Cherry Creek Reservoir Cyanobacteria Fact Sheet

What are Harmful Algal Blooms (HABs)?

Cyanobacteria often called “blue-green algae”, are not actually algae but actually photosynthetic bacteria naturally found in lakes, reservoirs, and rivers. Under certain conditions, they can multiply rapidly, forming harmful algal blooms (HABs). Some HABs produce toxins (cyanotoxins) that may pose risks to people, pets, livestock, and wildlife.

Why do HABs happen?

• Excess nutrients, specifically elevated phosphorus and nitrogen from runoff, fertilizers, or natural sources.
• Warm summer temperatures - Summer heat accelerates growth. 
• Calm, sunny weather encourages blooms. 
• Low inflow from Cherry Creek and Cottonwood Creek increases the time the water stays in the Reservoir. 
• Clear water and increased water transparency can lead to acquatic plant growth, which provides a place for algae to grow. 

Potential Risks

Not all blooms are toxic, but cyanotoxins can cause:

• Human health issues – Skin rashes, eye irritation, stomach illness, and, in severe cases, liver or nervous system impacts.
• Pet and livestock dangers – Animals may drink or swim in affected water.
• Wildlife impacts – Fish kills and reduced habitat quality.

Not all algae are bad

Algae are vital to reservoir health:

• Base of the food chain – Supporting insects, zooplankton, and fish.
• Oxygen production – Through photosynthesis.
• Fish health – Healthy primary productivity keeps fish well-fed.

Problems occur only when algae growth becomes excessive or toxins are produced.

What you can do

• Avoid contact with water that looks discolored, has scum, or smells unpleasant.
• Keep pets and livestock out of suspected bloom areas.
• Follow all posted advisories.
• Report suspected blooms to Colorado Parks and Wildlife.

Monitoring and Early Detection

Colorado Parks and Wildlife (CPW) operates an active HAB monitoring program at Cherry Creek Reservoir to protect public health. This program includes: 

• Regular visual inspections for signs of bloom activity.
• Sample collection and labratory analysis for cyanotoxins. 
• Risk assessment based on results. 
  • Below recretional threshold - No elevated risk, reservoir remains open for recreation with CAUTION signs posted. 
  • Above recreational threshold - Public health advisories issued, and access to affected areas may be restricted with WARNING signs posted.  

Ealry detection and testing allow CPW and partners to respond quickly, keeping the public informed and safe.

Resources

Colorado Environmental Public Health Tracking: Toxic algae

Colorado Department of Public Health & Environment: Toxic algae

Harmful Algal Blooms (HABs) in Water Bodies (EPA)