Updated Jul 2026
Iowa Lake Water Quality
Water quality grades for 170 monitored lakes across 75 counties.
Across Iowa, 170 lakes carry an active water-quality grade spread over 75 counties. The underlying measurements come from the Iowa Department of Natural Resources, alongside federal programs and volunteer monitoring networks. A-grade lakes are scarce in Iowa: about 2% of the dataset. Most lakes here sit in C or D — productive, often shallow, frequently algal in late summer.
Eutrophic conditions dominate — nutrient-loaded, productive, with algal pressure that shows up most years in late summer. These lakes tend to be shallow, agricultural-watershed, and over-warmed by the time July ends. The cleanest county on average is Floyd (grade B); the most-stressed is Pottawattamie. The gap between the top and bottom counties is large enough that "Iowa lake water quality" hides real local variation.
Grade Distribution
How Iowa’s 170 graded lakes break down from A (excellent) to F.
Top-Graded Iowa Lakes
The highest water-quality grades in Iowa — clarity, depth, and A-F grade at a glance.
West Okoboji Lake
Dickinson County, Iowa
See grade for West Okoboji Lake →Blue Pit Lake
Cerro Gordo County, Iowa
See grade for Blue Pit Lake →Nine Eagles Lake
Decatur County, Iowa
See grade for Nine Eagles Lake →Terra Park Lake
Polk County, Iowa
See grade for Terra Park Lake →Dale Maffitt Reservoir
Dallas County, Iowa
See grade for Dale Maffitt Reservoir →Rudd Lake
Floyd County, Iowa
See grade for Rudd Lake →Slipbluff Lake
Decatur County, Iowa
See grade for Slipbluff Lake →Lacey Keosauqua Lake
Van Buren County, Iowa
See grade for Lacey Keosauqua Lake →Yenruogis Pond
Keokuk County, Iowa
See grade for Yenruogis Pond →Lake Map
Explore Iowa Lakes
Iowa reports, fishing & travel
Iowa's cleanest and most-polluted lakes, ranked A–F
Species activity outlook by water temperature and season
Where to stay at Iowa's top lakes, each with its grade
Best-graded lakes within 50 miles of Des Moines
Best-graded lakes within 50 miles of Cedar Rapids
Reservoirs in Iowa
View all 123 reservoirs →123 of 170 Iowa lakes (72%) are man-made reservoirs in the USACE National Inventory of Dams. 91 carry a High or Significant hazard rating. The oldest matched dam dates to 1900.
EPA-Impaired Lakes in Iowa
View all 117 impaired lakes →117 of 170 Iowa lakes (69%) are officially listed as impaired under Clean Water Act §303(d) in the most recent EPA ATTAINS reporting cycle. 78 have an approved Total Maximum Daily Load (TMDL) restoration plan.
Iowa Fishing & Recreation
Licenses, regulations, records, and where to fish across Iowa.
Counties
All 75 Iowa 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.
Iowa Lake Water Quality Dataset (2026)
Every graded Iowa lake in one spreadsheet — A–F grade, water clarity, phosphorus, chlorophyll-a, trophic state, size, public access, and a link to each lake's report card.
Top 25 Cleanest Lakes
| Rank | Lake | County | Grade | Clarity |
|---|---|---|---|---|
| 1 | West Okoboji Lake | Dickinson | A | 15.7 ft |
| 2 | Blue Pit Lake | Cerro Gordo | A | 17.1 ft |
| 3 | Nine Eagles Lake | Decatur | A | 11 ft |
| 4 | Terra Park Lake | Polk | A | 11.5 ft |
| 5 | Dale Maffitt Reservoir | Dallas | B | 9.8 ft |
| 6 | Rudd Lake | Floyd | B | 8.1 ft |
| 7 | Slipbluff Lake | Decatur | B | 9 ft |
| 8 | Lacey Keosauqua Lake | Van Buren | B | 7.1 ft |
| 9 | Yenruogis Pond | Keokuk | B | 9.9 ft |
| 10 | Ada Hayden Heritage Park Lake | Story | B | 6.8 ft |
| 11 | Lambach Lake | Scott | B | 7.9 ft |
| 12 | Sand Lake | Marshall | B | 6.4 ft |
| 13 | Arrowhead Lake | Sac | C | 5.6 ft |
| 14 | Railroad Lake | Scott | C | 5.1 ft |
| 15 | Mariposa Lake | Jasper | C | 5.1 ft |
| 16 | Mormon Trail Lake | Adair | C | 6.2 ft |
| 17 | Willow Lake | Harrison | C | 4.8 ft |
| 18 | Lake Wapello | Davis | C | 7.5 ft |
| 19 | Oldham Lake | Monona | C | 5.1 ft |
| 20 | Little Sioux Park Lake | Woodbury | C | 5.7 ft |
| 21 | Green Castle Lake | Marshall | C | 4.6 ft |
| 22 | Poll Miller Park Lake | Lee | C | 5.9 ft |
| 23 | Kent Park Lake | Johnson | C | 4.8 ft |
| 24 | Upper Gar Lake | Dickinson | C | 3.9 ft |
| 25 | Big Spirit Lake | Dickinson | C | 5.5 ft |
25 Lakes Needing Attention
| Rank | Lake | County | Grade | Clarity |
|---|---|---|---|---|
| 1 | Okamanpeedan Lake | Emmet | F | 1 ft |
| 2 | Central Park Lake | Jones | F | 1.6 ft |
| 3 | Humeston Reservoir | Wayne | F | 1.6 ft |
| 4 | Rock Creek Beach Lake | Jasper | F | 1.1 ft |
| 5 | Lake Fisher | Davis | F | 2 ft |
| 6 | Corydon Reservoir | Wayne | F | 1.9 ft |
| 7 | Lpinelk T Lake | Hardin | F | 1.3 ft |
| 8 | Alice Wyth Lake | Black Hawk | F | 1.6 ft |
| 9 | Pine Lake South Beach | Hardin | F | 1.4 ft |
| 10 | Terry Trueblood Lake | Johnson | F | 1.1 ft |
| 11 | Volga Lake | Fayette | F | 2.1 ft |
| 12 | Lake Pahoja | Lyon | F | 1.7 ft |
| 13 | Pierce Creek Lake | Page | F | 2.2 ft |
| 14 | Blue Lake | Monona | F | 0.9 ft |
| 15 | Lake Meyer | Winneshiek | F | 3.1 ft |
| 16 | Little River Lake | Decatur | F | 2.3 ft |
| 17 | Windmill Lake | Taylor | F | 2.5 ft |
| 18 | Viking Lake | Montgomery | F | 3.3 ft |
| 19 | Mill Creek Lake | O'Brien | F | 3 ft |
| 20 | Lake Hendricks | Howard | F | 1.5 ft |
| 21 | Lake Sugema | Van Buren | F | 2.3 ft |
| 22 | Smith Lake | Kossuth | F | 2.4 ft |
| 23 | Ingham Lake | Emmet | F | 1.4 ft |
| 24 | Indian Lake | Van Buren | F | 1.2 ft |
| 25 | Williamson Pond Max Depth | Lucas | F | 1.6 ft |
Where to stay near Iowa
Top-rated places to stay near Iowa
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Source: EPA National Aquatic Resource Surveys, 2026.