Clark Canyon Reservoir
Beaverhead County, Montana6,606 acresView mapDirections
Nutrients and algae sit mid-range for this region.
- Mesotrophic
- Impaired · Hydrologic alteration
- #1 of 2 in the county
- Limited data
Clark Canyon Reservoir in Beaverhead County, Montana has a water quality grade of C (Fair). Water clarity: 7.4 ft visibility. Phosphorus: 31 µg/L. Chlorophyll-a (algae): 5.2 µg/L. Surface area: 6606 acres.
- Clarity
- 7.4 ft
- Area
- 6,606 acres
- Type
- Mesotrophic
- Phosphorus
- 31 µg/L
- Chlorophyll-a
- 5.2 µg/L
- Trend
- ↓ Declining
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Watch Clark Canyon Reservoir’s letter
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Clark Canyon Reservoir is a 6,606-acre mesotrophic lake in Beaverhead County, Montana. Water quality is graded C based on 1 sampling years through 2018. Overall water quality is declining over recent sampling years. Compared to the 1 other monitored lakes in Beaverhead County, Clark Canyon Reservoir ranks #1 for water quality.
Source: EPA Water Quality Portal sampling records, Montana DEQ, last sampled 2018-10-11. Grade methodology: LakeQuality methodology.
What Clark Canyon Reservoir is good for
From access and monitoring records, not editorial picks.
- Big water6,606 acres
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. Clark Canyon Reservoir is listed impaired under Clean Water Act §303(d) (IR 4C) for hydrologic alteration. Check local advisories before you swim.
Water Quality Grade: C (Fair)
Moderate clarity, visible to about 7.4 ft. Phosphorus 31 µg/L. Chlorophyll-a 5.2 µg/L. TSI 50. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 7.4 ft | Not graded |
| Phosphorus | 31 µg/L | Fair |
| Chlorophyll-a (Algae) | 5.2 µg/L | Not graded |
| Trophic State Index (TSI) | 50 | Mesotrophic |
Moderate nutrients, good water quality
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 6,606 acres |
No bathymetric survey has been published for Clark Canyon Reservoir, so its depth is not documented here.
Fishing Clark Canyon Reservoir
Water Quality Trend: ↓ Declining
Based on 4 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↓ Declining | -2.165 m/yr | 4 |
| Phosphorus | ↓ Declining | +11.5 µg/L/yr | 2 |
Reservoir Info (USACE NID)
Clark Canyon Reservoir is a man-made reservoir impounded by the Clark Canyon Dam (completed 1964), built primarily for flood risk reduction on the Beaverhead River; earth-type dam, 148 ft tall and 2,950 ft long.
- Surface area
- 6,606 ac
- Normal storage
- 178,062 ac-ft
- Max storage
- 328,979 ac-ft
- Drainage area
- 2,321 sq mi
- Hazard class
- High
- Owner
- BUREAU OF RECLAMATION
All listed purposes: Flood Risk Reduction;Irrigation.
Source: USACE National Inventory of Dams, NID ID MT00569 · Operator website
EPA Impairment Status
Clark Canyon Reservoir is officially listed as impaired under Clean Water Act §303(d) in the 2021 EPA reporting cycle (IR category 4C).
Causes of impairment
Source: EPA ATTAINS assessment unit MT41A002_010 · Official waterbody report
County Ranking
Ranked #1 of 2 lakes in Beaverhead County
Nearby Lakes in Beaverhead County
Frequently asked questions about Clark Canyon Reservoir
Where is Clark Canyon Reservoir?
- Clark Canyon Reservoir is a lake in Beaverhead County, Montana. LakeQuality grades it C for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Clark Canyon Reservoir?
- Clark Canyon Reservoir in Beaverhead County, Montana has a water quality grade of C (Fair). Water clarity: 7.4 ft visibility. Phosphorus: 31 µg/L. Chlorophyll-a (algae): 5.2 µg/L. Surface area: 6606 acres.
Is Clark Canyon Reservoir safe to swim in?
- The C is nutrients and algae, not a swimming clearance. Clark Canyon Reservoir is listed impaired under Clean Water Act §303(d) (IR 4C) for hydrologic alteration. Check local advisories before you swim.
Explore Clark Canyon Reservoir across LakeQuality
3 thermal springs within 11.6 mi, nearest McMenomey Ranch Spring
Species activity outlook by water temperature and season
Where to stay at Montana's top lakes, each with its grade
Moderate nutrients, good water quality
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Where this data comes from
180 measurements from 9 monitoring stations, 2015–2018. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Montana DEQ WQPB
MDEQ_WQ_WQX180 samples · 2015–2018
Analytical methods
- Phosphorus by Colorimetry
- 10200 H ~ Chlorophyll a-b-c Determination
Monitoring station identifiers (9)
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