Water quality data for 18 locations along Boulder Creek are collected monthly by the City of Boulder. Generally, water quality is excellent in samples from North and Middle Boulder Creeks near the headwaters. There is some degradation as the creek moves down Boulder Canyon. Water quality continues to decrease as the Creek passes through the City of Boulder and to the east of the City. The parameters of most concern to the health of Boulder Creek are ammonia, nitrate, and phosphorus. In addition, at some times at certain sites, there are elevated levels of fecal coliform. The major factors affecting the quality of Boulder Creek are geology, urban runoff, wastewater effluent, algal growth, and water diversions.

Water Quality Index for Boulder Creek
Read a summary of each water quality parameter in Boulder Creek
Access the Boulder Creek water quality database

Geology

Geology and topography are significant factors in changing water quality in the creek as it moves from mountains to plains. As Boulder Creek drops in elevation, water temperature warms up, along with air temperature. In the mountains, Boulder Creek passes over granitic and metamorphic rocks that aren’t very reactive; when it moves into the plains, the geology is dominated by sedimentary rocks, including sandstones and shales. Increases in pH, specific conductivity, alkalinity, and TDS reflect the more easily weathered, calcite-bearing sedimentary rocks. For more information on geology in the Boulder area, visit the BASIN geology page.

 

Urban Runoff

Human activity, such as urban and highway runoff, affects water quality in the creek as it passes through Boulder Canyon and the City of Boulder. The Boulder Creek Millennium Baseline (BCMB) Study found that fecal coliform concentrations were higher in Boulder Creek at 30^th Street (BC-30) and 61^st Street (BC-61) in Boulder in June 2000 than at any other site. Many factors could contribute to these elevated concentrations, including faulty sewer systems and animal waste left in or near the stream. Fecal coliform concentrations were 170 colonies/100 ml at these sites in June 2000; the primary recreation standard (swimming, kayaking, etc.) is 200 colonies/100 ml (for more on standards, see the BASIN Water Quality Standards discussion).

 

Wastewater Effluent

Effluent from the Wastewater Treatment Plant (WWTP), located just upstream of 75^th Street, also affects water quality. The WWTP removes a tremendous amount of the waste from our houses and businesses, and discharges the treated water to Boulder Creek (for more information on the WWTP, see the City of Boulder's Public Works webpage. The WWTP meets all state requirements for discharging to the creek. However, the plant cannot remove everything we contribute to the sanitary sewer. The June 2000 water quality data showed increases in specific conductivity, total dissolved solids (TDS), alkalinity, hardness, ammonia, phosphorus, nitrate, and nitrite at the 75^th Street sampling site (BC-75). Dissolved oxygen levels drop at BC-75, because the WWTP effluent contributes organic waste and nutrients, which encourage the growth of bacteria. These bacteria consume the waste and remove oxygen from the water.

 

Algal Growth

Downstream of the WWTP and the City of Boulder, the environment of Boulder Creek changes in several ways. There are fewer trees to shade the creek and prevent bank erosion, and the flow rate is lower, due to the many diversion from the Creek. These factors cause the creek water temperature to warm up. The nutrients (nitrate and phosphate) from the WWTP and from fertilizer runoff, along with higher temperatures and direct sunlight, cause significant plant growth to occur. The rapid photosynthesis that occurs during the day causes pH and dissolved oxygen levels to increase significantly. At night, when plants, bacteria, and animals are undergoing respiration, pH and dissolved oxygen levels drop. The high pH and temperature affect the toxicity of ammonia in the creek.  At high temperatures and pH (e.g., above 9), ammonia begins to convert to unionized ammonia, which is much more toxic to aquatic organisms than the ammonium ion.

 

Diversions

The degree to which water quality in the creek is affected is largely related to the flow rate. Downstream of the WWTP, this is particularly significant, because it affects the proportion of wastewater effluent to total flow in the creek. During times of high flow in the creek (e.g., from April to September, when flow in Boulder Creek is higher due to snowmelt runoff and thunderstorms) the greater amount of water dilutes chemical concentrations in the creek.  During low-flow, wastewater effluent can comprise over half of the total flow, and chemical concentrations are often higher. The flow rate in Boulder Creek is also largely affected by the many diversions that take water from it and return to the creek. During June 2000, the flow at BC-ORO (in Boulder Canyon at the USGS gauging station) had the highest flow (97 cfs) of those measured; at this point, North and Middle Boulder Creek  have joined forces. By the time the stream reached 30^th Street in Boulder, a significant amount of flow was diverted by several ditches. By the time Boulder Creek reached the Saint Vrain River, the flow in the creek was at its lowest (16 cfs).