In this study, air temperatures were collected between 1985 and 2016 and compared with water temperatures in four locations in the distribution system of Pasadena Water and Power (PWP), which received surface water imported into Pasadena between 2001 and 2016 from the Metropolitan Water District. The concentrations of chloramine residual and nitrite concentrations were collected between 2001 and 2016 from these five locations. The results indicate that the median nighttime temperature of the period 2009–2016 was 1.6 °C warmer than the period 1985–2000 and 0.5 °C warmer than the period 2001–2008. The median water temperature in the four distribution system samples increased by 0.8–1.4 °C depending on the location over the study period (p < 0.001). The median chloramine concentration fell significantly (p < 0.001) at three distribution system locations, and the nitrite concentrations increased significantly at all four distribution system locations (p < 0.001). As air temperature in the study area increased, water temperatures also increased resulting in the loss of disinfectant residual and the increase in the activity of ammonia-oxidizing bacteria. As this represented an increased risk to public health, PWP took additional steps to increase disinfectant residuals by adding chlorine and flushing stale water. In localities where climate change is most measurable, local water purveyors must adapt to warmer water to ensure stable concentrations of disinfectants.