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INTERPRETING GEOMORPHIC, BIOLOGICAL AND CHEMICAL STREAM DATA TO DETERMINE BIOTIC STRESSORS AND WRITE A TMDL FOR AN IMPAIRED TROUT STREAM IN THE ST. CROIX RIVER WATERSHED
Jason Naber, Emmons & Olivier Resources
Brown's Creek is located in the Brown's Creek Watershed District (BCWD) in the St. Croix River watershed in Central Minnesota. Brown's Creek was listed on the 303(d) list of impaired water bodies in 2002 for aquatic life impairment based on a low Index of Biotic Integrity (IBI) for its classification as a cold water stream. Brown's Creek has an approximate 19,000-acre watershed that includes a small portion of urban land uses and significant portion of rural and agricultural areas. The first step in addressing impairment was the stressor identification process, an analysis to identify factors causing the impairment. Stressor identification is an important step that guarantees that the solutions correctly target the most important causes. Data gathered between 2000 and 2008 were compiled and analyzed by a group of experts from a wide range of sciences. This analysis was used to assess the factors leading to the biological impairments of Brown's Creek. Monitoring data were evaluated against water quality standards, guidelines based on healthy streams, and the physiology of indicator organisms like insects and trout. Correlations, monitoring data, and models of causal pathways were compared to identify mechanisms that explain the biological impairment. All available evidence was investigated using the CADDIS (Causal Analysis/Diagnosis Decision Information System) system of the EPA. This process formalizes causal reasoning in a quantitative checklist that balances the strength of evidence from a variety of sources and is a record of the reasoning behind the scientific analysis. The stressors identified by the CADDIS process were high suspended sediment, high temperature, low dissolved oxygen, and pulses of high copper concentrations. Flow and load duration curves were used to evaluate under which flow regimes the standard exceedances occur. A P8 model was built to target subwatersheds contributing high sediment loads to the creek and to evaluate implementation practices to reduce future sediment loads. Geomorphic analyses of the stream found the bed and bank to be relatively stable concluding that sediment was being derived from the watershed. Temperature exceedances were observed primarily after summer thunderstorm events where runoff from paved surfaces entered the stream. Specific reaches of the stream were also identified as lacking adequate tree cover for shading. Copper exceedances were observed more sporadically and likely linked to applications of algaecides and fungicides in the watershed prior to runoff producing precipitation events. This TMDL is in the formal review process and is one of only a few biological TMDLs written in the State of Minnesota.