As we transition from the end of the summer heat into cooler temps and some early season rain, Sierra Streams Institute has received repeated requests for an update on the E. coli levels at Pioneer Park. We don’t mean to keep anyone in the dark, so here’s a quick update on where things stand!
E. coli Values and Trends This Summer
This graph shows the individual values and a rolling average of E. coli at Pioneer Park from July 16th to October 1st, 2025. Blue points represent the individual sample values of E. coli as a most probable number (MPN) for every sampling event performed by SSI. The yellow line represents the 30-day rolling average of E. coli values (MPN) from SSI sampling, and in red/orange is the EPA threshold for 30-day average E. coli values.
As you can see, the rolling average for E. coli levels hovered right above the threshold limit for most of the summer. The average even got close to dipping below the limit in mid-August, but unfortunately some higher individual sample values in late August kept us above the threshold. In the last few weeks we have also seen a few very high individual values that have pushed the 30-day rolling average even higher above the 30-day average EPA threshold.
When will the creek be safe again? What is causing the high E. coli values?
These are both great questions, and the unfortunate answer to both is that we just don’t know. That being said, SSI will continue to work closely with Nevada City to monitor E. coli values at Pioneer Park. SSI and the City still cannot say with certainty why the creek has maintained high E. coli values, or why these values spiked in the last few weeks. To answer this question would require an in-depth investigation that we don’t have the resources for at this time. However, your support of SSI through donations and memberships allows us to continue our monitoring efforts and hopefully find the resources to investigate the situation in the future. Please consider donating now to help us out!
In the meantime, we encourage you to revisit our original blog post which provides more detailed information about E. coli and resources you can use to safely recreate in natural waters as we enter the shoulder season. If you have follow up questions or concerns, we always encourage you to reach out via social media, giving us a call, or emailing the lab manager at lab@sierrastreamsinstitute.org.
Has the little creek been tested up stream to see where it begins
Great question, Charly! We sampled four locations on September 24th from below the Dog Park to upstream of the picnic table area. All of our samples that day came back with roughly the same values, meaning that within our sampling reach we did not identify an area where the problem “begins.” While we would love to perform more extensive sampling and see if we can identify a beginning point of the contamination, we do not currently have the resources to do more extensive testing, and further sampling upstream would require access to private property which we have not broached with property owners along the creek!