Updated August 2026
Best Swimming Lakes in Colorado
Pueblo Reservoir (Pueblo County) ranks #1 for swimming in Colorado — clear, low-nutrient water with low algae and public access. Below: the top 1 lakes in Coloradothat pass our swimming filter. No reading here is a swimming clearance; check CDPHE advisories the day you go.
- Clear, low-nutrient water (top of the state’s measured range)
- Chlorophyll-a below 10 µg/L
Swimming-friendly is a clarity-plus-algae problem: clear water and a low chlorophyll-a number. Pueblo Reservoir 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 & bacteria | Clarity | Algae | Phosphorus | Area |
|---|---|---|---|---|---|---|---|
| 1 | Pueblo Reservoir | Pueblo | Low 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 first for swimming in the 2026 Colorado sampling record, combining the clearest water with low algae and public access. The full top 25 above is ranked the same way.
How do you know if a lake is safe to swim in Colorado?
That is a call for CDPHE and county health departments, who post seasonal and same-day advisories — always check before you swim, especially in late summer when blue-green algae bloom risk peaks. The readings on this page show what past sampling found: Secchi depth above 6 feet is a strong sign of low suspended sediment, and chlorophyll-a below 10 µg/L means a low algae load. Neither is a clearance.
Are lakes in Colorado safe to swim in?
Whether a given lake is safe to swim in on a given day is answered by CDPHE and the county health department, not by a water-quality reading. The lakes ranked above are filtered to the clearest, lowest-nutrient readings in the sampling record with chlorophyll-a below 10 µg/L, which means they have tested cleaner and carried less algae than the state average. Always check posted advisories before swimming and avoid water after heavy rain, when bacteria 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; each lake page shows its own phosphorus and chlorophyll-a readings.
When is it safe to swim in lakes in Colorado?
Algae load in Colorado lakes is typically lowest from late spring through early summer (May–July), when water is cool. It rises in late summer and early fall (August–September) when warm water and accumulated nutrients drive cyanobacteria blooms. Avoid swimming after heavy rainfall, when bacteria spikes from runoff, and check CDPHE's current advisories the day you go.
What's the cleanest lake in Colorado?
The single cleanest lake on measured clarity and nutrients leads the Colorado cleanest-lakes ranking, linked below. The lakes on this page are filtered for swimming — the same clarity and nutrient readings, plus low chlorophyll-a (an active algae filter).
Our swimming filter
Lakes here have to pass three filters:
- Clear, low-nutrient water — the top of the state’s measured range for clarity, phosphorus and chlorophyll-a
- 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 & bacteria column adds a direct bacteria signal: where the EPA Water Quality Portal has enough E. coli or fecal-coliform samples, we show how often the lake has exceeded the EPA recreation 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 the bacteria record in the table rather than beside it.
The ranking within the filter is on combined readings (Secchi clarity + phosphorus + chlorophyll-a). Always check current CDPHE advisories before swimming.
Where to stay near Colorado swimming lakes
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Top-rated places to stay near Colorado swim lakes
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More on swimming water quality
The cleanest lakes for swimming, ranked on EPA nutrient and algae data — plus the one thing a reading 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 why a long-term reading 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 the cleanest 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.