An Evaluation of Designated Aquatic Life Uses of the South Fork Palouse River Watershed, WA

The South Fork Palouse River drains a 130 mi² watershed, originating to the east of Moscow, Idaho and flowing west to Colfax, Washington before entering the Palouse River. Land conversion, primarily to dry land agriculture and urban uses, has led to the elimination of wetland and riparian habitats throughout the watershed, affecting instream flows, channel sinuosity, and habitat diversity. As a result, segments of the streams are currently reported under each state’s §303(d) list for impairments that include dissolved oxygen, fecal coliform bacteria, polychlorinated biphenyls (PCBs) and dieldrin, pH, and temperature. These impairments and the subsequent development of total maximum daily loads (TMDLs) have prompted a reassessment of the watershed’s designated aquatic life uses. Thus, ecological conditions were evaluated over three distinct time periods: pre-European, 1975 to current, and current conditions.  Pre-European to current physical, chemical, and biological (fish and benthic macroinvertebrate) information was obtained through extensive literature reviews and compared with current data collected as part of a dedicated probabilistic survey in 2010. Altered streamflow and concomitant changes in water quality and sediment transport are the most notable changes over time. Current fish community structure reflects the influence of degraded habitat, contributing to favorable conditions for non-native introductions that comprise nearly 50 percent of the species collected during recent sampling efforts. Similar to historical accounts, native speckled dace (Rhinichthys osculus) and redside shiner (Richardsonius balteatus) are the dominant species; however, several native species appear extirpated. With exception to isolated populations of rainbow trout (Oncorhynchus mykiss), salmonines are restricted from the watershed by Palouse Falls, which effectively limits upstream movement from the Snake River drainage. Taxa comprising the family Chironomidae were the most abundant benthic macroinvertebrate collected, indicating little change from previous studies. Results suggest that the existing designated use does not reflect historical environmental conditions and may not be suitable for current biological communities.