Updated August 2026
Safest Lakes to Swim in Colorado
Pueblo Reservoir (Pueblo County) ranks #1 for swimming in Colorado — grade A water with low algae and public access. Below: the top 1 lakes in Coloradothat pass our swim-safety filter.
- Grade A–B water quality
- Chlorophyll-a below 10 µg/L
Swimming-friendly is a clarity-plus-algae problem: clear water and a low chlorophyll-a number. Pueblo Reservoir (A) tops the Colorado 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 | Advisories & grade | Clarity | Algae | Phosphorus | Area |
|---|---|---|---|---|---|---|---|
| 1 | Pueblo Reservoir | Pueblo | ALow algae | 6.5 ft | 4.9 µg/L | 15 µg/L | 5,671 acres |
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Frequently Asked Questions
What is the best lake to swim in Colorado?
Pueblo Reservoir in Pueblo County ranks #1 for swimming-safety in our 2026 Colorado 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 Colorado?
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. CDPHE 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 Colorado safe to swim in?
Most monitored Colorado 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 Colorado?
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 Colorado?
Colorado 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 Colorado?
The single cleanest lake by combined water-quality score leads our Colorado cleanest-lakes ranking, linked below. The lakes on this page are filtered specifically for swimming safety — same A/B grade requirement, plus low chlorophyll-a (active algae filter).
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 Advisories & grade 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. Where a lake carries an EPA 303(d) bacteria or recreation listing, that advisory runs above its letter in the table rather than beside it.
The ranking within the filter is by combined water-quality score (Secchi clarity + phosphorus + chlorophyll-a). Always check current CDPHE advisories before swimming.
Where to stay near Colorado swimming lakes
Book a cabin, resort, or rental near Colorado's cleanest swim lakes — live availability and prices for your beach weekend.
Top-rated places to stay near Colorado swim lakes
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More on swimming water quality
The highest-graded lakes for swimming, ranked on EPA nutrient and algae data — plus the one thing a letter grade cannot tell you the day you go.
What a swim advisory actually means, how E. coli monitoring works, how fast a lake clears after rain, and when a long-term grade is not a swim guide.
Typical lake surface temperatures month by month and state by state, from years of monitoring readings — when lakes warm up, how warm they get, and when they turn.
The month when well-graded lakes in each state are typically warmest, from long-run Water Quality Portal readings rather than this year’s weather.
Source: EPA National Aquatic Resource Surveys, 2026.