Division of Water Quality
Mantua Reservoir Recreational Monitoring
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.
Warning Advisory
Harmful Algae Present
- Do not swim or water ski
- Avoid areas of algae when boating
- Keep animals away
- Do not drink the water
- Clean fish well and discard guts
Updates
October 1, 2026: 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.
If you visit the area:
- Do not swim or water ski
- Avoid areas of algae when boating
- Clean fish well and discard skin and guts
- Keep animals away
- Don't drink the water
- Know how to recognize a harmful algal bloom
September 2, 2026: 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.
If you visit the area:
- Do not swim or water ski
- Avoid areas of algae when boating
- Clean fish well and discard skin and guts
- Keep animals away
- Don't drink the water
- Know how to recognize a harmful algal bloom
August 24, 2026: 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.
If you visit the area:
- Do not swim or water ski
- Avoid areas of algae when boating
- Clean fish well and discard skin and guts
- Keep animals away
- Don't drink the water
- Know how to recognize a harmful algal bloom
August 19, 2026: 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.
If you visit the area:
- Do not swim or water ski
- Avoid areas of algae when boating
- Clean fish well and discard skin and guts
- Keep animals away
- Don't drink the water
- Know how to recognize a harmful algal bloom
August 3, 2026: 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.
If you visit the area:
- Do not swim or water ski
- Avoid areas of algae when boating
- Clean fish well and discard skin and guts
- Keep animals away
- Don't drink the water
- Know how to recognize a harmful algal bloom
July 8, 2026: Harmful algal bloom (HAB) monitoring update
KEEP OUT: A harmful algal bloom is present at Mantua Reservoir. Algae in the water is producing high levels of dangerous toxins. Accidentally drinking this water may hurt or kill humans and animals. Any exposure to this water may cause illness.
If you visit the area:
- Do not swim or water ski
- Use caution if boating in this area and avoid areas of algae
- Use caution if fishing in this area
- Keep animals away
- Don't drink the water
- Know how to recognize a harmful algal bloom
June 17, 2026: 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.
If you visit the area:
- Do not swim or water ski
- Avoid areas of algae when boating
- Clean fish well and discard skin and guts
- Keep animals away
- Don't drink the water
- Know how to recognize a harmful algal bloom
June 10, 2026: 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.
If you visit the area:
- Do not swim or water ski
- Avoid areas of algae when boating
- Clean fish well and discard skin and guts
- Keep animals away
- Don't drink the water
- Know how to recognize a harmful algal bloom