St. Charles County, Missouri · 38.79°N · Water Quality Report Card
Lake St Louis
Lake St Louis in St. Charles County, Missouri has a water quality grade of F (Very Poor). Water clarity: 1 ft visibility. Phosphorus: 125.8 µg/L. Chlorophyll-a (algae): 40.6 µg/L.
- Clarity
- 1 ft
- Mean depth
- ~18 ft est.
- Type
- Hypereutrophic
- Trend
- ↓ Declining
Lake St Louis is a highly nutrient-loaded hypereutrophic lake in St. Charles County, Missouri with an estimated average depth of about 18 feet. Water quality is graded F based on 2 sampling years through 2024. Overall water quality is declining over recent sampling years. Compared to the 5 other monitored lakes in St. Charles County, Lake St Louis ranks #5 for water quality.
Source: EPA Water Quality Portal sampling records, Missouri DNR, last sampled 2024-09-15. Grade methodology: LakeGrade methodology.
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Swimming Safety
Avoid swimming, very poor water quality, potential algae toxins
Water Quality Grade: F (Very Poor)
Very murky, less than 1 ft of visibility. Phosphorus level: 125.8 µg/L. Chlorophyll-a: 40.6 µg/L. Trophic State Index: 73.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 1 ft | |
| Phosphorus | 125.8 µg/L | Poor |
| Chlorophyll-a (Algae) | 40.6 µg/L | Poor |
| Trophic State Index (TSI) | 73 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Lake Details
| Characteristic | Value |
|---|---|
| Average Depth | ~18 ft est. |
Depth marked “est.” is a modeled estimate from HydroLAKES, not a measured survey. Missouri has not published bathymetry for Lake St Louis.
Where to stay near Lake St Louis
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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 | -0.03 m/yr | 5 |
| Phosphorus | → Stable | -0.47 µg/L/yr | 5 |
| Chlorophyll-a | ↓ Declining | +1.19 µg/L/yr | 5 |
Location
County Ranking
Ranked #5 of 6 lakes in St. Charles County
Cleaner Lakes Within 30 Miles
Lake St Louis holds Grade F. 2 nearby lakes hold higher grades.
- CL. Ste. Louise LakeSt. Charles County · 0.7 mi · Two grade letters higher (F → C)
- CL Ste Louise LakeSt. Charles County · 0.9 mi · Two grade letters higher (F → C)
Nearby Lakes in St. Charles County
Explore Lake St Louis across LakeGrade
Species activity outlook by water temperature and season
Where to stay at Missouri's top lakes, each with its grade
Very high nutrients, dense algae, poor clarity
Missouri's cleanest and most-polluted lakes, ranked
Planning a trip around Gateway Arch?
Lake St Louis is about 35 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 Grade F Lakes in Missouri
Hypereutrophic Lakes in Missouri
More records, surveys & data sources
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
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth here is modelled, not surveyed — HydroLAKES v1.0 estimates lake depth from surrounding terrain rather than from soundings. Treat it as an approximation, and prefer a state bathymetric survey where one exists.
Where this data comes from
120 measurements from 1 monitoring station, 2020–2024. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Univ. of Missouri, Columbia
UMC120 samples · 2020–2024
Analytical methods
- Depth, Secchi disk in meters
- 4500 P E ~ Phosphorus in Water by Colorimetry- Ascorbic Acid Method
- 445.0 ~ EPA; Chlorophyll and Pheophytin in Algae by Fluorescence
Monitoring station identifiers (1)
Grade thresholds come from the U.S. EPA National Lakes Assessment 2022 Technical Support Document (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See how grades are calculated.
Data Sources
Water quality data from the EPA Water Quality Portal
Impairment status from EPA ATTAINS 303(d) database
Lake size & watershed from HydroLAKES (HydroSHEDS)
Grade thresholds from the EPA National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion
Most recent sample: 2024-09-15
Monitoring stations: 1