As the global population continues to shift into cities, urban hydrologic systems are becoming increasingly important drivers of overall water quality. Engineered waterways and impervious surfaces strongly influence baseline flow, peak flow, and the transport of pollutants in the urban environment. Between May 2016 -- May 2019, we systematically measured water temperature, pH, dissolved oxygen, biochemical oxygen demand, and the concentrations of nitrate-N, ammonia-N, and orthophosphate in the South Platte River in the Denver metropolitan area, Colorado, USA. We found that the overall water quality of the river decreased through the study area. In addition, there appear to be several hotspots with consistently poor water quality. While it is beyond the scope of this paper to determine the specific sources of the hotspots, it seems likely that wastewater treatment facilities contribute to elevated pollution levels. We also found that water quality was strongly influenced by season. Decreased natural flows during the late fall and winter lead to higher concentrations of nutrients and lower dissolved oxygen levels. Most of the samples collected in this study had nutrient levels that were out of compliance with state regulations for nitrogen and phosphorus concentrations while dissolved oxygen and temperature levels were better than threshold values. Urban hydrologic systems are complex and improving water-quality may be difficult. However, tightening of water-quality standards could result in positive changes to this system.