The Utah Department of Environmental Quality issued a warning advisory for Mantua Reservoir in Box Elder County after monitoring confirmed the presence of harmful algal blooms. The advisory, which has been in enough effect since at least June 10, 2026, warns that the algae may be producing dangerous toxins capable of making humans and animals sick.
Safety Precautions and Advisories
The Utah Department of Environmental Quality (DEQ) Division of Water Quality confirmed through monitoring updates on June 10, 2026, and June 17, 2026, that a harmful algal bloom is present at the reservoir. Located approximately five miles east of Brigham City near the edge of Mantua, the reservoir is currently under a warning advisory.
Health Risks and Instructions
Due to the presence of these blooms, state officials have issued specific instructions for anyone visiting the reservoir. The DEQ advises that people should not swim or water ski in the affected waters. Boaters are instructed to avoid areas containing visible algae during their activities.
Risks to Animals and Food Safety
The advisory also includes critical warnings regarding consumption and animal safety. The DEQ stated that visitors should not drink the water and must keep animals away from the reservoir to prevent illness or death. For those who fish in the area, officials recommend cleaning fish thoroughly and discarding the skin and guts.
Understanding Cyanobacteria
Harmful algal blooms, often referred to as blue-green algae, are caused by the rapid multiplication of cyanobacteria. According to the Utah Department of Health and Human Services, these aquatic bacteria form visible colonies consisting of millions of cells.
Environmental Growth Factors
The Utah Department of Health and Human Services notes that cyanobacteria thrive in environments characterized by warm temperatures, sunlight, and nutrient-rich water. When these conditions are met, the bacteria can reproduce rapidly, filling lakes, ponds, and ditches with cloudy, bluish-green growth. While these blooms can be toxic if swallowed in large amounts, they pose a significant threat to pets and other animals, which may become very sick or die upon contact.
Visual Identification Guide
Identifying a bloom can be difficult because their appearance varies significantly. The Utah Department of Environmental Quality has provided photo examples to help recreators recognize potential hazards. Blooms typically occur in standing water like reservoirs or ponds and may appear in shades of green, blue, red, pink, white, gray, or purple.
Appearance and Movement
The texture of these blooms can resemble various materials, including spilled paint, green globules, surface scum, foam, floating mats, or grass clippings. Some blooms may not be visible on the water's surface at all times because they rise and fall within the water column throughout the day. Additionally, wind, temperature, and weather conditions can cause these blooms to move or disperse quickly.
