Lake Nighthorse
La Plata County, Colorado1,548 acresView mapDirections
Nutrients and algae sit mid-range for this region.
↓ down from A at the last regrade
- Oligotrophic
- #1 of 2 in the county
- Limited data
Lake Nighthorse in La Plata County, Colorado has a water quality grade of C (Fair). Water clarity: 18.7 ft visibility. Phosphorus: 45 µg/L. Chlorophyll-a (algae): 0.7 µg/L. Surface area: 1548 acres.
- Clarity
- 18.7 ft
- Area
- 1,548 acres
- Type
- Oligotrophic
- Trend
- ↑ Improving
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Watch Lake Nighthorse’s letter
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Lake Nighthorse is a 1,548-acre cold, clear oligotrophic lake in La Plata County, Colorado. Water quality is graded C based on 3 sampling years through 2023. Overall water quality is improving over recent sampling years. Compared to the 1 other monitored lakes in La Plata County, Lake Nighthorse ranks #1 for water quality.
Source: EPA Water Quality Portal sampling records, CDPHE, last sampled 2023-09-13. Grade methodology: LakeQuality methodology.
What Lake Nighthorse is good for
From access and monitoring records, not editorial picks.
- Clear water18.7 ft of visibility
- Big water1,548 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. Lake Nighthorse is listed impaired under Clean Water Act §303(d) (IR 5) for metals (other than mercury). Check local advisories before you swim.
Water Quality Grade: C (Fair)
Crystal clear, you can see 18.7 ft down. Phosphorus 45 µg/L. Chlorophyll-a 0.7 µg/L. TSI 40. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 18.7 ft | Not graded |
| Phosphorus | 45 µg/L | Poor |
| Chlorophyll-a (Algae) | 0.7 µg/L | Good |
| Trophic State Index (TSI) | 40 | Oligotrophic |
Low nutrients, clear water. The letter is not a swimming clearance.
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 1,548 acres |
No bathymetric survey has been published for Lake Nighthorse, so its depth is not documented here.
Fishing Lake Nighthorse
Water Quality Trend: ↑ Improving
Based on 2 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Chlorophyll-a | ↑ Improving | -1.4 µg/L/yr | 2 |
Reservoir Info (USACE NID)
Lake Nighthorse is a man-made reservoir impounded by the Ridges Basin Dam (completed 2008), built primarily for water supply on the Basin Creek; earth-type dam, 273 ft tall and 1,600 ft long.
- Surface area
- 1,548 ac
- Normal storage
- 121,070 ac-ft
- Max storage
- 139,809 ac-ft
- Drainage area
- 13.88 sq mi
- Hazard class
- High
- Owner
- BUREAU OF RECLAMATION
Source: USACE National Inventory of Dams, NID ID CO02952 · Operator website
EPA Impairment Status
Lake Nighthorse is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 5).
Causes of impairment
Source: EPA ATTAINS assessment unit COSJAF22_A · Official waterbody report
County Ranking
Ranked #1 of 2 lakes in La Plata County
Nearby Lakes in La Plata County
Frequently asked questions about Lake Nighthorse
Where is Lake Nighthorse?
- Lake Nighthorse is a lake in La Plata County, Colorado. LakeQuality grades it C for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Lake Nighthorse?
- Lake Nighthorse in La Plata County, Colorado has a water quality grade of C (Fair). Water clarity: 18.7 ft visibility. Phosphorus: 45 µg/L. Chlorophyll-a (algae): 0.7 µg/L. Surface area: 1548 acres.
Is Lake Nighthorse safe to swim in?
- The C is nutrients and algae, not a swimming clearance. Lake Nighthorse is listed impaired under Clean Water Act §303(d) (IR 5) for metals (other than mercury). Check local advisories before you swim.
Explore Lake Nighthorse across LakeQuality
3 thermal springs within 16.9 mi, nearest Trimble Hot Spr, Stratten Warm Spr
Species activity outlook by water temperature and season
Where to stay at Colorado's top lakes, each with its grade
Low nutrients, clear water. The letter is not a swimming clearance.
Colorado's cleanest and most-polluted lakes, ranked
Planning a trip around Mesa Verde?
Lake Nighthorse is about 31 miles from Mesa Verde 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 Mesa Verde →More Colorado Lake Rankings
Other Grade C Lakes in Colorado
Oligotrophic Lakes in Colorado
Where this data comes from
29 measurements from 4 monitoring stations, 2021–2023. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Colorado Dept. of Public Health & Environment-WQCD
21COL001_WQX29 samples · 2021–2023
Analytical methods
- Secchi disk depth measurement
- Phosphorus by Colorimetry
- 10200 H ~ Chlorophyll a-b-c Determination
- Nitrogen in Water by Pyrolysis Detection
Monitoring station identifiers (4)
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