Mantua Reservoir Algal Bloom Monitoring 2020

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.

Update November 2, 2020

The Warning Advisory for Mantua Reservoir has been removed as a result of the end of the sampling and monitoring recreation season.

The Utah Division of Water Quality’s water recreation season runs from May 1 through October 31. At this time, no further monitoring or sampling for harmful algal blooms or waterborne pathogens at priority water bodies will be conducted until May 2021.

This does not mean that the potential for harmful algal blooms or waterborne pathogens has ceased. Please remain aware while recreating in or near water bodies and for information on how to keep yourself, your family, and your pets safe, please visit:

Update September 2, 2020

Monitoring and Sampling Update: On 8/26/2020, the DWQ HABs monitoring team visited Mantua Reservoir. The monitoring team observed a harmful algal bloom at the beach that appeared as grass clippings on or within the water surface and/or water column. Cyanotoxin and cell density samples were collected. Results show toxigenic cyanobacteria cell count density at 10,018 cells/mL and microcystin, a liver toxin, concentrations at 0.16 ug/L.

Update August 19, 2020

Mantua Reservoir was visited August 13, 2020. A surface sample consisted of >1.4M cells/ml of cyanobacteria. A depth-integrated composite sample consisted of 319K cells/ml of cyanobacteria. Both samples exceed the recommended recreational threshold for toxin-producing cyanobacteria concentration (100K cells/ml). The cyanobacteria bloom is dominated by Dolichospermum, a known toxin-producer. Cyanotoxin results are pending

Update August 14, 2020

August 4 Toxin Results

  • NW shore: 3.2 ug/L microcystin (surface)
  • Boat ramp: 0.43 ug/L microcystin (surface); n/d (depth-integrated composite)

Update August 6, 2020

The Bear River Health Department has issued a Warning Advisory for Mantua Reservoir and will post signs at the waterbody.

Drone footage taken on the morning of 08/06/2020 shows a lakewide surface bloom.

Update August 04, 2020

On August 04, 2020, the DWQ HABs monitoring team visited Mantua reservoir and observed a high density, lakewide bloom. Samples were collected at the NW Beach area and the Boat Ramp. Toxigenic cyanobacteria cell density at both locations were found to be between 5 to 10 times above the UWQ and Utah Department of Health recommended warning advisory threshold of 100,000 cells/mL. Toxin results are still pending.

Update July 31, 2020

Note: This is not a formal advisory level but rather an indication that a bloom may exist or become more severe based on available information from Utah DWQ and Utah Department of Health.

On Friday July 31, 2020 Brigham City Public works observed a high-density HAB along the shoreline and dock of the reservoir. The bloom appears to be the highest magnitude this season. DWQ sampling crews sampled a lower magnitude bloom on Monday, July 27th and those toxin results are pending from the EPA lab. Bear River Health Department has been informed.

Update July 24, 2020

On July 24, 2020 Toxin results from ChemTech-Ford Laboratories for came back non-detect for microcystin, anatoxin-a, cylindrospermopsin, and nodularian samples taken at the NE Shore of the reservoir on 07/21/2020.

Update July 21, 2020

On July 21, 2020, the DWQ HABs monitoring team visited Mantua reservoir and observed a HAB at the NE shore. The dominant HAB material consisted of spilled paint and isolated clumps. The bloom was located predominantly at the surface only (top few inches). Samples (surface, composite) were collected at this location.
A bloom was also observed at dock. The dominant HAB material consisted of spilled paint.

Update July 19, 2020

On July 19, 2020, DWQ was updated by the UDAF Animal Health Program that a dog died of liver necrosis and symptoms consistent with cyanobacteria toxicosis after swimming in Mantua Reservoir the first week of July. Necropsy results are still pending.