E.Coli Spike Closes Deer Creek at Pioneer Park

Nevada City, CA — Elevated levels of E. coli bacteria have been detected in Little Deer Creek, which runs through the heart of Pioneer Park in Nevada City. The contamination has prompted public health advisories warning residents and visitors to avoid contact with the creek, particularly in the area surrounding the popular dog park and children’s play spaces.

A sign posted at the creek on July 25, 2025 reads, “WARNING: STAY OUT OF CREEK. Bacteria levels recently exceeded the EPA’s good water quality standards in Little Deer Creek. It is recommended that humans and pets avoid contact with the creek. We will continue to monitor levels and post updates if there are changes.”

The contamination was first flagged in late July by the Sierra Streams Institute (SSI), a Nevada County–based nonprofit that has been monitoring water quality in the Deer Creek watershed for over two decades. According to SSI, the most recent testing revealed E. coli concentrations that exceed the U.S. Environmental Protection Agency (EPA) recreational safety threshold of 320 colony-forming units (CFU) per 100 milliliters of water — levels that pose a risk of illness to swimmers, children, and pets.

SSI’s concern is for public safety, especially as people use Pioneer Park recreationally during the summer. Once E. coli levels pass the EPA threshold, it becomes a health hazard to touch or wade in the water.

Understanding the Source

E. coli, a bacterium commonly found in the intestines of warm-blooded animals, typically enters waterways through fecal contamination. While the specific source of the bacteria spike is still being investigated, Sierra Streams Institute and city officials believe uncollected dog waste in the park is the most likely culprit.

When it rains, or when irrigation occurs, waste left on the ground can be swept directly into the creek. The problem becomes worse during late summer, when creek flows are low and temperatures are high, allowing bacteria to concentrate in stagnant sections of water.

While other potential sources — such as leaking sewer infrastructure or wildlife — haven’t been ruled out, testing data continues to point to human-associated sources within the park’s boundaries. 

A Pattern, Not an Isolated Incident

This is not the first time Little Deer Creek has faced water quality issues. Similar warnings were issued in previous summers, but 2025 has brought particularly high concern due to prolonged dry spells and increased recreational use. In July, Little Deer Creek was the only monitored waterway in the Nevada City area to surpass the EPA threshold during Sierra Streams’ routine sampling period.

In contrast, Deer Creek and other tributaries monitored by SSI remained within safe limits — a sign that the problem is hyper-localized within the Pioneer Park corridor. The elevated readings prompted the nonprofit to post signage at the park warning visitors not to wade, swim, or allow pets in the water.

Indeed, Deer Creek flows into Lake Wildwood which has had its bouts with E. coli in the past resulting in the hospitalization of a young Penn Valley boy in 2017.

The local government (Nevada City or Nevada County) does not fund SSI to do water testing, so other bodies of water might be at risk, but the public does not know. While SSI does monitor Deer Creek, it does not monitor Scott’s Flat Lake or the Yuba River, so their E. Coli levels are unknown. 

When Will It Be Safe Again?

SSI conducts weekly water quality sampling and has stated it will lift the advisory once E. coli levels consistently fall back below acceptable limits. According to EPA guidelines, there are two key thresholds to watch:

• A single-sample maximum of 320 CFU per 100 mL

• A 30-day average of 126 CFU per 100 mL

SSI’s Chemistry Lab Manager Melissa Grimm says, “This means that the initial Nevada City Public Notice that Little Deer Creek was “above the EPA threshold” means that there were concentrations of E. coli in the water greater than 320 CFU/100 mL of water during our single sampling event. During follow-up monitoring, the E. coli levels will need to drop below an 30-day average value of 126 CFU/100 mL OR if a sample is taken more than 30 days from a previous sample, bacteria needs to be lower than 320 CFU.”

If follow-up testing shows either of these criteria are met, the advisory may be removed. SSI will test weekly. 

In the meantime, parkgoers are advised to avoid any contact with the creek water, especially young children, seniors, or those with weakened immune systems.

Community and Government Response

The contamination has spurred broader discussions around environmental stewardship in Nevada City. While no official city enforcement action has been taken, local officials have acknowledged the need for better education and signage — particularly encouraging dog owners to clean up after their pets.

Additionally, SSI and other conservation partners are continuing work on the Little Deer Creek Watershed Restoration Project, a long-term effort aimed at improving stream health, reducing urban runoff, and strengthening flood resilience. This includes bank stabilization, vegetation restoration, and possibly future infrastructure upgrades to reduce pollution inputs.

Nevada City has also partnered with organizations like the Yuba Watershed Institute and the Bureau of Land Management on a complementary project — the Little Deer Creek Landscape Resilience Project — which focuses on wildfire risk reduction and forest health across the upper watershed. These environmental improvements may indirectly help water quality in the long term by reducing erosion and stormwater surges.

Representatives from Nevada City and Nevada County were asked to comment, but none had responded as of this writing.

What the Public Can Do

Residents and visitors can play a role in addressing the issue:

• Pick up dog waste and dispose of it in designated bins.

• Avoid using fertilizers or chemicals near storm drains or creeks.

• Report any signs of sewer leaks or unusual odors near the creek to city officials.

• Volunteer with SSI’s monitoring program, which trains community members to collect water samples and contribute to real-time data collection.

Little Deer Creek Still Under Active Health Advisory

As of late August, Little Deer Creek remains under an active health advisory, with updated results expected from SSI’s weekly sampling. The nonprofit urges community members to check their website and social media for current data before planning creekside activities.

The situation is a stark reminder of how urban use and natural systems intersect—and the importance of vigilance in preserving the health of local ecosystems, particularly those that serve as public gathering spaces.

For real-time updates on water quality, visit: https://sierrastreamsinstitute.org