Elevated nutrients, orthophosphate and nitrate, in the Lower Boise River (LBR) in southwest Idaho may be attributed to contamination from agriculture runoff draining into tributaries and/or waste water treatment plants (WWTP). To better understand the sources and associated spatial and temporal dynamics of nutrient loading to the LBR, a series of high resolution synoptic sampling events were conducted. Six separate synoptic sampling events were conducted along approximately 65 miles, from upstream near the city of Boise to the confluence with the Snake River, seasonally from July 2012 to October 2013 with a sampling increment of approximately 0.5 miles. The samples were then analyzed for the anions chloride, nitrate, orthophosphate, and sulfate. The resulting dataset shows overall trends of increasing concentrations for all anions with distance downstream, with chloride increasing an average of 17.2 mg L-1, nitrate by 3.5 mg L-1, orthophosphate by 0.15 mg L-1, and sulfate by 33.1 mg L-1. The most dramatic increases in nutrient concentrations are observed at WWTP discharges, with the greatest increases at the West Boise WWTP (Mile 17.1) with average increases of 0.32 mg L-1 for orthophosphate and 1.8 mg L-1 for nitrate, and the Caldwell plant (Mile 41.4) with average increases of 0.05 mg L-1 for orthophosphate and 1.7 mg L-1 for nitrate. Importantly, there was little evidence of significant increases in nutrient concentrations from any of the major tributaries not associated with WWTPs
