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LakeQuality

Data Report · Updated July 2026

The State of Colorado’s Lakes 2026

Of 89 monitored lakes in Colorado, 38.2% earn a water-quality grade of A or B and 50.6% score D or F (average grade C). Crystal Reservoir is the cleanest;

89
Monitored lakes graded
C
Average grade
38.2%
Grade A or B
0%
On impaired list

Colorado grade distribution

A
B
C
D
F
A: 15 (16.9%)B: 19 (21.3%)C: 10 (11.2%)D: 24 (27.0%)F: 21 (23.6%)

Cleanest lakes in Colorado

Highest combined water-quality score, 2026.

  1. 1.ACrystal Reservoir — Montrose County15.4 ft clarity · 16 µg/L P
  2. 2.ALake Granby — Grand County12.7 ft clarity · 17 µg/L P · 2 µg/L chl-a
  3. 3.BPueblo Reservoir — Pueblo County6.7 ft clarity · 10 µg/L P · 5 µg/L chl-a
  4. 4.BWolford Mountain Reservoir — Grand County8.3 ft clarity · 10 µg/L P · 2 µg/L chl-a
  5. 5.BGrand Lake — Grand County8.9 ft clarity · 14 µg/L P · 3 µg/L chl-a
  6. 6.BSummit Lake — Jackson County12.1 ft clarity · 20 µg/L P
  7. 7.BBlue Mesa Reservoir — Gunnison County11.5 ft clarity · 35 µg/L P · 5 µg/L chl-a
  8. 8.BWindy Gap Reservoir — Grand County53 µg/L P · 3 µg/L chl-a
  9. 9.BShadow Mountain Lake — Grand County7.8 ft clarity · 20 µg/L P · 5 µg/L chl-a
  10. 10.CWillow Creek Reservoir — Grand County3.9 ft clarity · 88 µg/L P · 2 µg/L chl-a

Most-polluted lakes

Lowest combined water-quality score among monitored lakes.

  1. 1.FProspect Lake — Weld County250 µg/L P · 43 µg/L chl-a
  2. 2.CWillow Creek Reservoir — Grand County3.9 ft clarity · 88 µg/L P · 2 µg/L chl-a
  3. 3.CStagecoach Reservoir — Routt County7.5 ft clarity · 34 µg/L P · 19 µg/L chl-a
  4. 4.BShadow Mountain Lake — Grand County7.8 ft clarity · 20 µg/L P · 5 µg/L chl-a
  5. 5.BSummit Lake — Jackson County12.1 ft clarity · 20 µg/L P
  6. 6.BBlue Mesa Reservoir — Gunnison County11.5 ft clarity · 35 µg/L P · 5 µg/L chl-a
  7. 7.BWindy Gap Reservoir — Grand County53 µg/L P · 3 µg/L chl-a
  8. 8.BPueblo Reservoir — Pueblo County6.7 ft clarity · 10 µg/L P · 5 µg/L chl-a
  9. 9.BWolford Mountain Reservoir — Grand County8.3 ft clarity · 10 µg/L P · 2 µg/L chl-a
  10. 10.BGrand Lake — Grand County8.9 ft clarity · 14 µg/L P · 3 µg/L chl-a

Worst algae hotspots

Highest measured chlorophyll-a (algae) concentration.

  1. 1.FProspect Lake — Weld County43 µg/L chlorophyll-a

Explore Colorado lakes

Lake Fishing Forecast

Species activity outlook by water temperature and season

Grades combine Secchi clarity, total phosphorus, and chlorophyll-a from the EPA Water Quality Portal and state agencies. See the full methodology, the national State of America’s Lakes report, or all Colorado lakes.

Cite this report: LakeQuality, “The State of Colorado’s Lakes 2026,” lakequality.org/report/co.