Updated Aug 2026
Wyoming Lake Water Quality
Water quality grades for 220 monitored lakes across 19 counties.
220 Wyoming lakes carry a grade. Most of the input data flows through monitoring run by the Wyoming Department of Environmental Quality, which feeds state, federal, and volunteer measurements into the EPA Water Quality Portal. A-grade lakes are scarce in Wyoming: about 0% 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. Sweetwater ranks highest on average county grade (A). Goshen sits lowest. With only 19 counties in the state, the cross-county spread is narrower than the within-county spread for most lakes.
Grade Distribution
All 220 Wyoming lakes in the index: 82 (37.3%) carry an A–F grade and 138 (62.7%) are not rated, because they lack enough recent nutrient or algae sampling to support a letter.
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Top-Graded Wyoming Lakes
The highest water-quality grades in Wyoming — clarity, depth, and A-F grade at a glance.
Lake Map
Explore Wyoming Lakes
Wyoming reports, fishing & travel
Reservoirs in Wyoming
View all 94 reservoirs →94 of 220 Wyoming lakes (43%) are man-made reservoirs in the USACE National Inventory of Dams. 60 carry a High or Significant hazard rating. The oldest matched dam dates to 1902.
EPA-Impaired Lakes in Wyoming
View all 18 impaired lakes →18 of 220 Wyoming lakes (8%) are officially listed as impaired under Clean Water Act §303(d) in the most recent EPA ATTAINS reporting cycle. 1 have an approved Total Maximum Daily Load (TMDL) restoration plan.
Wyoming Fishing & Recreation
Licenses, regulations, records, and where to fish across Wyoming.
Counties
All 19 Wyoming 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 | AFlaming Gorge Reservoir | Sweetwater | A | 22 ft | Oligotrophic | Not recorded |
| 2 | BPathfinder Reservoir Lacustrine Pelagic | Natrona | B | 10.7 ft | Mesotrophic | ✓ Public |
| 3 | BSeminoe Reservoir Riverine Pelagic | Carbon | B | 4.4 ft | Eutrophic | Not recorded |
| 4 | BAlcova Reservoir Transitional Pelagic | Natrona | B | 11.3 ft | Mesotrophic | ✓ Public |
| 5 | BPathfinder Reservoir Transitional Pelagic | Natrona | B | 11.1 ft | Mesotrophic | Not recorded |
| 6 | BPathfinder Reservoir Riverine Pelagic | Carbon | B | 5.9 ft | Mesotrophic | Not recorded |
| 7 | BAlcova Reservoir Lacustrine Pelagic | Natrona | B | 12.8 ft | Mesotrophic | Not recorded |
| 8 | BGlendo Reservoir Riverine Pelagic | Platte | B | 1.4 ft | Eutrophic | ✓ Public |
| 9 | BSeminoe Reservoir Lacustrine Pelagic | Carbon | B | 17.1 ft | Oligotrophic | ✓ Public |
| 10 | BGlendo Reservoir Lacustrine Pelagic | Platte | B | 5.2 ft | Eutrophic | ✓ Public |
| 11 | BSeminoe Reservoir Transitional Pelagic | Carbon | B | 11.4 ft | Mesotrophic | Not recorded |
| 12 | BAlcova Reservoir Riverine Pelagic | Natrona | B | 5.8 ft | Eutrophic | Not recorded |
| 13 | BGreyrocks Reservoir | Platte | B | 5.4 ft | Eutrophic | ✓ Public |
| 14 | BCrystal Reservoir | Laramie | B | 11.6 ft | Oligotrophic | ✓ Public |
| 15 | BWheatland Reservoir | Platte | B | 8.9 ft | Mesotrophic | ✓ Public |
| 16 | BGranite Springs Reservoir | Laramie | B | 11.8 ft | Oligotrophic | ✓ Public |
| 17 | BLake Desmet Lpd | Johnson | B | 39.1 ft | Oligotrophic | ✓ Public |
| 18 | BKeyhole Reservoir | Crook | B | 7.7 ft | Mesotrophic | ✓ Public |
| 19 | BGuernsey Reservoir | Platte | B | 4.9 ft | Eutrophic | Not recorded |
| 20 | BKeyhole Reservoir | Crook | B | 2.5 ft | Eutrophic | ✓ Public |
| 21 | BKeyhole Reservoir | Crook | B | 10.6 ft | Mesotrophic | ✓ Public |
| 22 | BLake Owen | Albany | B | 8.1 ft | Mesotrophic | ✓ Public |
| 23 | BRob Roy Reservoir | Albany | B | 11.2 ft | Oligotrophic | ✓ Public |
| 24 | BLake Desmet Rtpd | Johnson | B | 34.6 ft | Oligotrophic | Not recorded |
| 25 | BGlendo Reservoir Transitional Pelagic | Platte | B | 5.3 ft | Eutrophic | ✓ Public |
25 Lakes Needing Attention
25 lakes25 graded lakes
| # | Lake | County | Grade | Clarity |
|---|---|---|---|---|
| 1 | DRawlins Reservoir | Carbon | D | No recent samples |
| 2 | DYellowtail Reservoir Rpd | Big Horn | D | No recent samples |
| 3 | DBrooks Lake | Fremont | D | No recent samples |
| 4 | DBrooks Lake | Fremont | D | 6.2 ft |
| 5 | DBrooks Lake | Fremont | D | 6 ft |
| 6 | DBrooks Lake | Fremont | D | 6.7 ft |
| 7 | DBrooks Lake | Fremont | D | 6.3 ft |
| 8 | DHereford Reservoir | Laramie | D | 3.7 ft |
| 9 | DBump-Sullivan Reservoir | Goshen | D | 0.2 ft |
| 10 | DGoshen Hole Reservoir | Goshen | D | 1.1 ft |
| 11 | DHorseshoe Reservoir | Big Horn | D | 1.8 ft |
| 12 | DFesto Lake | Platte | D | 0.8 ft |
| 13 | DSloans Lake | Laramie | D | 2 ft |
| 14 | DHawk Springs Reservoir | Goshen | D | 1.1 ft |
| 15 | CSouth Crow Reservoir | Laramie | C | No recent samples |
| 16 | CUnnamed Reservoir | Big Horn | C | No recent samples |
| 17 | CCity Reservoir | Park | C | No recent samples |
| 18 | CBighorn Lake | Big Horn | C | No recent samples |
| 19 | CBrooks Lake | Fremont | C | 6.7 ft |
| 20 | BRawlins Peaking Reservoir | Carbon | B | No recent samples |
| 21 | BUpper North Crow Reservoir | Laramie | B | No recent samples |
| 22 | BTwin Lakes Reservoir D | Sheridan | B | No recent samples |
| 23 | BSawmill Reservoir D | Sheridan | B | No recent samples |
| 24 | BHanna Water Supply Lake | Carbon | B | No recent samples |
| 25 | BHanna Reservoir | Carbon | B | No recent samples |
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.