Touchet River,Washington, USA

The Touchet River in eastern Washington State is the site of the first causal assessment in the arid Northwest for salmonids using the U.S. Environmental Protection Agency's (USEPA's) Stressor Identification process. Seven candidate causes that affect salmonid density and macroinvertebrate abundance and diversity were considered: toxics, warm water temperature, sedimentation, low dissolved oxygen, alkaline pH, reduced detritus, and reduced habitat complexity. Candidate causes were evaluated using several types of evidence of preceding causation, co-occurrence, sufficiency, and alteration along with evaluation of the consistency of that evidence and consistency with other assessments. Evidence was scored, and the body of evidence was weighed based on credibility, strength, diversity, and coherence. Warm water temperature and sedimentation were highly probable causes of altered biological condition. Low dissolved oxygen and alkaline pH were also a problem for some areas but were less severe than temperature and sediment. Water removal and reduced habitat complexity and canopy cover were not directly causal but could affect sedimentation and temperature. This case study is noteworthy for using assemblage symptomology associated with temperature, sediment, and detritus as a type of evidence and for physiographically matching reference sites for comparisons and evaluation of natural and cumulative anthropogenic stressor gradients in the absence of state biological criteria.