ScholarWorks at University of Montana

Nutrient Loading and Proposed BMP’s for Browns Gulch

Nutrients are essential to support stream ecosystems, however, if present in excess may lead to algal blooms, excessive aquatic weeds, and alternation of natural aquatic ecosystems. Silver Bow Creek (SBC), the headwater stream of the Clark Fork River, is listed as impaired for nutrients (total nitrogen (TN) and total phosphorus (TP)), by the Montana Department of Environmental Quality. Browns Gulch is a major tributary to SBC, and drains agricultural and forested lands. To meet target nutrient TMDL concentrations in SBC, the tributary load inputs of TN and TP must be reduced by 93% in Browns Gulch. To identify the sources of nutrients, surface water samples were collected and analyzed for TN and TP at three flow stages from locations distributed along the stream. Browns Gulch water quality data suggests that runoff from agricultural lands (during spring and summer) is the main source of TN and TP loads. For all the flow stages, TN and TP loads increase from up to down-stream. Irrigated and grazed areas correspond with a sharp increase in the stream nutrient load. Specific conductivity and alkalinity concentrations were highly correlated with TP concentrations at each flow stage. The Lowland Creek Volcanics are the predominant geologic formation in the Browns Gulch watershed and may be contributing consistent low levels of TP via groundwater. To reduce agricultural nutrient inputs, riparian buffer zones and off-stream winter pastures are recommended. It is believed that effective implementation of the proposed BMPs will significantly reduce tributary nutrient load inputs to SBC.