Kolob Reservoir
Washington County, UtahView mapDirections
Nutrients and algae sit mid-range for this region.
↓ down from A at the last regrade
- Mesotrophic
- #8 of 10 in the county
- Limited data
Kolob Reservoir in Washington County, Utah has a water quality grade of C (Fair). Water clarity: 10.5 ft visibility. Phosphorus: 14.3 µg/L. Chlorophyll-a (algae): 2.1 µg/L.
- Clarity
- 10.5 ft
- Type
- Mesotrophic
- Phosphorus
- 14.3 µg/L
- Chlorophyll-a
- 2.1 µg/L
- Trend
- ↑ Improving
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Watch Kolob Reservoir’s letter
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Kolob Reservoir is a mesotrophic lake in Washington County, Utah. Water quality is graded C based on 2 sampling years through 2025. Overall water quality is improving over recent sampling years. Compared to the 9 other monitored lakes in Washington County, Kolob Reservoir ranks #8 for water quality.
Source: EPA Water Quality Portal sampling records, Utah DWQ, last sampled 2025-08-18. Grade methodology: LakeQuality methodology.
What Kolob Reservoir is good for
From access and monitoring records, not editorial picks.
- Clear water10.5 ft of visibility
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 C is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
Water Quality Grade: C (Fair)
Good clarity, visible to about 10.5 ft. Phosphorus 14.3 µg/L. Chlorophyll-a 2.1 µg/L. TSI 41. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 10.5 ft | Not graded |
| Phosphorus | 14.3 µg/L | Good |
| Chlorophyll-a (Algae) | 2.1 µg/L | Fair |
| Trophic State Index (TSI) | 41 | Mesotrophic |
Moderate nutrients, good water quality
Fishing Kolob Reservoir
Water Quality Trend: ↑ Improving
Based on 2 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↑ Improving | +0.092 m/yr | 2 |
| Chlorophyll-a | ↑ Improving | -0.5 µg/L/yr | 2 |
Reservoir Info (USACE NID)
Kolob Reservoir is a man-made reservoir impounded by the Kolob Creek (completed 1956), built primarily for irrigation on the Kolob Creek; earth-type dam, 61 ft tall and 686 ft long.
- Normal storage
- 5,586 ac-ft
- Max storage
- 7,189 ac-ft
- Drainage area
- 8 sq mi
- Hazard class
- High
- Owner
- WASHINGTON COUNTY WATER CONSERVANCY DIST
Source: USACE National Inventory of Dams, NID ID UT00164 · Operator website · Matched by proximity (0.18 km)
County Ranking
Ranked #8 of 10 lakes in Washington County
Nearby Lakes in Washington County
Frequently asked questions about Kolob Reservoir
Where is Kolob Reservoir?
- Kolob Reservoir is a lake in Washington County, Utah. LakeQuality grades it C for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Kolob Reservoir?
- Kolob Reservoir in Washington County, Utah has a water quality grade of C (Fair). Water clarity: 10.5 ft visibility. Phosphorus: 14.3 µg/L. Chlorophyll-a (algae): 2.1 µg/L.
Is Kolob Reservoir safe to swim in?
- The C is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
Explore Kolob Reservoir across LakeQuality
20.8 mi away · recorded at 108°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
Utah's cleanest and most-polluted lakes, ranked
Planning a trip around Zion?
Kolob Reservoir is about 9 miles from Zion National Park. Crowds there swing hard by season, so the month you pick changes both how busy the park is and what lodging costs.
Best time to visit Zion →More Utah Lake Rankings
Other Grade C Lakes in Utah
Mesotrophic Lakes in Utah
Where this data comes from
18 measurements from 1 monitoring station, 2019–2025. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Utah Department Of Environmental Quality
UTAHDWQ_WQX18 samples · 2019–2025
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.