During the warmer months of the year (April to September), water temperature increases gradually as Boulder Creek moves downstream from the mountains to the plains (for example, see August, 1999). This is primarily a result of regional air temperature variations. The lowest water temperatures of the North Boulder Creek/Boulder Creek samples recorded from April to September were at the two sampling points farthest upstream, North Boulder Creek at Lakewood Reservoir (NBC-Lw) and North Boulder Creek Falls (NBC-FALLS). Warmest temperatures recorded from April to September were at the sampling sites farthest downstream (Boulder Creek above Coal Creek (BC-aCC) and Boulder Creek below Coal Creek (BC-bCC)). Temperatures at these sites sometimes neared 30°C (86°F). Exposed to the sun during the long summer days, the water at the sites on the plains become very warm, especially when flow rates are low and the water moves slower. The City of Boulder has worked to cool the waters by planting vegetation (to provide shade) along some areas of lower Boulder Creek.

During the colder months of the year (October to March), water temperature of Boulder Creek increases gradually from the mountains to just upstream of the Boulder Wastewater Treatment Plant (WWTP), spikes significantly at the 75th Street sampling site, and then decreases gradually to the confluence of Boulder Creek and Coal Creek (for example, see February, 1999). Lowest winter temperatures were observed at the sampling sites in the mountains (North Boulder Creek at Lakewood Reservoir, North Boulder Creek Falls, and the Orodell station), where temperatures were often near freezing. The warmest temperatures in Boulder Creek during winter were observed at 75th Street. This sampling site is just downstream of where effluent from the WWTP enters Boulder Creek. Water discharged from the WWTP is warm, because the waste water entering the plant (coming from our homes and businesses) is warm. When this water is discharged and mixes with the water in the creek, it warms Boulder Creek. The water temperature then gradually decreases as Boulder Creek moves downstream and receives flow from other sources and mixes with cold air. However, the water does not return to the ambient temperature of Boulder Creek above the WWTP.

Colorado Department of Public Health and Environment Water Quality Control Division (CDPHE-WQCD) regulations (5 CCR 1002-31) for temperature depend on how the water body is classified. Upstream of the confluence of South Boulder Creek and Boulder Creek, Boulder Creek is classified as a Class 1 Cold Water stream. Class 1 Cold Water streams should never have temperatures above 20°C (68°F). All of the stream samples upstream of South Boulder Creek were less than 20°C in the period analyzed (1998-2000). Downstream of the confluence of South Boulder Creek and Boulder Creek, Boulder Creek is classified as a Class 1 Warm Water stream. CDPHE-WQCD regulations state that Class 1 Warm Water streams should never have temperatures exceeding 30°C (86°F). No temperatures above 30° C were recorded in lower Boulder Creek in the period analyzed (1998 to 2000).