Multiple sampling campaigns were conducted near Boulder, Colorado, to quantify constituent concentrations and loads in Boulder Creek and its tributary, South Boulder Creek. Diel sampling was initiated at approximately 1100 hours on September 17, 2019, and continued until approximately 2300 hours on September 18, 2019. During this time period, samples were collected at two locations on Boulder Creek approximately every 3.5 hours to quantify the diel variability of constituent concentrations at low flow. Synoptic sampling campaigns on South Boulder Creek and Boulder Creek were conducted October 15-18, 2019, to develop spatial profiles of concentration, streamflow, and load. Numerous main stem and inflow locations were sampled during each synoptic campaign using the simple grab technique (17 main stem and 2 inflow locations on South Boulder Creek; 34 main stem and 17 inflow locations on Boulder Creek). Streamflow at each main stem location was measured using acoustic doppler velocimetry. Bulk samples from all sampling campaigns were processed within one hour of sample collection. Processing steps included measurement of pH and specific conductance, and filtration using 0.45-micron filters. Laboratory analyses were subsequently conducted to determine dissolved and total recoverable constituent concentrations. Filtered samples were analyzed for a suite of dissolved anions using ion chromatography. Filtered, acidified samples and unfiltered acidified samples were analyzed by inductively coupled plasma-mass spectrometry and inductively coupled plasma-optical emission spectroscopy to determine dissolved and total recoverable cation concentrations, respectively. This data release includes three data tables, three photographs, and a kmz file showing the sampling locations. Additional information on the data table contents, including the presentation of data below the analytical detection limits, is provided in a Data Dictionary.