Lake St Louis
St. Charles County, Missouri563 acresView mapDirections
Missouri DNR publishes the water-quality measurements for Lake St Louis. Here is what has been measured.
- Impaired · Algal growth
- Limited data
Lake St Louis in St. Charles County, Missouri has water-quality measurements published by Missouri DNR, shown here as reported. Water clarity: 2.7 ft visibility. Phosphorus: 50 µg/L. Chlorophyll-a (algae): 74.3 µg/L. Surface area: 563 acres.
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. Methodology: how these readings are compared.
Swimming conditions
No readingLimited data
Nutrient and algae readings are 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.
Whether Lake St Louis is safe to swim in today is a call for the Missouri Department of Natural Resources and the county health department, who post current advisories. LakeQuality does not issue swimming clearances.
Water quality measurements for Lake St Louis
These are the measurements Missouri DNR publishes for Lake St Louis, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Very murky, less than 2.7 ft of visibility. Phosphorus 50 µg/L. Chlorophyll-a 74.3 µg/L. TSI 65.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 2.7ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 50µg/L | Measured, not scored The nutrient that most limits algae growth in freshwater. More phosphorus, more algae potential. |
| Chlorophyll-a (algae) | 74.3µg/L | Measured, not scored The green pigment in algae — a direct measure of how much algae is in the water. |
| Trophic state (TSI)Carlson's 0–100 index of how much algae a lake can support, from clarity, phosphorus and chlorophyll-a. | 65 | Eutrophic OligotrophicMesotrophicEutrophicHypereutrophic |
High nutrients, frequent algae, reduced clarity
Ask about Lake St Louis
Answers come straight from the public measurements and survey records on this page.
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.
Column height runs from this lake’s coldest month (62°F) to its warmest (86°F).
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
Surface area
563 acres
No bathymetric survey has been published for Lake St Louis, so its depth is not documented here.
Stays via Stay22
Stays near Lake St Louis
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Fishing Lake St Louis
Water Quality Trend: Worsening
Based on 5 years of monitoring (2020–2024).
Water clarity
Worsening5 years
Phosphorus
Stable+1.2 µg/L/yr5 years
Chlorophyll-a
Worsening+2.21 µg/L/yr5 years
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
Designated uses not supporting: Aquatic life.
Source: EPA ATTAINS assessment unit MO_7054.00 · Official waterbody report
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. Its water-quality measurements are published by Missouri DNR and shown on this page as reported.
What is the water quality of Lake St Louis?
- Lake St Louis in St. Charles County, Missouri has water-quality measurements published by Missouri DNR, shown here as reported. 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?
- Nutrient and algae readings are 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
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Missouri's cleanest lakes, ranked on agency measurements
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 lakes in Missouri
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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)
EPA condition classes from the National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See where the data comes from and data sources.
Other records on this page
Impairment status from EPA ATTAINS 303(d) database