Algae Blooms in Sweetwater Reservoir

A Naturally Occurring Process That May Affect Water Taste and Odor

 

Sweetwater Authority manages seasonal algae blooms in the Sweetwater Reservoir, a natural and common occurrence among water agencies that rely on surface reservoir water. While algae blooms can temporarily affect the taste and odor of water, they are naturally occurring and do not pose a public health risk.  

What are Algae Blooms?  
Algae are naturally occurring organisms found in all surface waters, including freshwater systems and oceans. When conditions become favorable, algae populations can grow rapidly, resulting in a bloom. These conditions typically include higher nutrient inputs from runoff, warmer ambient air and surface water temperatures, and increased daylight hours, all of which contribute to accelerated algae growth.  

Optimal conditions for algae growth typically occur between April and October. Although the duration and intensity of algae blooms can vary from year to year, these events do not impact the safety of surface water for drinking or other uses. 

Why Do These Blooms Occur in the Sweetwater Reservoir?  

Sweetwater Reservoir is considered a eutrophic reservoir, meaning it is nutrient rich and highly biologically productive. During the spring, warmer temperatures and longer daylight hours cause the reservoir to thermally stratify, with the upper layer of water warming and becoming stable. This upper layer remains warm, oxygen rich, and shallow enough for sunlight to penetrate, creating ideal conditions for algae to photosynthesize and grow.

In contrast, the deeper bottom layer remains cooler and often becomes anoxic (low in dissolved oxygen). Under these conditions, nutrients stored in the sediments can be released into the water column, providing an additional food source for algae. Together, these factors create favorable conditions for algal blooms to develop during the spring and summer months. 

In the fall, cooler temperatures and shorter days cause the surface water to cool and mix with the deeper water, creating more uniform conditions throughout the reservoir in a process known as fall turnover. Although algal blooms can still occur during the fall and winter, they are generally less common because of cooler water temperatures, reduced sunlight, and cooler water temperatures.  

What Does This Mean for Your Water? 
Water agencies that rely on reservoir supplies often experience algae blooms. While these blooms do not pose a public health risk, they can temporarily affect water taste and odor.  

How is Sweetwater Authority Managing Algae Blooms? 
Sweetwater Authority carefully manages and continuously monitors its water supply to ensure it remains safe for public use. During algal bloom season, Sweetwater Authority takes proactive measures to reduce temporary impacts on water taste and odor.  

These proactive measures include:  

  • Imported water: Wholesaler water may be purchased and either fully treated or blended with reservoir water to help mitigate taste-and-odor impacts. 
  • Reservoir algaecide application: Copper sulfate is an industry-standard algaecide that’s widely used by water agencies to reduce and control algal blooms, helping maintain and improve water quality. Its application is conducted under applicable regulatory permits and in accordance with established environmental and drinking water safety standards. When properly applied, copper sulfate doesn’t impact the safety of drinking water. Sweetwater Authority is also evaluating more environmentally friendly algaecide alternatives, including hydrogen peroxide-based products. 

Specific measures put in place can vary year to year and are based on the intensity of the algae bloom. These measures are also performed in an environmentally safe manner and follow strict regulatory requirements.  

On May 6, 2026, the Authority applied copper sulfate to Sweetwater Reservoir to control taste- and odor-causing algae.

Learn more about Sweetwater Authority’s efforts to protect your water.  View the Sweetwater Authority Fact Sheet.

For more information regarding algae treatment and

For more information regarding algae treatment and odor, please call Melissa Molina at (619) 409-6813 or Natalia Blake at (619) 409-6826.

To report an issue:

The Authority’s annual water quality report is available in English here (PDF), in Spanish here (PDF), and in Tagalog here.