Updated Aug 2026
Colorado Lake Water Quality
Water quality grades for 118 monitored lakes across 49 counties.
Colorado has 118 graded lakes in our dataset, sampled across 49 counties, drawing on monitoring by the Water Quality Control Division of the Colorado Department of Public Health and Environment plus federal and volunteer measurements. A-grade lakes are scarce in Colorado: about 8% of the dataset. Most lakes here sit in C or D — productive, often shallow, frequently algal in late summer.
Mesotrophic is the dominant trophic class — a workable middle ground between the pristine oligotrophic deep lakes and the algal-pressured eutrophic shallow ones. Pueblo ranks highest on average county grade (A). Adams sits lowest. With only 49 counties in the state, the cross-county spread is narrower than the within-county spread for most lakes.
Grade Distribution
All 118 Colorado lakes in the index: 60 (50.8%) carry an A–F grade and 58 (49.2%) are not rated, because they lack enough recent nutrient or algae sampling to support a letter.
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Top-Graded Colorado Lakes
The highest water-quality grades in Colorado — clarity, depth, and A-F grade at a glance.
Lake Map
Explore Colorado Lakes
Colorado reports, fishing & travel
Reservoirs in Colorado
View all 80 reservoirs →80 of 118 Colorado lakes (68%) are man-made reservoirs in the USACE National Inventory of Dams. 74 carry a High or Significant hazard rating. The oldest matched dam dates to 1892.
EPA-Impaired Lakes in Colorado
View all 56 impaired lakes →56 of 118 Colorado lakes (47%) are officially listed as impaired under Clean Water Act §303(d) in the most recent EPA ATTAINS reporting cycle. 29 have an approved Total Maximum Daily Load (TMDL) restoration plan.
Colorado Fishing & Recreation
Licenses, regulations, records, and where to fish across Colorado.
Counties
All 49 Colorado counties, ranked by average lake water quality. Compare them against every county nationwide in the county rankings, browse the full directory of graded lakes, or pick a different state.
Top 25 Cleanest Lakes
25 lakes25 graded lakes
| # | Lake | County | Grade | Clarity | Trophic | Access |
|---|---|---|---|---|---|---|
| 1 | APueblo Reservoir | Pueblo | A | 6.5 ft | Mesotrophic | ✓ Public |
| 2 | ALake Granby | Grand | A | 12.7 ft | Oligotrophic | Not recorded |
| 3 | ALon Hagler Reservoir | Larimer | A | 4.6 ft | Mesotrophic | ✓ Public |
| 4 | ABoyd Lake | Larimer | A | 4.6 ft | Mesotrophic | ✓ Public |
| 5 | ACrystal Reservoir | Montrose | A | 15.4 ft | Mesotrophic | Not recorded |
| 6 | ADouglas Reservoir | Larimer | A | 4.1 ft | Mesotrophic | ✓ Public |
| 7 | AHighline Lake | Mesa | A | 3 ft | Mesotrophic | ✓ Public |
| 8 | AGreen Mountain Reservoir | Summit | A | 11.5 ft | Oligotrophic | Not recorded |
| 9 | ALake John | Jackson | A | 13.8 ft | Oligotrophic | ✓ Public |
| 10 | AElkhead Reservoir | Moffat | A | 5.2 ft | Mesotrophic | ✓ Public |
| 11 | BChatfield Reservoir | Jefferson | B | 6.2 ft | Eutrophic | ✓ Public |
| 12 | BChambers Lake | Larimer | B | 5.1 ft | Mesotrophic | Not recorded |
| 13 | BLittle Eldorado Lake | San Juan | B | No recent samples | Oligotrophic | Not recorded |
