Upper Stillwater Reservoir
Duchesne County, Utah329 acresView mapDirections
Nutrients and algae sit mid-range for this region.
↓ down from B at the last regrade
- Mesotrophic
- #6 of 9 in the county
- Limited data
Upper Stillwater Reservoir in Duchesne County, Utah has a water quality grade of C (Fair). Water clarity: 8.7 ft visibility. Phosphorus: 11.1 µg/L. Chlorophyll-a (algae): 2.8 µg/L. Surface area: 329 acres.
- Clarity
- 8.7 ft
- Area
- 329 acres
- Type
- Mesotrophic
- Phosphorus
- 11.1 µg/L
- Chlorophyll-a
- 2.8 µg/L
- Trend
- ↑ Improving
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Watch Upper Stillwater Reservoir’s letter
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Upper Stillwater Reservoir is a 329-acre mesotrophic lake in Duchesne County, Utah. Water quality is graded C based on 2 sampling years through 2024. Overall water quality is improving over recent sampling years. Compared to the 8 other monitored lakes in Duchesne County, Upper Stillwater Reservoir ranks #6 for water quality.
Source: EPA Water Quality Portal sampling records, Utah DWQ, last sampled 2024-07-29. Grade methodology: LakeQuality methodology.
What Upper Stillwater Reservoir is good for
From access and monitoring records, not editorial picks.
- Camping1 public campground nearby
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)
Moderate clarity, visible to about 8.7 ft. Phosphorus 11.1 µg/L. Chlorophyll-a 2.8 µg/L. TSI 42. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 8.7 ft | Not graded |
| Phosphorus | 11.1 µg/L | Good |
| Chlorophyll-a (Algae) | 2.8 µg/L | Fair |
| Trophic State Index (TSI) | 42 | Mesotrophic |
Moderate nutrients, good water quality
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 329 acres |
No bathymetric survey has been published for Upper Stillwater Reservoir, so its depth is not documented here.
Recreation & Access
Public campgrounds near Upper Stillwater Reservoir
- Upper Stillwater0.6 km
Nearest public campgrounds in the USGS National Map inventory, within about 3 miles. An empty list means USGS records no campground that close, which is not the same as camping being unavailable. Where the source gives an official reservation link, it is used; the cabin and hotel search does not cover campsites. More camping lakes
Fishing Upper Stillwater Reservoir
Water Quality Trend: ↑ Improving
Based on 2 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↑ Improving | +1.45 m/yr | 2 |
| Phosphorus | ↑ Improving | -0.8 µg/L/yr | 2 |
| Chlorophyll-a | ↑ Improving | -5 µg/L/yr | 2 |
Reservoir Info (USACE NID)
Upper Stillwater Reservoir is a man-made reservoir impounded by the Upper Stillwater Dam (completed 1987), built primarily for irrigation on the Rock Creek; concrete-type dam, 292 ft tall and 2,660 ft long.
- Surface area
- 329 ac
- Normal storage
- 33,123 ac-ft
- Max storage
- 35,253 ac-ft
- Drainage area
- 98.9 sq mi
- Hazard class
- High
- Owner
- DEPARTMENT OF THE INTERIOR
All listed purposes: Irrigation;Recreation;Water Supply.
Source: USACE National Inventory of Dams, NID ID UT00608 · Operator website
County Ranking
Ranked #6 of 9 lakes in Duchesne County
Nearby Lakes With Higher Grades
Upper Stillwater Reservoir holds Grade C. One nearby lake has a higher nutrient-and-algae grade — useful for a swim day, not a reason to skip lodging here.
Nearby Lakes in Duchesne County
Frequently asked questions about Upper Stillwater Reservoir
Where is Upper Stillwater Reservoir?
- Upper Stillwater Reservoir is a lake in Duchesne County, Utah. LakeQuality grades it C for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Upper Stillwater Reservoir?
- Upper Stillwater Reservoir in Duchesne County, Utah has a water quality grade of C (Fair). Water clarity: 8.7 ft visibility. Phosphorus: 11.1 µg/L. Chlorophyll-a (algae): 2.8 µg/L. Surface area: 329 acres.
Is Upper Stillwater Reservoir safe to swim in?
- The C is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
Are there campgrounds on Upper Stillwater Reservoir?
- Upper Stillwater Reservoir has 1 public campground from USGS The National Map within 5 km, including Upper Stillwater. This is a location join, not a booking. Cabins and hotels are listed separately; campsite reservations use the official link when the source provides one.
Explore Upper Stillwater Reservoir across LakeQuality
10.3 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
Utah's cleanest and most-polluted lakes, ranked
More Utah Lake Rankings
Other Grade C Lakes in Utah
Summit County · 7.5 ft
Wasatch County · 20 ft
Wasatch County · 17.1 ft
Wasatch County · 16.4 ft
Wasatch County · 15.6 ft
Wasatch County · 8.2 ft
Mesotrophic Lakes in Utah
Where this data comes from
15 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_WQX15 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.
Other records on this page
Public campgrounds from USGS The National Map Structures, within about 3 miles