Skip to main content

Oregon State Flag An official website of the State of Oregon »

Harmful Algal Blooms


Harmful algal blooms are caused by high concentrations of certain types of algae that can produce toxic compounds. These blooms can cause sickness and death in humans, pets and livestock who come in contact with or drink the water and also can result in hypoxia (low oxygen) in water bodies, which can kill fish and other wildlife. Oregon has several documented cases of dogs dying and humans becoming ill. The Oregon Health Authority is the agency responsible for posting warnings and educating the public about harmful algal blooms. Once a waterbody is identified as having a harmful algal bloom, DEQ is responsible for investigating the causes, identifying sources of pollution and writing a pollution reduction plan.

Harmful algal blooms have occurred in a number of Oregon’s lakes, reservoirs and rivers. The blooms look different depending on local conditions. They can appear green, blue-green or reddish brown and form foam, slicks, scum or mats.    

DEQ and the Oregon Health Authority are partnering to develop a plan for coordinating, receiving and analyzing water samples for harmful algal blooms at DEQ’s environmental laboratory in Hillsboro at no cost to water suppliers subject to new rules. Read more in the Harmful Algal Blooms FAQs document.  

DEQ's TMDL harmful algal bloom strategy

DEQ developed a Harmful Algal Bloom strategy in 2011 to improve its abilities to address HABs and identify needs to improve this approach. 

Most often harmful algal blooms in freshwaters are caused by cyanobacteria (aka blue-green algae). Algae are simple, often single-celled, plants that are naturally occurring and form the base of the food webs. A small percentage of algae can produce toxic compounds.

Nutrient pollution, warm water, high pH, stagnant water and lots of sunlight can lead to excessive blooms. Introduction of invasive species, such as non-native fish species, may also lead to excessive blooms. Nutrient pollution can come from wastewater treatment plants, residential on-site wastewater treatment systems, agricultural, urban and forestry runoff and natural sources. Introduced fish species also can recycle nutrients within a lake, allowing for more intense blooms. Warm water, high pH, stagnant water and sunlight are conditions that are harder to control in lakes and large rivers than nutrient pollution.

More information about specific lakes can be found in the Atlas of Oregon Lakes.​

Most lakes and reservoirs have a designated management agency which is responsible for managing recreation or drinking water. These agencies include the U.S. Forest Service, U.S. Army Corp of Engineers, lake associations and local government agencies. When a potential harmful algal bloom is observed, these DMAs will collect samples, document conditions and potentially post preliminary warning signs.

When no DMA is identified, or when the DMA is not willing to respond, OHA may ask DEQ to monitor the bloom and collect and ship water samples to an appropriate lab for analysis. Sampling results are shared with OHA which will issue a recreational public health advisory when appropriate. OHA issues a press release, posts advisory information on its website and provides advisory signs.

OHA will lift an advisory when sampling indicates there is a low risk to public health. At lakes or river reaches with known recurring problems, management agencies may post year-round signs with educational information about harmful algal blooms.​

DEQ has the regulatory responsibility for restoring lakes and rivers to support recreation. DEQ tracks water bodies that don’t achieve water quality standards and develops pollution reduction plans. OHA and DEQ jointly develop drinking water protection plans. The state programs regulate pollution sources through water quality permits, licenses and certifications and nonpoint pollution source control. ​​

Protecting high quality waters from harmful algal blooms is achieved by addressing the causes. DEQ does not allow discharge of wastewater to lakes or reservoirs. However, much more work needs to be done to identify waters at risk of developing harmful blooms. ​​

Who to contact about harmful algal blooms

For questions or concerns about harmful algal blooms and human health or advisories, contact OHA. Also see OHA's website for a current list of waterbodies that have harmful algal bloom warnings.