Safest Lakes to Swim in Iowa
West Okoboji Lake (Dickinson County) ranks #1 for swimming in Iowa — grade A water with low algae and public access. Below: the top 12 lakes in Iowathat pass our swim-safety filter (grade A/B, chlorophyll-a below 10 µg/L).
Swimming-friendly is a clarity-plus-algae problem: clear water and a low chlorophyll-a number. West Okoboji Lake (A) tops the Iowa list on those combined measures.
The swimming-best list filters more aggressively on chlorophyll-a than the cleanest list — even very clear lakes can have harmful-algal-bloom risk under specific conditions, so the swimming ranking is more conservative on that front.
| Rank | Lake | County | Grade | Swim Safety | Clarity | Algae | Phosphorus | Area |
|---|---|---|---|---|---|---|---|---|
| 1 | West Okoboji Lake | Dickinson | A | Bacteria D · advisory | 15.7 ft | 2.6 µg/L | — | - |
| 2 | Blue Pit Lake | Cerro Gordo | A | Low algae | 17.1 ft | 4 µg/L | — | - |
| 3 | Nine Eagles Lake | Decatur | A | Bacteria F · advisory | 11 ft | 3.3 µg/L | — | 59 acres |
| 4 | Terra Park Lake | Polk | A | Low algae | 11.5 ft | 4.7 µg/L | — | - |
| 5 | Dale Maffitt Reservoir | Dallas | B | Low algae | 9.8 ft | 2.5 µg/L | — | 230 acres |
| 6 | Rudd Lake | Floyd | B | Low algae | 8.1 ft | 4.5 µg/L | — | - |
| 7 | Slipbluff Lake | Decatur | B | Low algae | 9 ft | 4.5 µg/L | — | 18 acres |
| 8 | Lacey Keosauqua Lake | Van Buren | B | Bacteria B | 7.1 ft | 4.7 µg/L | — | 23 acres |
| 9 | Yenruogis Pond | Keokuk | B | Low algae | 9.9 ft | 6.2 µg/L | — | - |
| 10 | Ada Hayden Heritage Park Lake | Story | B | Low algae | 6.8 ft | 5.3 µg/L | — | - |
| 11 | Lambach Lake | Scott | B | Low algae | 7.9 ft | 5.1 µg/L | — | 9 acres |
| 12 | Sand Lake | Marshall | B | Low algae | 6.4 ft | 3.6 µg/L | — | - |
Frequently Asked Questions
What is the best lake to swim in Iowa?
West Okoboji Lake in Dickinson County ranks #1 for swimming-safety in our 2026 Iowa dataset, combining grade-A water clarity with low algae and public access. The full top 25 above ranks the swim-safest lakes statewide.
How do you know if a lake is safe to swim in Iowa?
Three signals: (1) water clarity — Secchi depth above 6 feet is a strong sign of low suspended sediment and pathogens; (2) chlorophyll-a below 10 µg/L means low risk of harmful algae bloom; (3) a current grade of A or B from year-round sampling. Iowa DNR and county health departments post seasonal advisories — always check before you swim, especially in late summer when blue-green algae bloom risk peaks.
Are lakes in Iowa safe to swim in?
Most monitored Iowa lakes are safe to swim in under normal conditions, but water quality varies widely. The lakes ranked above are filtered to grade A and B with chlorophyll-a below 10 µg/L — meaning they consistently test cleaner and have lower algae bloom risk than the state average. Always check posted advisories before swimming and avoid water after heavy rain (bacterial contamination spikes from runoff).
What lakes have algae blooms in Iowa?
Algae blooms (especially blue-green / cyanobacteria) are most common in shallow, warm, nutrient-rich (eutrophic) lakes during late summer. Lakes with high phosphorus and chlorophyll-a measurements are at higher risk. The lakes on this page are filtered to exclude high-algae lakes. For the opposite list, see our most-polluted-lakes trend page.
When is it safe to swim in lakes in Iowa?
Iowa lakes are typically safest to swim in late spring through early summer (May–July), when water is cool and algae bloom risk is lowest. Risk rises in late summer and early fall (August–September) when warm water and accumulated nutrients drive cyanobacteria blooms. Always avoid swimming after heavy rainfall (bacterial spikes from runoff) and check current state DNR advisories the day you go.
What's the cleanest lake in Iowa?
The single cleanest lake by combined water-quality score is featured on the Iowa cleanest-lakes page. The lakes here are filtered specifically for swimming safety — same A/B grade requirement, plus low chlorophyll-a (active algae filter). For the broader cleanliness ranking, see /best/cleanest/ia.
Our swimming-safety filter
We don't just rank by overall grade. Lakes here have to pass three filters:
- Grade A or B overall water quality (top 40% statewide)
- Chlorophyll-a below 10 µg/L — actively low algae density, low risk of harmful blue-green blooms
- Multi-year sampling data — excludes lakes with limited sampling history (data confidence)
The Swim Safety column adds a direct bacteria signal on top of the grade: where the EPA Water Quality Portal has enough E. coli or fecal-coliform samples, we show a historical swim-safety grade (how often the lake has exceeded the safe-swimming threshold). It is a record of past testing, never a live "safe today" reading — bacteria spike fast after rain.
The ranking within the filter is by combined water-quality score (Secchi clarity + phosphorus + chlorophyll-a). Always check current Iowa DNR advisories before swimming.
Where to swim near a Iowa city
Looking for a lake close to home? These pages rank the safest swim lakes within driving distance of Iowa's largest cities.
Where to stay near Iowa swimming lakes
Book a cabin, resort, or rental near Iowa's cleanest swim lakes — live availability and prices for your beach weekend.
Top-rated places to stay near Iowa swim lakes
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Source: EPA National Aquatic Resource Surveys, 2026.