Otter Tail River Watershed Stressor Identification Report

In 2016 and 2017, the MPCA conducted biological monitoring at several stations throughout the Otter Tail River Watershed (OTRW). An Index of Biological Integrity (IBI) score was calculated for each fish (F-IBI) and macroinvertebrate (M-IBI) monitoring visit. The biological monitoring results for the OTRW were then assessed to identify individual stream reaches that were not supporting a healthy fish and/or macroinvertebrate assemblage. A reach with a low IBI score(s) (i.e., below an established threshold) is considered "impaired" (i.e., unable to support its designated beneficial use) for aquatic life. A total of six reaches were determined to have a F-IBI and/or M-IBI impairment in the OTRW, including segments of the Otter Tail River, Pelican River, Toad River, and Judicial Ditch 2. This report identifies the probable causes, or "stressors", that are likely contributing to the biological impairments in the OTRW. Five candidate causes were examined as potential stressors for the biologically impaired reaches: loss of longitudinal connectivity, flow regime instability, insufficient physical habitat, high suspended sediment, and low dissolved oxygen (DO). Causal analysis was then performed to determine and evaluate connections between each candidate cause and the biological impairments.