| 14 | BLake Granby Dam Bottom | Grand | B | No recent samples | Mesotrophic | Not recorded |
| 15 | BBoyd Lake Bottom | Larimer | B | No recent samples | Mesotrophic | Not recorded |
| 16 | CWillow Creek Reservoir | Grand | C | 3.9 ft | Mesotrophic | Not recorded |
| 17 | CWolford Mountain Reservoir | Grand | C | 7.5 ft | Oligotrophic | Not recorded |
| 18 | CGrand Lake | Grand | C | 8.7 ft | Mesotrophic | Not recorded |
| 19 | CSkaguay Reservoir | Teller | C | 12.5 ft | Mesotrophic | ✓ Public |
| 20 | CPearl Lake | Routt | C | 14.1 ft | Oligotrophic | ✓ Public |
| 21 | CAntero Reservoir | Park | C | 14.8 ft | Oligotrophic | Not recorded |
| 22 | CLake Nighthorse | La Plata | C | 18.7 ft | Oligotrophic | Not recorded |
| 23 | CIsland Lake | Delta | C | 16.7 ft | Oligotrophic | Not recorded |
| 24 | CRampart Reservoir | El Paso | C | 28.5 ft | Oligotrophic | Not recorded |
| 25 | CTrinidad Reservoir | Las Animas | C | 6.6 ft | Oligotrophic | ✓ Public |
25 Lakes Needing Attention
25 lakes25 graded lakes
| # | Lake | County | Grade | Clarity |
|---|---|---|---|---|
| 1 | FMilton Outlet Lake | Weld | F | No recent samples |
| 2 | FBarr Outlet Lake | Adams | F | No recent samples |
| 3 | FNorth Sterling Reservoir | Logan | F | 1.1 ft |
| 4 | FProspect Lake | Weld | F | No recent samples |
| 5 | FProspect Lake | El Paso | F | 1.6 ft |
| 6 | DSky Pond Hypolimnion | Larimer | D | No recent samples |
| 7 | DTwin Lake West Lower | Lake | D | No recent samples |
| 8 | DStagecoach Lake | Routt | D | No recent samples |
| 9 | DHorse Creek Outlet Lake | Weld | D | No recent samples |
| 10 | DWindy Gap Reservoir | Grand | D | No recent samples |
| 11 | DSteamboat Lake | Routt | D | 6.6 ft |
| 12 | DTurquoise Reservoir | Lake | D | 16.1 ft |
| 13 | DHaviland Lake | La Plata | D | 13.8 ft |
| 14 | DVega Reservoir | Mesa | D | 3.9 ft |
| 15 | DBig Meadows Reservoir | Mineral | D | 7.2 ft |
| 16 | DEggleston Lake | Delta | D | 4.9 ft |
| 17 | DBeaver Creek Reservoir | Rio Grande | D | 6.2 ft |
| 18 | DJumbo Reservoir | Logan | D | 2.1 ft |
| 19 | DUnion Reservoir | Weld | D | 3.2 ft |
| 20 | DJackson Reservoir | Morgan | D | 2.1 ft |
| 21 | DElevenmile Reservoir | Park | D | 13.1 ft |
| 22 | DDowdy Lake | Larimer | D | 8.2 ft |
| 23 | DDeweese Reservoir | Custer | D | 5.2 ft |
| 24 | DDillon Reservoir | Summit | D | 11.6 ft |
| 25 | DWindsor Lake | Weld | D | 2 ft |
Lake Match
Which lake suits how you actually use one?
Six questions — swimming, fishing, quiet, cabins, drive time — and a shortlist of graded lakes that fit, with the water quality behind each one.
Source: EPA National Aquatic Resource Surveys and the EPA Water Quality Portal.
More on lake water quality
The top-graded lake in every state we cover, picked grade-first with water clarity as the tie-breaker — plus the states whose winner is still not clean.
State-by-state Secchi champions: the clearest measured lake in each U.S. state we grade, which is not the same as the cleanest nutrient grade.
A data snapshot of the lakes we grade: how the A–F distribution breaks down, the clearest and the greenest states, and where the water is improving.
How lakes are graded A–F on nutrients and algae using EPA ecoregion condition classes, what each letter means in the water, and why an A is not a clearance.