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Mantua Reservoir is located in Box Elder County, approximately 5 miles east of Brigham City, at the edge of the town of Mantua. DEQ’s recreational water quality monitoring checks for two health concerns: Harmful Algal Blooms (HABs) and Waterborne Pathogens (E. coli). We post monitoring updates on this page as soon as we can, but our map (also found on HABs.utah.gov) will always have the most recent information.
Updates
November 1, 2025: Recreational water quality monitoring update
Recreational water quality monitoring has ended for the season. Monitoring will resume in June or July 2026. A harmful algal bloom may still be present at this waterbody. Know what to look for and always check before recreating.
October 10, 2025: Harmful algal bloom (HAB) monitoring update
BE CAREFUL: A harmful algal bloom is present at Mantua Reservoir. Algae in the water may be producing dangerous toxins that can make humans and animals sick.
Bear River Health Department has issued a WARNING ADVISORY.
If you visit Mantua Reservoir:
- Do not swim or water ski
- Avoid areas of algae when boating
- Clean fish well and discard guts
- Keep animals away
- Don’t drink the water
- Know how to recognize a harmful algal bloom
September 9, 2025: Harmful algal bloom (HAB) monitoring update
BE CAREFUL: A harmful algal bloom is present at Mantua Reservoir. Algae in the water may be producing dangerous toxins that can make humans and animals sick.
Bear River Health Department has issued a WARNING ADVISORY.
If you visit Mantua Reservoir:
- Do not swim or water ski
- Avoid areas of algae when boating
- Clean fish well and discard guts
- Keep animals away
- Don’t drink the water
- Know how to recognize a harmful algal bloom
July 29, 2025: Harmful algal bloom (HAB) monitoring update
BE CAREFUL: A harmful algal bloom is present at Mantua Reservoir. Algae in the water may be producing dangerous toxins that can make humans and animals sick.
Bear River Health Department has issued a WARNING ADVISORY.
If you visit Mantua Reservoir:
- Do not swim or water ski
- Avoid areas of algae when boating
- Clean fish well and discard guts
- Keep animals away
- Don’t drink the water
- Know how to recognize a harmful algal bloom
July 7, 2025: Harmful algal bloom (HAB) monitoring update
BE CAREFUL: A harmful algal bloom is present at Mantua Reservoir. Algae in the water may be producing dangerous toxins that can make humans and animals sick.
Bear River Health Department has issued a WARNING ADVISORY.
If you visit Mantua Reservoir:
- Do not swim or water ski
- Avoid areas of algae when boating
- Clean fish well and discard guts
- Keep animals away
- Don’t drink the water
- Know how to recognize a harmful algal bloom
June 30, 2025: Harmful algal bloom (HAB) monitoring update
BE CAREFUL: A harmful algal bloom is present at Mantua Reservoir. Algae in the water may be producing dangerous toxins that can make humans and animals sick.
Bear River Health Department has issued a WARNING ADVISORY.
If you visit Mantua Reservoir:
- Do not swim or water ski
- Avoid areas of algae when boating
- Clean fish well and discard guts
- Keep animals away
- Don’t drink the water
- Know how to recognize a harmful algal bloom
June 12, 2025: Harmful algal bloom (HAB) monitoring update
GOOD NEWS: Our monitoring team did not see any visible signs of a harmful algal bloom. Learn more about harmful algal blooms.
Sites visited: Boat ramp, NE near Indian Trail road and Dike road.