California’s lakes, rivers, reservoirs, and other waterways provide opportunities for recreation, fishing, wildlife habitat, and community connection. But beneath the surface, environmental conditions can sometimes create a less visible public health concern: harmful algal blooms (HABs).
Harmful algal blooms occur when certain types of algae or cyanobacteria grow rapidly and produce conditions or toxins that can harm people, animals, and aquatic ecosystems. California environmental agencies continue to monitor these events and provide guidance to help communities recognize potential risks.
What Are Harmful Algal Blooms?
Harmful algal blooms are excessive growths of algae or cyanobacteria in bodies of water. While not every algal bloom is harmful, some can produce toxins that affect humans and animals.
These blooms can occur in freshwater environments such as lakes, reservoirs, rivers, and streams. California's environmental health agencies maintain monitoring and reporting programs to help identify potential HAB events and communicate health information to the public.
HABs can sometimes appear as:
- Green, blue-green, or brown discoloration
- Surface scum or mats
- Water that looks unusually cloudy
- Paint-like or spilled-material appearances
- Unusual odors
However, appearance alone is not always enough to determine whether a bloom is harmful.
Why Are HABs an Environmental Health Concern?
Certain harmful algal blooms can produce toxins that may affect people and animals.
People can potentially be exposed through activities such as swimming, wading, or other contact with contaminated water. Pets can also be at risk if they drink contaminated water or come into contact with affected areas.
For this reason, environmental monitoring and public awareness are important tools for reducing unnecessary exposure.
California's HAB program provides information on reported blooms, health effects, and precautions for people and pets.
California Continues to Monitor Water-Related Health Risks
Recent activity from California's Office of Environmental Health Hazard Assessment demonstrates the state's continuing focus on water-related environmental health.
For example, OEHHA issued a fish consumption advisory for Twin Lakes in Mono County on August 11, 2026, based on mercury levels found in fish. The agency also reminded anglers that fish-consumption advisories are separate from shorter-term advisories related to harmful algal blooms.
This distinction is important because environmental health risks in waterways can come from different sources. A lake may have long-term concerns involving contaminants in fish while also requiring separate monitoring for short-term biological hazards.
Protecting Yourself Around California Waterways
One of the simplest ways to reduce potential exposure is to stay informed before entering a lake, river, or other recreational waterway.
Before swimming, fishing, or allowing pets near the water:
- Check for current water-quality advisories.
- Look for posted warning signs.
- Avoid water that appears unusually discolored or has visible scum.
- Keep children and pets away from suspected blooms.
- Do not allow pets to drink from questionable water.
- Follow local health department and environmental agency guidance.
- Check California's harmful algal bloom reporting resources when visiting recreational waterways.
When an advisory is posted, following the guidance is an important part of protecting individual and community health.
Why Environmental Monitoring Matters
Harmful algal blooms demonstrate why environmental health requires ongoing observation.
Water quality can change as environmental conditions change. Temperature, nutrients, rainfall, runoff, and other factors can influence aquatic ecosystems and contribute to conditions that support algal growth.
Monitoring allows environmental and public health professionals to identify potential problems, investigate reported illnesses, and communicate precautions to communities.
California's freshwater HAB program is designed in part to support this type of coordinated monitoring and response.
Protecting People, Pets, and the Environment
Environmental health is about more than protecting people from immediate hazards. It also means understanding the relationship between healthy ecosystems and healthy communities.
California's waterways support wildlife, recreation, agriculture, tourism, and local communities. Protecting these resources requires collaboration between environmental professionals, public agencies, water managers, researchers, and the public.
Awareness is an important part of that effort.
Knowing what a harmful algal bloom is, recognizing potential warning signs, and checking current advisories can help people make safer decisions when enjoying California's waterways.
Environmental Health Starts With Awareness
Harmful algal blooms may not always be obvious, but their potential impact makes water-quality awareness an important part of environmental health.
As Californians continue to enjoy lakes, rivers, reservoirs, and coastal environments, staying informed about changing water conditions can help protect people, pets, and ecosystems.
At Definitive Safety Group, we believe environmental health starts with awareness, prevention, and informed action.
Protecting our communities means understanding the environmental risks around us—and taking practical steps to reduce them.
Conclusion
California's waterways are valuable environmental and community resources. Keeping them safe requires continued monitoring, responsible stewardship, and public awareness.
Harmful algal blooms are a reminder that environmental hazards are not always visible or predictable. By checking advisories, recognizing potential warning signs, and following public health guidance, Californians can make safer choices while enjoying the state's natural resources.
Healthy water supports healthy communities—and protecting our waterways is everyone's responsibility.
