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.
- Clarity
- 2.7 ft
- Typical in August
- 81°F
- Area
- 563 acres
- Trend
- ↓ Declining
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Why Lake St Louis has no grade
We grade a lake only when its monitoring record clears the evidence bar: 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: LakeGrade methodology.
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Swimming Safety
Not ratedLimited data
Not rated for swimming — there is not enough monitoring data to judge this lake
Water quality measurements for Lake St Louis
These readings exist but do not meet the evidence bar for an A–F grade — see why.
Very murky, less than 2.7 ft of visibility. Phosphorus level: 50 µg/L. Chlorophyll-a: 74.3 µg/L. Trophic State Index: 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
Where to stay near Lake St Louis
Compare lakeside cabins, hotels, and vacation rentals near Lake St Louis in St. Charles County, Missouri — live availability and prices for your trip. Switch the map to Satellite to see the lake and how close each stay is to the water.
Top-rated places to stay near Lake St Louis
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 whole trip? See where to stay at Missouri’s top lakes →
Water Temperature
Lake St Louis typically sits around 81°F at the surface in August. 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.
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) — there is not enough recent sampling to assign a letter.
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?
- Not rated for swimming — there is not enough monitoring data to judge this lake. Lake St Louis is not rated for water quality — its monitoring record does not meet the evidence bar. Algae concentration (chlorophyll-a) is 74.3 µg/L. Always check local health department advisories before swimming.
What is the water temperature of Lake St Louis?
- Lake St Louis typically sits around 81°F at the surface in August, 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 LakeGrade
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
Other Lakes in Missouri
Eutrophic Lakes in Missouri
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
Lake size & watershed from HydroLAKES (HydroSHEDS)