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Unrestricted cattle access to streams in traditionally pastoral regions has been linked to increased concentrations of bacteria, suspended sediments and associated contaminants in streams. However, there is a dearth of data available regarding the impact of cattle access to streams in poorly drained landscapes of the Midwest. In this study, we investigate changes in water quality on a 1005 m long stream section impacted by cattle grazing on the upper 130 m. We monitor discharge, water quality [nitrate, ammonium, total Kjeldahl nitrogen (TKN), total phosphorus (TP), total suspended sediments (TSS), turbidity, Escherichia coli] and chloride, atrazine, silica and major cation concentrations over a 12-month period. Cattle access to the stream does not significantly affect nitrate concentration but leads to large increases in TKN (fourfold increase), TP (fivefold increase), ammonium (fourfold increase), TSS (11-fold increase), turbidity (13-fold increase) and E. coli (36-fold increase) in the summer/fall period. In particular, E. coli concentration in the stream in the summer/fall period exceeds 235 colony forming unit (CFU)/100 ml 64% of the time upstream from the section impacted by cattle, but exceeds this same threshold 89% of the time immediately downstream. Despite the negative impact of cattle access to the stream on water quality, data indicate that dilution, in-stream processes, and natural stream geometry downstream from the impacted section help mitigate this pollution. We expect that this study will be an incentive for policy makers to promote stream rehabilitation and develop more stringent guidelines limiting cattle access to streams in many Midwestern states and other regions with poorly drained soils where the impact of cattle access to streams on water quality is often ignored.
