The Nevada Department of Health and Human Services has issued Harmful Algal Bloom (HAB) Watch Advisories for Lahontan Reservoir and Eagle Valley Reservoir following visual observations of algae resembling surface cyanobacteria.

Officials warned that hazardous conditions are possible or present, urging people and pets to exercise caution when in contact with water and to avoid areas where algae accumulate.

Harmful algal blooms form when naturally occurring blue-green algae multiply rapidly, causing discoloration, scum, and mats on the water. These microorganisms, known as cyanobacteria, are closely related to bacteria and can photosynthesize like algae. While most algal blooms are not toxic, some cyanobacteria produce nerve or liver toxins. Toxicity can be unpredictable, as a single species of algae can have both toxic and non-toxic strains.

Officials recommend avoiding contact with any floating mats, scums, and discolored water, as it is difficult to determine whether an algal bloom is harmful. Blooms in Nevada typically occur in freshwater bodies such as lakes, ponds, streams, and rivers, particularly in late summer and early fall when conditions are favorable. These conditions include high nutrient levels, abundant sunlight, warm water temperatures, and stagnant or slow-moving waters.

If these conditions persist for several days, cyanobacteria can multiply to form large blooms that may cover an entire lake or collect in smaller areas. Blooms generally dissipate after one or two weeks.

Lahontan Reservoir is located in northern Nevada along the Carson River and is part of the Bureau of Reclamation’s Newlands Irrigation Project for watering farmland in Lahontan Valley. Lahontan Dam was built in 1905 and water used for irrigation first occurred in 1916. Additionally, water is transferred from the Truckee River at Derby Dam to Lahontan Reservoir via the Truckee Canal.

Eagle Valley Reservoir was created in 1965 and is located within Spring Valley State Park. It occurs within the Colorado River watershed and the outflow, Meadow Valley Wash, enters the Muddy River and eventually into Lake Mead.