Gunlock Reservoir
Washington County, UtahView mapDirections
Clarity, phosphorus, and algae all sit in the cleanest band for this region.
↑ up from B at the last regrade
- Mesotrophic
- Impaired · Nutrients (phosphorus, nitrogen)
- #1 of 10 in the county
- Public access
- Limited data
Gunlock Reservoir in Washington County, Utah has a water quality grade of A (Excellent). Water clarity: 4.6 ft visibility. Phosphorus: 21 µg/L. Chlorophyll-a (algae): 2.9 µg/L.
- Clarity
- 4.6 ft
- Type
- Mesotrophic
- Phosphorus
- 21 µg/L
- Chlorophyll-a
- 2.9 µg/L
- Trend
- → Stable
- Public access
- Yes
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Watch Gunlock Reservoir’s letter
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Gunlock Reservoir is a mesotrophic lake in Washington County, Utah. Water quality is graded A based on 2 sampling years through 2025. Limited fish-species data available. Public access is available with 1 public boat landing. Compared to the 9 other monitored lakes in Washington County, Gunlock Reservoir ranks #1 for water quality.
Source: EPA Water Quality Portal sampling records, Utah DWQ, last sampled 2025-08-19. Grade methodology: LakeQuality methodology.
What Gunlock Reservoir is good for
From access and monitoring records, not editorial picks.
- Boat launch1 documented ramp
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. Gunlock Reservoir is listed impaired under Clean Water Act §303(d) (IR 4A) for nutrients (phosphorus, nitrogen). Check local advisories before you swim.
Water Quality Grade: A (Excellent)
Murky, only visible to about 4.6 ft. Phosphorus 21 µg/L. Chlorophyll-a 2.9 µg/L. TSI 48. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 4.6 ft | Not graded |
| Phosphorus | 21 µg/L | Good |
| Chlorophyll-a (Algae) | 2.9 µg/L | Good |
| Trophic State Index (TSI) | 48 | Mesotrophic |
Moderate nutrients, good water quality
Recreation & Access
- Gunlock State ParkPublic
Named access points in the USGS National Boat Ramps inventory, within about 3 miles. More lakes with boat ramps
Fishing Gunlock Reservoir
Water Quality Trend: → Stable
Based on 4 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | → Stable | -0.036 m/yr | 4 |
| Chlorophyll-a | → Stable | +0.05 µg/L/yr | 2 |
Reservoir Info (USACE NID)
Gunlock Reservoir is a man-made reservoir impounded by the Gunlock (completed 1970), built primarily for irrigation on the Santa Clara River; earth-type dam, 117 ft tall and 1,400 ft long.
- Normal storage
- 10,884 ac-ft
- Max storage
- 15,041 ac-ft
- Drainage area
- 306 sq mi
- Hazard class
- High
- Owner
- LOWER GUNLOCK RESERVOIR CORPORATION
Source: USACE National Inventory of Dams, NID ID UT00129 · Operator website
EPA Impairment Status
Gunlock Reservoir is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 4A).
Causes of impairment
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit UT-L-15010008-001_00 · Official waterbody report
County Ranking
Ranked #1 of 10 lakes in Washington County
Nearby Lakes in Washington County
Frequently asked questions about Gunlock Reservoir
Where is Gunlock Reservoir?
- Gunlock Reservoir is a lake in Washington County, Utah. LakeQuality grades it A for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Gunlock Reservoir?
- Gunlock Reservoir in Washington County, Utah has a water quality grade of A (Excellent). Water clarity: 4.6 ft visibility. Phosphorus: 21 µg/L. Chlorophyll-a (algae): 2.9 µg/L.
Is Gunlock Reservoir safe to swim in?
- The A is nutrients and algae, not a swimming clearance. Gunlock Reservoir is listed impaired under Clean Water Act §303(d) (IR 4A) for nutrients (phosphorus, nitrogen). Check local advisories before you swim.
Are there boat ramps on Gunlock Reservoir?
- Gunlock Reservoir has 1 documented boat landing in the USGS National Boat Ramps inventory. Named access points include Gunlock State Park. Public access is listed as yes.
Explore Gunlock Reservoir across LakeQuality
4 thermal springs within 15.8 mi, nearest Veyo Hot Spring
19.2 mi away · cable-pulled, no boat wake
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
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Other Grade A Lakes in Utah
Mesotrophic Lakes in Utah
Where this data comes from
38 measurements from 1 monitoring station, 2013–2025. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Utah Department Of Environmental Quality
UTAHDWQ_WQX38 samples · 2013–2025
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.
Other records on this page
Impairment status from EPA ATTAINS 303(d) database
Named boat-access points from the USGS National Boat Ramps inventory