Lake St. Louis is a 563-acre productive, nutrient-rich eutrophic lake in St. Charles County, Missouri. Water quality is graded D based on 3 sampling years through 2020. Overall water quality is declining over recent sampling years. Compared to the 10 other monitored lakes in St. Charles County, Lake St. Louis ranks #7 for water quality.
Source: EPA Water Quality Portal sampling records, Missouri DNR, last sampled 2020-08-24. Grade methodology: LakeGrade methodology.
Watch Lake St. Louis water quality
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Swimming Safety
Swimming not recommended, poor water quality with high algae risk
Water Quality Grade: D, Poor
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 | F |
| Phosphorus | 50 µg/L | C |
| Chlorophyll-a (Algae) | 74.3 µg/L | F |
| Trophic State Index (TSI) | 65 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 563 acres |
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 | -0.055 m/yr | 5 |
| Phosphorus | → Stable | +1.2 µg/L/yr | 5 |
| Chlorophyll-a | ↓ Declining | +2.21 µg/L/yr | 5 |
Water Temperature
Lake St. Louis averages about 82°F at the surface in summer, 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.
Location
County Ranking
Ranked #7 of 11 lakes in St. Charles County
Cleaner Lakes Within 30 Miles
Lake St. Louis holds Grade D. 2 nearby lakes hold higher grades.
- DL. Ste. Louise LakeSt. Charles County · 1.3 mi · Higher overall water quality score
- DL. Ste. Louise LakeSt. Charles County · 1.8 mi · Higher overall water quality score
Nearby Lakes in St. Charles County
Explore Lake St. Louis across LakeGrade
More Missouri Lake Rankings
Other Grade D Lakes in Missouri
Eutrophic Lakes in Missouri
Compare Lake St. Louis With Nearby Lakes
More records, surveys & data sources
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
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth is a modeled estimate.
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)
Grading methodology based on Metropolitan Council standards
Most recent sample: 2020-08-24
Monitoring stations: 1