This data release provides data in support of an assessment of water quality and discharge in the Herring River at the Chequessett Neck Road dike in Wellfleet, Massachusetts, from November 2015 to September 2017. The assessment was a cooperative project among the U.S. Geological Survey, National Park Service, Cape Cod National Seashore, and the Friends of Herring River to characterize environmental conditions prior to a future removal of the dike. It is described in U.S. Geological Survey (USGS) Scientific Investigations Report "Assessment of Water Quality and Discharge in the Herring River, Wellfleet, Massachusetts, November 2015 – September 2017." This data release is structured as a set of comma-separated values (CSV) files, each of which contains information on data source (or laboratory used for analysis), USGS site identification (ID) number, beginning date of time of observation or sampling, ending date and time of observation or sampling and data such as flow rate and analytical results. The CSV files include calculated tidal daily flows (Flood_Tide_Tidal_Day.csv and Ebb_Tide_Tidal_Day.csv) that were used in Huntington and others (2020) for estimation of nutrient loads. Tidal daily flows are the estimated mean daily discharges for two consecutive flood and ebb tide cycles (average duration: 24 hours, 48 minutes). The associated date is the day on which most of the flow occurred. CSV files contain quality assurance data for water-quality samples including blanks (Blanks.csv), replicates (Replicates.csv), standard reference materials (Standard_Reference_Material.csv), and atmospheric ammonium contamination (NH4_Atmospheric_Contamination.csv). One CSV file (EWI_vs_ISCO.csv) contains data comparing composite samples collected by an automatic sampler (ISCO) at a fixed point with depth-integrated samples collected at equal width increments (EWI). One CSV file (Cross_Section_Field_Parameters.csv) contains field parameter data (specific conductance, temperature, pH, and dissolved oxygen) collected at a fixed location and data collected along the cross sections at variable water depths and horizontal distances across the openings of the culverts at the Chequessett Neck Road dike. One CSV file (LOADEST_Bias_Statistics.csv) contains data that include estimated natural log of load, model residuals, Z-scores, and seasonal model residuals for winter (December, January, and February); spring (March, April and May); summer (June, July and August); and fall (September, October, and November). The data release also includes a data dictionary (Data_Dictionary.csv) that provides detailed descriptions of each field in each CSV file, including: data filename; laboratory or data source; U.S. Geological Survey site ID numbers; data types; constituent (analyte) U.S. Geological Survey parameter codes; descriptions of parameters; units; methods; minimum reporting limits; limits of quantitation, if appropriate; method reference citations; and minimum, maximum, median, and average values for each analyte. The data release also includes an abbreviations file (Abbreviations.pdf) that defines all the abbreviations in the data dictionary and CSV files. Note that the USGS site ID includes a leading zero (011058798) and some of the parameter codes contain leading zeros, so care must be taken in opening and subsequently saving these files in other formats where leading zeros may be dropped.