Cleanest Lakes in Iowa: Top 25 Ranked by Water Quality
The single cleanest lake in Iowa is West Okoboji Lake, with the highest combined water quality score in our 2026 dataset (grade A). Cleanliness is measured by Secchi depth (clarity), total phosphorus, and chlorophyll-a — all sourced from EPA Water Quality Portal and state agency sampling. The full top 25 ranked list appears below.
Filtered to remove lakes with limited data (<3 sampling years). Sorted by overall water quality score.
The "cleanest" ranking on LakeGrade is composite: water clarity (Secchi), total phosphorus, and chlorophyll-a — the three core inputs to the LakeGrade rubric — taken together. The cleanest Iowa lake on the current data is West Okoboji Lake (A).
The top of the cleanest-lakes list is dominated by deep, glacial, cold-water basins — the same basins that produce the A-grade bracket overall. The pattern is not coincidence: depth and watershed land-use are the dominant inputs to water-clarity and nutrient-loading both.
| Rank | Lake | County | Grade | Clarity | Phosphorus | Chlorophyll-a | Area |
|---|---|---|---|---|---|---|---|
| 1 | West Okoboji Lake | Dickinson | A | 15.7 ft | — | 2.6 µg/L | - |
| 2 | Blue Pit Lake | Cerro Gordo | A | 17.1 ft | — | 4 µg/L | - |
| 3 | Nine Eagles Lake | Decatur | A | 11 ft | — | 3.3 µg/L | 59 acres |
| 4 | Terra Park Lake | Polk | A | 11.5 ft | — | 4.7 µg/L | - |
| 5 | Dale Maffitt Reservoir | Dallas | B | 9.8 ft | — | 2.5 µg/L | 230 acres |
| 6 | Rudd Lake | Floyd | B | 8.1 ft | — | 4.5 µg/L | - |
| 7 | Slipbluff Lake | Decatur | B | 9 ft | — | 4.5 µg/L | 18 acres |
| 8 | Lacey Keosauqua Lake | Van Buren | B | 7.1 ft | — | 4.7 µg/L | 23 acres |
| 9 | Yenruogis Pond | Keokuk | B | 9.9 ft | — | 6.2 µg/L | - |
| 10 | Ada Hayden Heritage Park Lake | Story | B | 6.8 ft | — | 5.3 µg/L | - |
| 11 | Lambach Lake | Scott | B | 7.9 ft | — | 5.1 µg/L | 9 acres |
| 12 | Sand Lake | Marshall | B | 6.4 ft | — | 3.6 µg/L | - |
| 13 | Arrowhead Lake | Sac | C | 5.6 ft | — | 7.1 µg/L | 110 acres |
| 14 | Railroad Lake | Scott | C | 5.1 ft | — | 8.3 µg/L | 29 acres |
| 15 | Mariposa Lake | Jasper | C | 5.1 ft | — | 8.9 µg/L | 19 acres |
| 16 | Mormon Trail Lake | Adair | C | 6.2 ft | — | 9.7 µg/L | 33 acres |
| 17 | Willow Lake | Harrison | C | 4.8 ft | — | 9.5 µg/L | 19 acres |
| 18 | Lake Wapello | Davis | C | 7.5 ft | — | 13 µg/L | 287 acres |
| 19 | Oldham Lake | Monona | C | 5.1 ft | — | 8.5 µg/L | 16 acres |
| 20 | Little Sioux Park Lake | Woodbury | C | 5.7 ft | — | 5.7 µg/L | - |
| 21 | Green Castle Lake | Marshall | C | 4.6 ft | — | 6.9 µg/L | 17 acres |
| 22 | Poll Miller Park Lake | Lee | C | 5.9 ft | — | 7.7 µg/L | 15 acres |
| 23 | Kent Park Lake | Johnson | C | 4.8 ft | — | 11.8 µg/L | 29 acres |
| 24 | Upper Gar Lake | Dickinson | C | 3.9 ft | — | 13.1 µg/L | - |
| 25 | Big Spirit Lake | Dickinson | C | 5.5 ft | — | 16.3 µg/L | - |
Clearest lakes in Iowa by water clarity
“Clearest” ranks by Secchi depth — how far you can see into the water — which can differ from the combined cleanliness score above. The clearest water in Iowa is Blue Pit Lake at 17.1 ft of Secchi visibility.
