The objective of this report was to evaluate the environmental data available for the Ann River watershed to diagnose the probable causes of biological impairment. Numerous candidate causes for impairment were evaluated using U.S. Environmental Protection Agency's (EPA's) Causal Analysis/Diagnosis Decision Information System (CADDIS), Minnesota Pollution Control Agency's (MPCA's) biological Total Maximum Daily Load (TMDL) protocols, and weight of evidence analysis. The results of the Stressor Identification analysis pointed to five probable causes for the biological impairment in the Ann River. Loss of habitat due to sedimentation appears to be most problematic in the lower reaches of the river, which are lower in gradient and serve as depositional areas for sediment from upstream sources. Observations made during stream reconnaissance efforts indicate that agricultural land-uses (primarily cattle grazing) are a significant source of sediment delivery in the watershed. In addition, historical logging, and the use of the Ann River as a log driving waterway may also play a role in present day sediment dynamics. Sediment deposition in the lower Ann River reduced pool and riffle habitat quality, which resulted in a lack of game fish and fish species that depend on coarse substrates for feeding and reproduction. Synoptic longitudinal and continuous (diurnal) measurements for dissolved oxygen were conducted at monitoring stations on the Ann River and Little Ann River during the summers of 2007 and 2008. The data indicates that dissolved oxygen concentrations in the Ann River occasionally drop below the standard of five mg/L during mid to late summer months. Most of the standard violations occurred in the early morning hours before sunrise. This stressor appears to be systemic in nature (watershed-wide) and may be linked to an altered flow regime, increased water temperatures resulting from a lack of stream shading, and climactic events (i.e. drought). Changes in channel morphology are considered a candidate stressor in the Ann River watershed due to the degradation of riparian buffers and habitat. This disturbance appears to be causing increases in channel width to depth ratios, loss of pool and riffle habitat, decreases in stream shading and woody debris inputs. Channel widening, gully formation, and other erosion processes within the stream corridor appear to be contributing higher than normal sediment loads to the river. Several impoundment structures located in the Ann River watershed may be altering stream flow and/or impeding fish passage. Extreme low flows were observed in 2007 and 2008 as much of the area experienced drought condition. Although sustaining adequate base flows are an issue that many undisturbed streams in the region face as well, we hypothesize that the impoundments are exacerbating low flow conditions and potentially stressing fish and invertebrate life in the river. Further evaluation of these conditions is needed to determine if they were the result of climactic events or flow alteration due to the impoundment of Ann Lake. These five stressors and their connections to biological impairments on the Ann River will be evaluated in this report. The EPA's Stressor Identification (SID) and CADDIS will be used to determine key stressors and their sources.
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Minnesota Pollution Control Agency (St. Paul, Minnesota)
Minnesota Pollution Control Agency