Mantua Reservoir N
Box Elder County, UtahView mapDirections
Clarity, phosphorus, and algae all sit in the cleanest band for this region.
↑ up from C at the last regrade
- Oligotrophic
- Impaired · Algal growth
- #2 of 15 in the county
- Limited data
Mantua Reservoir N in Box Elder County, Utah has a water quality grade of A (Excellent). Water clarity: 3 ft visibility. Phosphorus: 11 µg/L. Chlorophyll-a (algae): 1.8 µg/L.
- Clarity
- 3 ft
- Type
- Oligotrophic
- Phosphorus
- 11 µg/L
- Chlorophyll-a
- 1.8 µg/L
- Trend
- ↓ Declining
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Watch Mantua Reservoir N’s letter
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Mantua Reservoir N is a cold, clear oligotrophic lake in Box Elder County, Utah. Water quality is graded A based on 2 sampling years through 2021. Overall water quality is declining over recent sampling years. Compared to the 14 other monitored lakes in Box Elder County, Mantua Reservoir N ranks #2 for water quality.
Source: EPA Water Quality Portal sampling records, Utah DWQ, last sampled 2021-06-01. Grade methodology: LakeQuality methodology.
Swimming Safety
Grade onlyLimited data
Based on clarity and algae grades from past sampling — we do not hold live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
The A is nutrients and algae, not a swimming clearance. Mantua Reservoir N is listed impaired under Clean Water Act §303(d) (IR 5) for algal growth, nutrients (phosphorus, nitrogen), oxygen depletion (low DO), temperature. Check local advisories before you swim.
Water Quality Grade: A (Excellent)
Very murky, less than 3 ft of visibility. Phosphorus 11 µg/L. Chlorophyll-a 1.8 µg/L. TSI 46. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 3 ft | Not graded |
| Phosphorus | 11 µg/L | Good |
| Chlorophyll-a (Algae) | 1.8 µg/L | Good |
| Trophic State Index (TSI) | 46 | Oligotrophic |
Low nutrients, clear water. The letter is not a swimming clearance.
Fishing Mantua Reservoir N
Water Quality Trend: ↓ Declining
Based on 6 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↑ Improving | +0.082 m/yr | 6 |
| Phosphorus | ↓ Declining | +20.64 µg/L/yr | 3 |
| Chlorophyll-a | ↓ Declining | +19.23 µg/L/yr | 3 |
EPA Impairment Status
Mantua Reservoir N is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 5).
Causes of impairment
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit UT-L-16010204-033_00 · Official waterbody report · Matched by proximity (0.39 km)
County Ranking
Ranked #2 of 15 lakes in Box Elder County
Nearby Lakes in Box Elder County
Frequently asked questions about Mantua Reservoir N
Where is Mantua Reservoir N?
- Mantua Reservoir N is a lake in Box Elder County, Utah. LakeQuality grades it A for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Mantua Reservoir N?
- Mantua Reservoir N in Box Elder County, Utah has a water quality grade of A (Excellent). Water clarity: 3 ft visibility. Phosphorus: 11 µg/L. Chlorophyll-a (algae): 1.8 µg/L.
Is Mantua Reservoir N safe to swim in?
- The A is nutrients and algae, not a swimming clearance. Mantua Reservoir N is listed impaired under Clean Water Act §303(d) (IR 5) for algal growth, nutrients (phosphorus, nitrogen), oxygen depletion (low DO), temperature. Check local advisories before you swim.
Explore Mantua Reservoir N across LakeQuality
13.2 mi away · recorded at 129°F
Species activity outlook by water temperature and season
Where to stay at Utah's top lakes, each with its grade
Low nutrients, clear water. The letter is not a swimming clearance.
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Where this data comes from
53 measurements from 1 monitoring station, 2015–2021. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Utah Department Of Environmental Quality
UTAHDWQ_WQX53 samples · 2015–2021
Analytical methods
- Field Measurements
- Phosphorus by Colorimetry
- 10200 H ~ Chlorophyll a-b-c Determination
Monitoring station identifiers (1)
Grade thresholds from the EPA National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See how grades are calculated and data sources.
Other records on this page
Impairment status from EPA ATTAINS 303(d) database