Streamflow and phosphorus concentrations were monitored in the Assabet River in central Massachusetts in order to evaluate concentrations and loads in the river before, during, and after changes in the amount of total phosphorus that was discharged to the river from three wastewater-treatment plants. At four locations the U.S. Geological Survey collected weekly flow-proportional, composite samples of water from the Assabet River for analysis of concentrations of total phosphorus and orthophosphate. Streamflow and concentration data were used to estimate total phosphorus and orthophosphate loads in the river and compare them with total phosphorus load outputs from three wastewater-treatment plants. Data were collected from October 2008 through April 2014. The data are analyzed in a report that describes changes in phosphorus concentrations and loads during the study period. This data release consists of tables of (1) instream daily streamflow, concentrations of total phosphorus and orthophosphate from weekly composite samples, and daily load estimates for the four monitoring stations on the Assabet River; (2) daily outflow, daily measured and estimated total phosphorus and concentrations, and daily total phosphorus load estimates from the three wastewater-treatment plants; and (3) concentrations of total phosphorus and orthophosphate, and associated daily mean streamflows, for quality-control samples collected for the project.