Lake St Louis
St. Charles County, Missouri563 acresView mapDirections
Not enough samples to grade — no nutrient or algae measurement meets the evidence bar (at least 3 samples within 10 years). No letter yet. Here is what we do have.
- Impaired · Algal growth
- Limited data
Lake St Louis in St. Charles County, Missouri is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 2.7 ft visibility. Phosphorus: 50 µg/L. Chlorophyll-a (algae): 74.3 µg/L. Surface area: 563 acres.
- Clarity
- 2.7 ft
- Typical in September
- 78°F
- Area
- 563 acres
- Phosphorus
- 50 µg/L
- Chlorophyll-a
- 74.3 µg/L
- Trend
- ↓ Declining
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Stays near Lake St Louis
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Watch Lake St Louis’s letter
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Why Lake St Louis has no grade
We grade a lake only when the monitoring record supports a letter: at least three phosphorus or chlorophyll-a samples in the last ten years, scored against EPA National Lakes Assessment thresholds for this lake’s ecoregion. For Lake St Louis, no nutrient or algae measurement meets the evidence bar (at least 3 samples within 10 years). Sampling here has recorded water clarity, phosphorus and algae (chlorophyll-a).
Not Rated is not an F. It is EPA Integrated Reporting Category 3 — insufficient information. The water here may be excellent or poor; nobody has measured enough of it to say. Everything below that has been measured is still reported on this page.
Sampling is run by Missouri DNR and volunteer programs, and published through the EPA Water Quality Portal. Read the full grading methodology.
Lake St Louis is a 563-acre lake in St. Charles County, Missouri. Overall water quality is declining over recent sampling years.
Source: EPA Water Quality Portal sampling records, Missouri DNR, last sampled 2020-08-24. Grade methodology: LakeQuality methodology.
Swimming Safety
Not ratedLimited data
The letter is not a swimming clearance. Lake St Louis is listed impaired under Clean Water Act §303(d) (IR 5) for algal growth. Check local advisories before you swim.
Water quality measurements for Lake St Louis
These readings exist but are not enough for an A–F grade — see why.
Very murky, less than 2.7 ft of visibility. Phosphorus 50 µg/L. Chlorophyll-a 74.3 µg/L. TSI 65.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 2.7 ft | Not graded |
| Phosphorus | 50 µg/L | Not graded |
| Chlorophyll-a (Algae) | 74.3 µg/L | Not graded |
| Trophic State Index (TSI) | 65 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Water Temperature
Lake St Louis typically sits around 78°F at the surface in September. Summer average is about 82°F. Usually warmest in Jul (about 86°F) and coldest in Apr (about 62°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 33 field readings (2016–2020) via the EPA Water Quality Portal. See the full Lake St Louis water temperature breakdown →
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 563 acres |
No bathymetric survey has been published for Lake St Louis, so its depth is not documented here.
Fishing Lake St Louis
Water Quality Trend: ↓ Declining
Based on 5 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↓ Declining | -0.055 m/yr | 5 |
| Phosphorus | → Stable | +1.2 µg/L/yr | 5 |
| Chlorophyll-a | ↓ Declining | +2.21 µg/L/yr | 5 |
Reservoir Info (USACE NID)
Lake St Louis is a man-made reservoir impounded by the Lake St. Louis Dam (completed 1973), built primarily for recreation on the PERUQUE CREEK; earth-type dam, 47 ft tall and 900 ft long.
- Surface area
- 563 ac
- Normal storage
- 6,300 ac-ft
- Max storage
- 14,650 ac-ft
- Drainage area
- 56.41 sq mi
- Hazard class
- High
- Owner
- LAKE ST LOUIS COMM ASSOC
Source: USACE National Inventory of Dams, NID ID MO10545 · Operator website
EPA Impairment Status
Lake St Louis is officially listed as impaired under Clean Water Act §303(d) in the 2022 EPA reporting cycle (IR category 5).
Causes of impairment
Source: EPA ATTAINS assessment unit MO_7054.00 · Official waterbody report
Graded lakes near Lake St Louis
Lake St Louis has no grade, but these lakes within a short drive do.
Nearby Lakes in St. Charles County
Frequently asked questions about Lake St Louis
Where is Lake St Louis?
- Lake St Louis is a lake in St. Charles County, Missouri. LakeQuality has not rated this lake (UNRATED) — no nutrient or algae measurement meets the evidence bar (at least 3 samples within 10 years).
What is the water quality grade for Lake St Louis?
- Lake St Louis in St. Charles County, Missouri is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 2.7 ft visibility. Phosphorus: 50 µg/L. Chlorophyll-a (algae): 74.3 µg/L. Surface area: 563 acres.
Is Lake St Louis safe to swim in?
- The letter is not a swimming clearance. Lake St Louis is listed impaired under Clean Water Act §303(d) (IR 5) for algal growth. Check local advisories before you swim.
What is the water temperature of Lake St Louis?
- Lake St Louis typically sits around 78°F at the surface in September, from 33 EPA Water Quality Portal field readings (2016–2020). That is a typical value for the month, not a live reading.
Explore Lake St Louis across LakeQuality
Depth, room, public access, and the state wake-sport rules
Species activity outlook by water temperature and season
Where to stay at Missouri's top lakes, each with its grade
Missouri's cleanest and most-polluted lakes, ranked
Planning a trip around Gateway Arch?
Lake St Louis is about 33 miles from Gateway Arch National Park. Crowds there swing hard by season, so the month you pick changes both how busy the park is and what lodging costs.
Best time to visit Gateway Arch →More Missouri Lake Rankings
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Where this data comes from
3 measurements from 1 monitoring station, 2020. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Missouri Dept. of Natural Resources
MDNR3 samples · 2020
Analytical methods
- Depth, Secchi disk in meters
- Nitrogen and Phosphorus, Total, Whole-water, Alkaline Persulfate Digest
- 445.0 ~ EPA; Chlorophyll and Pheophytin in Algae by Fluorescence
Monitoring station identifiers (1)
Grade thresholds from the EPA National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See how grades are calculated and data sources.
Other records on this page
Impairment status from EPA ATTAINS 303(d) database