Taking Action to Protect Lake Elizabeth from Extreme Heat

09 Sep Taking Action to Protect Lake Elizabeth from Extreme Heat

In early July 2024, Lake Elizabeth in Fremont’s Central Park experienced a significant ecological challenge.

An extended period of triple-digit temperatures caused the lake’s water temperature to rise and oxygen levels to plummet. The result was a fish die-off event that startled the community and spurred the City of Fremont into action.

Dozens of fish washed up along the shoreline, a stark reminder of how vulnerable the lake can be during extreme weather. While minor fish die-offs have occurred periodically in the past, this event was far more severe and required a deeper look at the underlying causes.

Lake Elizabeth’s depth plays a key role in its sensitivity to extreme heat. Created in 1969, the lake is only about four feet deep on average, with a maximum depth of just under six feet. Shallow waters heat up rapidly, and without the mixing action seen in deeper lakes, oxygen cannot circulate effectively. When temperatures soar, fish and other aquatic life can run short of the oxygen they need to survive.

Measuring water quality parameters across the lake

In response to the fish die-off, the City partnered with experts from Applied Marine Sciences (AMS) to measure water quality parameters across the lake. Tests confirmed that dissolved oxygen levels were dangerously low—ranging from just 0–2 mg/L, far below what fish require. 

The good news: tests for harmful algal blooms came back negative.

To strengthen their understanding, scientists also collected environmental DNA samples to assess the remaining fish populations and gain insight into the overall health of the lake’s ecosystem.

With harmful algae ruled out, the culprit became clear: extreme heat combined with the lake’s shallow depth resulted in oxygen depletion. The challenge now is finding long-term solutions that can help protect Lake Elizabeth against future fish die-off events—especially as climate change brings more frequent and intense extreme heat.

One promising innovation is the recent installation of floating islands.

These engineered platforms are planted with native vegetation that serves multiple purposes:

– Shade and refuge: The islands provide cooler areas in the water column where fish can retreat during heat stress.

– Natural filtration: Plant roots absorb excess nutrients that would otherwise feed algae growth.

– Oxygenation: Roots and stems help release oxygen into the water, improving overall water quality.

– Wildlife habitat: The islands create miniature ecosystems, supporting insects, birds, and aquatic organisms that, in turn, benefit the lake’s biodiversity.

By piloting floating islands, the City is taking an important step toward protecting both the ecological health of Lake Elizabeth and the recreational value of Central Park.

Visitors rely on the lake not only as a scenic backdrop, but also as a space for community gatherings, exercise, and relaxation. Ensuring that the lake remains resilient in the face of environmental stressors is essential to preserving its role as the heart of the park.

This project represents more than just an environmental fix—it’s a forward-looking investment in community well-being. Through science, innovation, and collaboration, the City is working to ensure that Lake Elizabeth remains a thriving, healthy ecosystem for generations to come.

For more information about the City’s Lake Elizabeth Water Quality Improvement Pilot Project, visit city.fremont.gov/lakeelizabethwaterqualityproject.