News Flash

Nevada County News

Posted on: September 20, 2019

Report of Possible Blue Green Algae in Holding Pond Near Combie Lake

Blue Green Algae_News Flash

The Central Valley Regional Water Quality Control Board (CVRWQCB), Harmful Algae Bloom (HAB) Division Scientist released the report of a possible (not confirmed) Cynaobacteria Bloom occurring in a holding pond just above Combie Lake. The lake/reservoir and pond is owned and operated by the Nevada County Irrigation District. NID is currently working with the CVRWQCB with any testing and we recommend that people, especially children and pets, avoid direct contact with the water located in the holding pond or subsequent tributaries.

Blue Green Algae (Cyanobacteria) FAQ

The blue green algae (BGA), is a type of bacteria called cyanobacteria; are an ancient family of photosynthetic organisms. The fossil record shows that BGA has existed for around 3.5 billion years. It is thought to be one of the first organisms able to carry out photosynthesis.

BGA periodically “blooms,” that is, creates floating mats, forming what is commonly known as “pond scum.” These blooms can be blue-green, olive green, grey-green, yellow–brown or purple to red. The bright lime green algal mats commonly seen in our fresh water bodies are green algae, not BGA.

Blooms are most likely to form during these conditions:

  • the wind is quiet or mild
  • the water is warm but not hot (60 to 86 degrees F, 18 – 25 ºC))
  • the water harbors an abundance of the nutrients nitrogen and phosphorus (i.e., from agricultural or urban runoff (fertilizers) or failing sewage disposal systems).

The occurrence of BGA toxins in the freshwater environment is unpredictable. Blooms may persist for up to seven days but the resulting toxins may last for as long as three weeks. BGA move up and down within the water column and thus may not always float to the surface. Currents and surface winds can push them toward the land, causing poison-filled cells to accumulate in a thick layer near the leeward shore. Low flow river conditions in the summer and fall may result in large build-ups of BGA. When algae cells die or are damaged, toxins may be released at levels harmful to pets and livestock if they drink from impacted waterways. The algal blooms look like green, blue-green, white or brown foam, scum or mats floating on the water. Recreational exposure to toxic blue-green algae can cause eye irritation, allergic skin rash, mouth ulcer, vomiting, diarrhea, and cold and flu-like symptoms.

Predicting where and when toxic blooms will form is difficult if not impossible. The best alternative is to prevent exposure to the BGA blooms and toxins, especially by children or dogs. Green or blue-green scum of algae blooms in the water or against shorelines is an indication that toxic conditions may exist.

People who recreate in areas with a water body should follow these guidelines:

  • Avoid wading or swimming in water where algae blooms are visible and avoid dense mats of algae. Closely supervise young children, as they are more at risk due to their small body size.
  • Do not drink, eat or handle the algae and avoid ingesting the river water.
  • Do not allow pets to swim in or drink river water that is heavily infested with algae.
  • Swimmers should shower and pets be rinsed with tap water immediately after bathing.
  • Use water-resistant gloves to remove unwanted algae from shorelines.
  • Ranchers should not allow livestock to drink water from contaminated rivers or streams

For more information, please visit the CVRWQCB HAB Website:

Additional Info...
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