- 1. Blue Pit Lake — 17.1 ft, Cerro Gordo County
- 2. West Okoboji Lake — 15.7 ft, Dickinson County
- 3. Terra Park Lake — 11.5 ft, Polk County
- 4. Nine Eagles Lake — 11 ft, Decatur County
- 5. Yenruogis Pond — 9.9 ft, Keokuk County
Frequently Asked Questions
What is the cleanest lake in Iowa?
West Okoboji Lake has the highest water quality score in our 2026 Iowa dataset (grade A), based on combined Secchi clarity, phosphorus, and chlorophyll-a measurements. Cleanliness here means low nutrient load, deep water clarity, and minimal algae — measured directly from EPA and state DNR samples.
What is the most polluted lake in Iowa?
Among monitored Iowa lakes, Okamanpeedan Lake ranks among the most polluted by combined phosphorus and chlorophyll-a load. See our full ranking on the most-polluted-lakes trend page for state-by-state details.
Where in Iowa has the cleanest water?
Iowa's clearest waters are concentrated in deeper, lower-nutrient glacial basins with forested or low-agriculture watersheds. Among the current top-ranked lakes above, counties like Dickinson, Decatur, and Scott produce the highest-scoring lakes.
What is the clearest lake in Iowa?
Blue Pit Lake is the clearest lake in Iowa by water clarity, with a Secchi depth of 17.1 feet — the deepest transparency in our 2026 dataset. "Clearest" ranks by Secchi depth (how far you can see into the water) specifically, while "cleanest" combines clarity with phosphorus and chlorophyll-a. See our full clearest-lakes ranking for Iowa for the top 25 by Secchi clarity.
Are these lakes safe to swim in?
A and B grade lakes are typically safe for swimming under normal conditions. Chlorophyll-a below 10 µg/L means low risk of harmful algal blooms. Always check posted advisories before swimming — water quality changes seasonally, especially in late summer when blue-green algae bloom risk peaks.
How are these grades calculated?
Each lake is scored on three signals: Secchi depth (water clarity in feet), total phosphorus (nutrient load in µg/L), and chlorophyll-a (algae concentration in µg/L). Each gets a 0–100 sub-score relative to Iowa state norms. The three sub-scores average into the overall score, which maps to an A–F letter grade. Methodology details live on our methodology page.
What are the top 5 cleanest lakes in the US?
There is no single national ranking of cleanest lakes — each state agency uses different metrics. Crater Lake (Oregon) is widely cited as the clearest US lake, with Secchi depths over 30 m. Lake Tahoe and Lake Superior are also top-tier nationally. Within Iowa, the lakes ranked above are the top 25 by our standardized 2026 grading.
How we rank cleanest lakes
Cleanliness here means three things working together:
- Secchi clarity — how deep you can see into the water (in feet). Greater depth = clearer water = lower suspended sediment and algae.
- Total phosphorus — the nutrient that fuels algae. Lower phosphorus levels mean less algae growth potential.
- Chlorophyll-a — the green pigment in algae. Direct measurement of how much algae is currently in the water.
Each signal is normalized against Iowa state norms and combined into a single 0–100 score. The lakes above are sorted by that combined score.Full methodology →
Where to stay near Iowa's cleanest lakes
Compare cabins, resorts, and rentals near the clearest, lowest-algae lakes in Iowa — live availability and prices for your trip.
Top-rated places to stay near Iowa's cleanest lakes
Availability and prices from Stay22 partners (Booking.com, Vrbo, Expedia). We may earn a commission from bookings, at no extra cost to you.
Planning a trip to Iowa's cleanest lakes?
Get lake water-quality report cards plus the best cabin & lodging deals near Iowa's cleanest lakes — straight to your inbox. No spam, unsubscribe anytime.
Source: EPA National Aquatic Resource Surveys, 2026.