E. coli exceedances are a common problem across the Front Range, Colorado, and the United States. Streams monitored by KICP periodically exceed levels that the U.S. Environmental Protection Agency deems safe for recreational use. Typical sources of E. coli in urban streams include wildlife, pets, trash, and human influence.
Results
E. coli
Key Takeaways
Escherichia coli (E. coli) is a type of bacteria found naturally in the environment and in the digestive tracts of animals and humans. E. coli is often harmless, but persistently high levels can indicate an increased risk of illness when recreating in a waterbody. KICP carefully monitors E. coli in the watershed.
Recreate safely: Exposure to E. coli and other microorganisms is a risk when wading or swimming in any natural water body. Most types of E. coli do not cause illness, but there are strains that can cause gastrointestinal illness, and E. coli can be an indicator of other pathogens that can make people sick.View E. coli Graphs
Background
Escherichia coli (E. coli) is a large and diverse group of bacteria that typically live in the digestive tracts of warm-blooded animals. Most strains of E. coli are harmless, but some can make you sick. Measuring for E. coli is easier than measuring for all possible pathogens in waterbodies, so it is widely used as an indicator of the presence of bacteria and viruses that can make people sick.
E. coli concentrations provide information about the possible presence of pathogens but not about where the E. coli is coming from. E. coli impairments are commonly found in urban streams along the Front Range, Colorado, and across the country. Urban areas tend to have higher frequency and intensity of E. coli contamination for a variety of reasons that include stormwater runoff, increased populations, pets, and leaking wastewater infrastructure.
Geometric means, also referred to as "geomeans," are used to report E. coli trends as a more accurate representation of a central tendency for skewed data, which is common in bacterial counts. Geometric means reduce the impact of extreme values or outliers, offering a more reliable measure of typical contamination levels over time which is particularly useful for assessing long-term water quality trends and compliance with standards.
Learn More in the Keep It Clean Partnership's E. coli Fact Sheet
Analysis
Bacteria concentrations can vary dramatically within short periods. Monthly means by stream reach indicate that they tend to be highest in late summer and fall. Warmer temperatures and lower flows present during this period are favorable for E. coli growth. Analysis of 2024 E. coli conditions included review of monthly averages across stream reaches and annual averages by location, which are both useful for identifying trends. Due to highly fluctuating concentrations, graphs have been separated out for each stream in the sections below for improved visibility of results and trends. Colorado Department of Public Health and Environment’s (CDPHE) recreational E. coli standard for primary contact waters is 126 #CFU/100ml based on a 2-month static geometric mean.
Averaged values on the graphs in this section are calculated using a geometric mean calculation, as opposed to a standard average, but are not based on a 2-month rolling static geometric mean. Rolling geometric means can be viewed for each monitoring location on the time series graphs accessible from the Interactive Map tool.
Coal Creek
Coal Creek reached higher summer levels in 2024 than Boulder or St. Vrain Creeks. In general, the source of these higher levels in Coal Creek can be traced to Rock Creek, where measurements exceeding the upper limit of the test for E. coli were reported at several Rock Creek stations in June and August, as well as levels above 1900 CFU/100mL in July.
Average monthly E. coli levels peaked in November due to a monitoring result above the upper detection on Rock Creek near 104th (station 4b-RC), the effects of which can be seen in downstream stations on Rock Creek and into Coal Creek. The specific cause of this spike is not certain, however, rainfall was significantly higher than normal in 2024, and urbanized runoff as a result of the rainfall is a likely factor.
St. Vrain
Levels of E. coli in the St. Vrain have been fairly consistent over time, peaking in the summer months as expected, but at levels near or slightly above the 126 CFU/100mL threshold for primary contact waters at most locations. The highest levels are consistently seen at station M8.9 (@ Golden Ponds), which is above the urban corridor and warrants further investigation.
Boulder Creek
Boulder Creek E. coli concentrations in 2024 generally followed the seasonal patterns seen in the five-year monthly averages. However, concentrations in June, July, and November were notably higher than the corresponding five-year averages. The specific cause of these elevated values has not been determined. One correlation noted is that rainfall was significantly higher than normal in 2024 and urbanized runoff as a result of the rainfall may have contributed to the November spike. Similar correlations with rainfall or streamflows cannot be seen, however, in June or July.
The annual average E. coli concentrations by location generally followed the historical five-year pattern, which typically increases from upstream to downstream. The most notable increases occurred in the City of Boulder’s urban corridor (between stream miles 0 and 5) and downstream of the Coal Creek confluence (stream mile 16). In 2024, several individual monitoring locations exceeded their five-year averages, with the most pronounced exceedances observed at 11-BC and BC-aWRRF.