Black Cnyn Reservoir Ns of
Gem County, Idaho1,100 acresView mapDirections
Clarity, phosphorus, and algae all sit in the cleanest band for this region.
↑ up from B at the last regrade
- Mesotrophic
- #2 of 2 in the county
- Public access
Black Cnyn Reservoir Ns of in Gem County, Idaho has a water quality grade of A (Excellent). Water clarity: 7.5 ft visibility. Phosphorus: 29.5 µg/L. Chlorophyll-a (algae): 3.1 µg/L. Surface area: 1100 acres.
- Clarity
- 7.5 ft
- Area
- 1,100 acres
- Type
- Mesotrophic
- Phosphorus
- 29.5 µg/L
- Chlorophyll-a
- 3.1 µg/L
- Trend
- → Stable
- Public access
- Yes
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Watch Black Cnyn Reservoir Ns of’s letter
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Black Cnyn Reservoir Ns of is a 1,100-acre mesotrophic lake in Gem County, Idaho. Water quality is graded A based on 2 sampling years through 2023. Water clarity is improving over recent sampling years. Limited fish-species data available. Public access is available. Compared to the 1 other monitored lakes in Gem County, Black Cnyn Reservoir Ns of ranks #2 for water quality.
Source: EPA Water Quality Portal sampling records, IDEQ, last sampled 2023-08-17. Grade methodology: LakeQuality methodology.
What Black Cnyn Reservoir Ns of is good for
From access and monitoring records, not editorial picks.
- Big water1,100 acres
Swimming Safety
Grade onlyLow algae
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. Check local advisories before you swim.
Water Quality Grade: A (Excellent)
Moderate clarity, visible to about 7.5 ft. Phosphorus 29.5 µg/L. Chlorophyll-a 3.1 µg/L. TSI 48. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 7.5 ft | Not graded |
| Phosphorus | 29.5 µg/L | Good |
| Chlorophyll-a (Algae) | 3.1 µg/L | Good |
| Trophic State Index (TSI) | 48 | Mesotrophic |
Moderate nutrients, good water quality
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 1,100 acres |
| Public Access | Yes |
No bathymetric survey has been published for Black Cnyn Reservoir Ns of, so its depth is not documented here.
Fishing Black Cnyn Reservoir Ns of
Water Quality Trend: → Stable
Based on 6 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↑ Improving | +0.07 m/yr | 5 |
| Phosphorus | → Stable | -0.19 µg/L/yr | 6 |
| Chlorophyll-a | ↓ Declining | +0.34 µg/L/yr | 5 |
Reservoir Info (USACE NID)
Black Cnyn Reservoir Ns of is a man-made reservoir impounded by the Black Canyon Diversion Dam (completed 1924), built primarily for hydroelectric on the Payette River; concrete-type dam, 183 ft tall and 1,039 ft long.
- Surface area
- 1,100 ac
- Normal storage
- 29,822 ac-ft
- Max storage
- 31,200 ac-ft
- Drainage area
- 2,680 sq mi
- Hazard class
- High
- Owner
- BUREAU OF RECLAMATION
All listed purposes: Hydroelectric;Irrigation.
Source: USACE National Inventory of Dams, NID ID ID00282 · Operator website · Matched by proximity (0.52 km)
County Ranking
Ranked #2 of 2 lakes in Gem County
Nearby Lakes in Gem County
Frequently asked questions about Black Cnyn Reservoir Ns of
Where is Black Cnyn Reservoir Ns of?
- Black Cnyn Reservoir Ns of is a lake in Gem County, Idaho. LakeQuality grades it A for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Black Cnyn Reservoir Ns of?
- Black Cnyn Reservoir Ns of in Gem County, Idaho has a water quality grade of A (Excellent). Water clarity: 7.5 ft visibility. Phosphorus: 29.5 µg/L. Chlorophyll-a (algae): 3.1 µg/L. Surface area: 1100 acres.
Is Black Cnyn Reservoir Ns of safe to swim in?
- The A is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
Explore Black Cnyn Reservoir Ns of across LakeQuality
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Species activity outlook by water temperature and season
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Where this data comes from
36 measurements from 1 monitoring station, 2012–2023. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Bureau of Reclamation
1119USBR_WQX36 samples · 2012–2023
Analytical methods
- Depth, Secchi disk depth
- Zebra/Quagga Mussel APHA 10200G Modified
- Phosphorus by Single Reagent Colorimetry
- 10200 H ~ Chlorophyll a-b-c Determination
- Turbidity by Nephelometry
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.