The purpose of this data release is to provide the original data, analysis methods, and nitrogen loading models in support of a study of the upper South Platte River annual total nitrogen loads attributed to atmospheric deposition of reactive nitrogen during 2017-2018. The data release includes water-quality and stream discharge data and associated predictive regression models used in the estimation of South Platte River nitrogen loads upstream from Weldona, Colorado and sub-basin runoff coefficients for the reach between Chatfield Dam and the South Platte River at Denver gage. The water-quality data were obtained from monthly unfiltered grab samples collected by the Denver Department of Public Health and Environment and analyzed by Standard Methods 4500-NO2-B (nitrite); 4500-NO3-F (nitrate); and 4500-NORG-B+NH3 (Total Kjeldahl Nitrogen). Stream discharge data were obtained for gaging stations from the National Water Information System and Colorado Division of Water Resources with two exceptions as follows. The Englewood wastewater reclamation facility discharge data were supplied by the facility. Discharge values for site N25E were calculated by subtracting the discharge from Cherry Creek at Denver (site W17) from the discharge at South Platte River at Denver. In addition to supplying the data used to calculate nitrogen loads and sub-basin runoff coefficients, this data release also provides the equations and diagnostic statistics for the models developed using the LOADEST software configured for R (rloadest package). Data users are referred to R help to define the many diagnostic statistics presented.