As part of the Triangle Area Water Supply Monitoring Program, the U.S. Geological Survey conducted a study to evaluate the occurrence and distribution of mercury in 14 streams and 15 reservoirs in the Triangle area of North Carolina. This Data Release contains the associated data described in the Scientific Investigations Report, "Occurrence and Distribution of Mercury in Streams and Reservoirs in the Triangle Area of North Carolina, July 2007 - June 2009". Water samples were collected seasonally between July 2007 and June 2009 for laboratory analysis of mercury, organic carbon, chlorophyll a, sulfate, and suspended sediment. Field measurements of dissolved oxygen, pH, water temperature, and specific conductance were also conducted at the sites. Bed sediment was sampled at most sites for analysis of mercury, organic carbon, and weight percent of particles smaller than sand-sized (0.0625 millimeters). Quality-assurance samples, including blanks (field, equipment, and ambient) and replicates of all analytes, were collected to quantify bias and precision of the analytical concentrations.