Grand Lake
Grand County, ColoradoView mapDirections
Nutrients and algae sit mid-range for this region.
↓ down from B at the last regrade
- Mesotrophic
- #5 of 7 in the county
- 25 ft max depth
Grand Lake in Grand County, Colorado has a water quality grade of C (Fair). Water clarity: 8.7 ft visibility. Phosphorus: 14 µg/L. Chlorophyll-a (algae): 2.7 µg/L.
- Clarity
- 8.7 ft
- Max depth
- ~25 ft est.
- Type
- Mesotrophic
- Trend
- ↑ Improving
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Watch Grand Lake’s letter
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Grand Lake is a mesotrophic lake in Grand County, Colorado with an estimated average depth of about 25 feet. Water quality is graded C based on 2 sampling years through 2024. Overall water quality is improving over recent sampling years. Compared to the 6 other monitored lakes in Grand County, Grand Lake ranks #5 for water quality.
Source: EPA Water Quality Portal sampling records, CDPHE, last sampled 2024-01-30. Grade methodology: LakeQuality methodology.
Swimming Safety
Grade onlyModerate
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. Grand Lake 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)
Moderate clarity, visible to about 8.7 ft. Phosphorus 14 µg/L. Chlorophyll-a 2.7 µg/L. TSI 43. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 8.7 ft | Not graded |
| Phosphorus | 14 µg/L | Good |
| Chlorophyll-a (Algae) | 2.7 µg/L | Fair |
| Trophic State Index (TSI) | 43 | Mesotrophic |
Moderate nutrients, good water quality
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | ~25 ft est. |
Depth marked “est.” is a modeled maximum from GLOBathy (Khazaei et al. 2022, CC0), not a measured survey. Colorado has not published bathymetry for Grand Lake.
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth here is modelled, not surveyed — HydroLAKES v1.0 estimates lake depth from surrounding terrain rather than from soundings. Treat it as an approximation, and prefer a state bathymetric survey where one exists.
Fishing Grand Lake
Water Quality Trend: ↑ Improving
Based on 3 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↑ Improving | +5.25 m/yr | 3 |
| Phosphorus | ↑ Improving | -1 µg/L/yr | 3 |
| Chlorophyll-a | ↑ Improving | -0.3 µg/L/yr | 2 |
EPA Impairment Status
Grand Lake 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 COUCUC12_E · Official waterbody report
County Ranking
Ranked #5 of 7 lakes in Grand County
Nearby Lakes in Grand County
Frequently asked questions about Grand Lake
Where is Grand Lake?
- Grand Lake is a lake in Grand County, Colorado. LakeQuality grades it C for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Grand Lake?
- Grand Lake in Grand County, Colorado has a water quality grade of C (Fair). Water clarity: 8.7 ft visibility. Phosphorus: 14 µg/L. Chlorophyll-a (algae): 2.7 µg/L.
Is Grand Lake safe to swim in?
- The C is nutrients and algae, not a swimming clearance. Grand Lake is listed impaired under Clean Water Act §303(d) (IR 5) for metals (other than mercury). Check local advisories before you swim.
How deep is Grand Lake?
- No published bathymetric survey covers Grand Lake, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 25 feet, but that is a modelled estimate derived from surrounding terrain rather than a measurement on the water, so treat it as approximate.
Explore Grand Lake across LakeQuality
14 mi away · 25 ft
Species activity outlook by water temperature and season
Where to stay at Colorado's top lakes, each with its grade
Moderate nutrients, good water quality
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Planning a trip around Rocky Mountain?
Grand Lake is about 10 miles from Rocky Mountain 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 Rocky Mountain →More Colorado Lake Rankings
Other Grade C Lakes in Colorado
Mesotrophic Lakes in Colorado
Where this data comes from
3,673 measurements from 2 monitoring stations, 2020–2024. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- USGS Colorado Water Science Center
USGS-CO3,673 samples · 2020–2024
Analytical methods
- Secchi disk method
- Phosphorus, LL,wu,persulfate(CO)
- YSI EXO, 470/685/40,S bacillaris
- Plant pigments, sonication & 445
- YSI EXO turbidity sensor, FNU
Monitoring station identifiers (2)
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
Lake size & watershed from HydroLAKES (HydroSHEDS)