Cold weather doesn’t immediately eliminate the danger posed by algal blooms

November 1, 2022
Brigitta Gruenberg, Division of Public Health

Each fall as the weather cools, the Department of Health and Welfare gets questions from anglers eager to cast lines in lakes or rivers where water quality warnings were made.

Water quality warnings are often because of detection of harmful cyanobacterial blooms (“harmful algal blooms”), caused by cyanobacteria and the toxins they can produce. Cyanobacterial blooms, which can be made worse by such things as nutrient pollution and warm water, can have public health, environmental, and economic effects.

As days shorten and the weather cools, it’s the final phase of the annual bloom season. Cyanobacteria populations are still present —but are beginning to decline. However, cyanobacteria do not disappear, and a portion of the bacteria will remain near the surface of a water body even when most of the population settles to the bottom.

Idahoans who love fishing, swimming, and dog walking  should consider avoiding areas that were recently still blooming and keep pets away. The toxins the bacteria produce are not visible and may remain in the water after the bloom has vanished.

This summer and fall, a number of warnings or advisories were issued for cyanobacterial  blooms in Idaho, most recently for Cascade Lake on the Payette River and Henry’s Lake at the headwaters of the Henry’s Fork of the Snake River.

As a general rule for anglers eager to return to their favorite fishing holes or dog walkers who want to let their pets swim, a body of water should not be considered free and clear until it is removed from the Public Health Warnings map at Get Healthy Idaho. Even then, advisories will be removed when toxins meet human health criteria, and dogs can become ill at lower levels, so relying solely on a public health advisory may not protect your pets.

Harmful algal blooms are a health concern throughout the United States. According to a recent report by the Centers for Disease Control and Prevention, 13 states reported 227 harmful algal blooms that resulted in illness in 95 humans and at least 1,170 animals in 2020—the most recent year reported by the agency in depth. Of the affected animals, 94 percent died from their illnesses.

The bottom line for a number of Idaho waterways is that we can expect cyanobacteria don’t immediately disappear with cold weather and will be in the surface water for a while longer. We’re likely to see green patches of water until Idaho’s rivers and lakes ice over.

More resources:

Idaho Recreational Water Health Advisories:

Idaho Department of Health and Welfare FAQ about harmful algal blooms:

CDC resources about harmful algal blooms:

Idaho Department of Environmental Quality about harmful algal blooms:

Brigitta Gruenberg is the Environmental Health Program manager in the Division of Public Health.  

The Idaho Department of Health and Welfare is dedicated to strengthening Idahoans' health, safety, and independence. Learn more at

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