Clearwater River Watershed Stressor Identification Report

The Minnesota Pollution Control Agency (MPCA) follows a watershed approach to systematically
monitor and assess surface water quality in each of the state’s 80 major watersheds. The Lost River AUID 646 (Section 4.8) was investigated because the M-IBI score at site 05RD046 was lower during the 2015 sampling event than during the 2005 sampling event. The AUID passed the fish and macroinvertebrate IBI; however, signs of biological community change were evident in the macroinvertebrate community. Lower Badger Creek was also investigated during the stressor identification process. Originally, the AUID was failing for fish and full support for macroinvertebrates. Additional information was collected in 2016 that placed this AUID in the Modified Use category based on an updated MSHA score. The fish are now meeting the standard. This AUID is discussed later in the report (Section 4.10). This report identifies six candidate causes, or “stressors,” that are likely contributing to the nine aquatic life impairments throughout the Clearwater River watershed. Those candidate causes are: low dissolved oxygen (DO), flow alteration, lack of physical habitat, eutrophication (elevated phosphorus levels combined with high daily DO flux), and lack of longitudinal connectivity (prohibiting fish passage). A causal analysis procedure was performed to evaluate connections between each of the six candidate causes and the aquatic life impairments. Table 1 provides a summary of the stressors identified in the biologically impaired reaches of the Clearwater River watershed.