Two synoptic sampling campaigns were conducted near Breckenridge, Colorado, to quantify metal loading to Illinois Gulch, a tributary of the Blue River. The first campaign, conducted in August 2016, was designed to determine the degree to which Illinois Gulch loses water to the underlying mine workings, and to determine the effect of these losses on observed metal loads. The second campaign, conducted in September 2017, was designed to evaluate metal loading within Iron Springs, a subwatershed that was responsible for the majority of the metal loading observed in 2016. A continuous, instream injection of a sodium bromide (NaBr) tracer was initiated at the head of the respective study reaches several days prior to both synoptic sampling campaigns and maintained throughout the duration of each study. Bromide concentrations were subsequently used to determine streamflow in gaining stream reaches using the tracer-dilution method, and as an indicator of hydrologic connections between the Illinois Gulch and subsurface mine workings. Streamflow losses to the mine workings were quantified using a series of magnesium chloride slug additions conducted in August 2016, wherein specific conductivity readings were used as a surrogate for the tracer concentration. Study results indicate that Illinois Gulch loses water in the vicinity of the Puzzle Extension Shaft, and that water leaving the stream enters the subsurface mine workings. These losses are evidenced by the results of the slug additions and the elevated bromide concentrations observed at a collapsed mine portal in the Iron Springs subwatershed (Willard Adit 1). The primary sources of metal loading to the overall Illinois Gulch study reach include diffuse springs and groundwater near the toe of the Iron Springs mine dump and Willard Adit 1. This data release consists of 8 tables: Table 1, Locations of sampling sites for the 2016 and 2017 campaigns Table 2, Synoptic sampling results, August 18, 2016 Table 3, Synoptic sampling results, September 7, 2017 Table 4, Streamflow measurements, August 2016 Table 5, Bromide time series, August 2016 and September 2017 Table 6, Slug addition conductivity data, August 2016 Table 7, Slug addition results, August 2016 Table 8, Spatial profiles of streamflow and metal load, August 2016 and September 2017