Moon Lake
Duchesne County, Utah773 acresView mapDirections
Clarity, phosphorus, and algae all sit in the cleanest band for this region.
↑ up from B at the last regrade
- Mesotrophic
- #3 of 9 in the county
Moon Lake in Duchesne County, Utah has a water quality grade of A (Excellent). Water clarity: 9.5 ft visibility. Phosphorus: 11.2 µg/L. Chlorophyll-a (algae): 2.7 µg/L. Surface area: 773 acres.
- Clarity
- 9.5 ft
- Area
- 773 acres
- Type
- Mesotrophic
- Phosphorus
- 11.2 µg/L
- Chlorophyll-a
- 2.7 µg/L
- Trend
- ↓ Declining
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Watch Moon Lake’s letter
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Moon Lake is a 773-acre mesotrophic lake in Duchesne County, Utah. Water quality is graded A based on 2 sampling years through 2024. Overall water quality is declining over recent sampling years. Compared to the 8 other monitored lakes in Duchesne County, Moon Lake ranks #3 for water quality.
Source: EPA Water Quality Portal sampling records, Utah DWQ, last sampled 2024-08-21. Grade methodology: LakeQuality methodology.
Swimming Safety
Grade onlyLow algae
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. Check local advisories before you swim.
Water Quality Grade: A (Excellent)
Moderate clarity, visible to about 9.5 ft. Phosphorus 11.2 µg/L. Chlorophyll-a 2.7 µg/L. TSI 41. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 9.5 ft | Not graded |
| Phosphorus | 11.2 µg/L | Good |
| Chlorophyll-a (Algae) | 2.7 µg/L | Good |
| Trophic State Index (TSI) | 41 | Mesotrophic |
Moderate nutrients, good water quality
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 773 acres |
No bathymetric survey has been published for Moon Lake, so its depth is not documented here.
Fishing Moon Lake
Water Quality Trend: ↓ Declining
Based on 3 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↓ Declining | -0.181 m/yr | 3 |
| Phosphorus | ↑ Improving | -0.6 µg/L/yr | 2 |
| Chlorophyll-a | ↓ Declining | +1.6 µg/L/yr | 2 |
Reservoir Info (USACE NID)
Moon Lake is a man-made reservoir impounded by the Moon Lake Dam (completed 1938), built primarily for irrigation on the Lake Fork River; earth-type dam, 101 ft tall and 1,108 ft long.
- Surface area
- 773 ac
- Normal storage
- 49,500 ac-ft
- Max storage
- 49,500 ac-ft
- Drainage area
- 112.25 sq mi
- Hazard class
- High
- Owner
- BUREAU OF RECLAMATION
Source: USACE National Inventory of Dams, NID ID UT10128 · Operator website
County Ranking
Ranked #3 of 9 lakes in Duchesne County
Nearby Lakes in Duchesne County
Frequently asked questions about Moon Lake
Where is Moon Lake?
- Moon Lake is a lake in Duchesne County, Utah. LakeQuality grades it A for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Moon Lake?
- Moon Lake in Duchesne County, Utah has a water quality grade of A (Excellent). Water clarity: 9.5 ft visibility. Phosphorus: 11.2 µg/L. Chlorophyll-a (algae): 2.7 µg/L. Surface area: 773 acres.
Is Moon Lake safe to swim in?
- The A is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
Explore Moon Lake across LakeQuality
19.5 mi away · recorded at 79°F
Species activity outlook by water temperature and season
Where to stay at Utah's top lakes, each with its grade
Moderate nutrients, good water quality
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Where this data comes from
39 measurements from 1 monitoring station, 2017–2024. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Utah Department Of Environmental Quality
UTAHDWQ_WQX39 samples · 2017–2024
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.