WET personnel collected water quality samples in the Upper and Lower Jefferson River TMDL Planning Area for Montana DEQ during the spring and summer of 2011. The spring event was conducted in June 2011 near the peak flow period which coincided with record or near-record flows at many sites within the watershed. The summer sampling was conducted in September 2011 during low-flow conditions. WET personnel were responsible for contacting landowners and establishing access to sites prior to conducting the field sampling. The data collection effort included sampling for field constituents (water temperature, pH, specific conductance, dissolved oxygen), metals (total recoverable and dissolved), and total suspended sediment. A total of 35 sites were sampled throughout the watershed from the Jefferson River and numerous tributary streams. Samples for quality assurance were also collected, including trip blanks, field blanks and duplicate samples. Discharge was measured at each location using a combination of methods, including flow meter, USGS gauges, or the float method when the stream could not be safely waded. Data were evaluated for compliance with quality control/quality assurance benchmarks established in the project sampling and analysis plan, including representiveness, comparability, completeness, sensitivity, precision, accuracy, and bias. The final data set was uploaded to the state’s water quality database (EqUIS).