Koosharem Reservoir
Sevier County, UtahView mapDirections
Nutrients and algae run well above the regional average.
↓ down from D at the last regrade
- Hypereutrophic
- Impaired · Nutrients (phosphorus, nitrogen)
- #7 of 7 in the county
- Limited data
Koosharem Reservoir in Sevier County, Utah has a water quality grade of F (Algae-prone). Water clarity: 7.2 ft visibility. Phosphorus: 194 µg/L. Chlorophyll-a (algae): 58.4 µg/L.
- Clarity
- 7.2 ft
- Type
- Hypereutrophic
- Phosphorus
- 194 µg/L
- Chlorophyll-a
- 58.4 µg/L
- Trend
- ↓ Declining
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Stays near Koosharem Reservoir
Compare lakeside cabins, hotels, and vacation rentals near Koosharem Reservoir in Sevier County, Utah — live availability and prices for your trip. Switch the map to Satellite to see the lake and how close each stay is to the water. The water-quality grade measures nutrients and algae, not lodging — cabins and rentals around Koosharem Reservoir are listed below.
Top-rated places to stay near Koosharem Reservoir
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Watch Koosharem Reservoir’s letter
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Koosharem Reservoir is a highly nutrient-loaded hypereutrophic lake in Sevier County, Utah. Water quality is graded F based on 1 sampling years through 2020. Overall water quality is declining over recent sampling years. Public access is available with 1 public boat landing. Compared to the 6 other monitored lakes in Sevier County, Koosharem Reservoir ranks #7 for water quality.
Source: EPA Water Quality Portal sampling records, Utah DWQ, last sampled 2020-08-26. Grade methodology: LakeQuality methodology.
What Koosharem 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 F is nutrients and algae, not a swimming clearance. Koosharem Reservoir is listed impaired under Clean Water Act §303(d) (IR 5) for nutrients (phosphorus, nitrogen), oxygen depletion (low DO), pH / acidity, temperature. Check local advisories before you swim.
Water Quality Grade: F (Algae-prone)
Moderate clarity, visible to about 7.2 ft. Phosphorus 194 µg/L. Chlorophyll-a 58.4 µg/L. TSI 66. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 7.2 ft | Not graded |
| Phosphorus | 194 µg/L | Not graded |
| Chlorophyll-a (Algae) | 58.4 µg/L | Poor |
| Trophic State Index (TSI) | 66 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Recreation & Access
- Koosharem Reservoir Recreation SiteSemi-private
Named access points in the USGS National Boat Ramps inventory, within about 3 miles. More lakes with boat ramps
Fishing Koosharem Reservoir
Water Quality Trend: ↓ Declining
Based on 3 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | → Stable | -0.033 m/yr | 3 |
| Chlorophyll-a | ↓ Declining | +6.4 µg/L/yr | 2 |
Reservoir Info (USACE NID)
Koosharem Reservoir is a man-made reservoir impounded by the Koosharem (completed 1919), built primarily for irrigation on the Otter Creek; earth-type dam, 30 ft tall and 1,950 ft long.
- Normal storage
- 3,044.6 ac-ft
- Max storage
- 7,369.8 ac-ft
- Drainage area
- 63 sq mi
- Hazard class
- High
- Owner
- KOOSHAREM IRRIGATION COMPANY
Source: USACE National Inventory of Dams, NID ID UT00165 · Operator website
EPA Impairment Status
Koosharem Reservoir 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-16030002-011_00 · Official waterbody report
County Ranking
Ranked #7 of 7 lakes in Sevier County
Nearby Lakes in Sevier County
Frequently asked questions about Koosharem Reservoir
Where is Koosharem Reservoir?
- Koosharem Reservoir is a lake in Sevier County, Utah. LakeQuality grades it F for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Koosharem Reservoir?
- Koosharem Reservoir in Sevier County, Utah has a water quality grade of F (Algae-prone). Water clarity: 7.2 ft visibility. Phosphorus: 194 µg/L. Chlorophyll-a (algae): 58.4 µg/L.
Is Koosharem Reservoir safe to swim in?
- The F is nutrients and algae, not a swimming clearance. Koosharem Reservoir is listed impaired under Clean Water Act §303(d) (IR 5) for nutrients (phosphorus, nitrogen), oxygen depletion (low DO), pH / acidity, temperature. Check local advisories before you swim.
Are there boat ramps on Koosharem Reservoir?
- Koosharem Reservoir has 1 documented boat landing in the USGS National Boat Ramps inventory. Named access points include Koosharem Reservoir Recreation Site.
Explore Koosharem Reservoir across LakeQuality
4 thermal springs within 18.4 mi, nearest Red Hill Hot Spring
Species activity outlook by water temperature and season
Where to stay at Utah's top lakes, each with its grade
Very high nutrients, dense algae, poor clarity
Utah's cleanest and most-polluted lakes, ranked
Planning a trip around Capitol Reef?
Koosharem Reservoir is about 39 miles from Capitol Reef 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 Capitol Reef →More Utah Lake Rankings
Other Grade F Lakes in Utah
Hypereutrophic Lakes in Utah
Where this data comes from
22 measurements from 1 monitoring station, 2014–2020. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Utah Department Of Environmental Quality
UTAHDWQ_WQX22 samples · 2014–2020
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