Mantua Reservoir

Warning Advisory

  • Do not swim or water ski in this area
  • Do not ingest the water
  • Keep pets and livestock away
  • Clean fish well and discard guts
  • Avoid areas of scum when boating

Report a Bloom

24-Hour DEQ Environment Incidents Line: (801) 536-4123

Call Utah Poison Control Center

If you believe you or your pet have been exposed to a harmful algal bloom, call (800) 222-1222.

Visit the 2019 Photo Gallery

Get BloomWatch App

Help us track cyanobacteria blooms with your smartphone. Go to for more info.

Update August 19, 2019

Mantua Reservoir, August 15, 2019

Mantua Reservoir, August 15, 2019

The Bear River Health Department (BRHD) issued a Warning Advisory for Mantua Reservoir on August 19, 2019, after samples collected in the reservoir last week showed high cyanobacteria cell-count densities. Health officials are waiting for follow-up samples before taking further action. BRHD has posted signs around the reservoir to alert the public about the Warning Advisory.

Cyanobacteria cell-count concentrations from samples collected by the Division of Water Quality (DWQ) at the public dock at Mantua Reservoir on August 15, 2019, were approximately 11.5 million cells per milliliter (cells/mL). These levels exceed the recreation health-based threshold for a Danger Advisory. Aphanizomenon and Dolichospermum were the primary taxa in the samples. Both are toxin-producing species, particularly for microcystin and anatoxin-a.

Toxin test results were detect for anatoxin-a, which exceeds the recreation health-based threshold for a Warning Advisory. Microcystin levels were well below the health advisory threshold.

A Warning Advisory indicates a moderate relative probability of acute health risk, cell-count density of 20,000 – 10 million cells per milliliter (cells/ml), microcystin levels of 4-2,000 micrograms per liter (µg/L), or anatoxin-a levels above non-detect.

Data Summary

Surface sample

  • Sample date: August 15, 2019
  • Microcystin: 0.33 micrograms per liter (µg/L)
  • Anatoxin-a: 0.17 µg/L
  • Cell counts: 11,447,127 cells per milliliter (cells/mL)

Elbow-depth integrated composite sample

  • Sample Date: August 15, 2019
  • Microcystin: <0.10 micrograms per liter (µg/L)
  • Anatoxin-a: <0.10 µg/L
  • Cell Counts: 116,859 cells per milliliter (cells/mL)
Mantua Reservoir, August 15, 2019

Mantua Reservoir, August 15, 2019

Update August 15, 2019

A possible harmful algal bloom (HAB) at Mantua Reservoir was observed by Brigham City staff the week of August 12, 2019. The Division of Water Quality (DWQ) sent a staff member to visit the reservoir on August 15, 2019, to collect samples and observe water conditions after DWQ was notified on August 14, 2019, that the reservoir might be experiencing a bloom.

DWQ won’t be able to confirm the presence of cyanobacteria in Mantua Reservoir until it receives sample results back from the Utah Public Health Lab (UPHL). Lab results for toxins are expected back on August 16, 2019, and for cyanobacteria cell count concentrations on August 19, 2019.

Mantua Reservoir, August 15, 2019

Brigham City notified city residents of the possible HAB via email and the city’s Facebook page. Information shared with residents noted that cyanobacteria and cyanotoxins have been detected periodically in the reservoir and included precautions to protect human and animal health. Users of Mantua Reservoir are advised by Brigham City to avoid areas of scum, not drink the water, keep pets and livestock away, and thoroughly clean fish and properly discard guts.

DWQ will post lab results as soon as they become available.