Mallard Lake is a productive, nutrient-rich eutrophic lake in St. Louis County, Missouri. Water quality is graded F based on 3 sampling years through 2024. Overall water quality is declining over recent sampling years. Compared to the 8 other monitored lakes in St. Louis County, Mallard Lake ranks #3 for water quality.
Source: EPA Water Quality Portal sampling records, Missouri DNR, last sampled 2024-09-19. 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 1.9 ft of visibility. Phosphorus level: 84 µg/L. Chlorophyll-a: 32.2 µg/L. Trophic State Index: 67.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 1.9 ft | F |
| Phosphorus | 84 µg/L | D |
| Chlorophyll-a (Algae) | 32.2 µg/L | F |
| Trophic State Index (TSI) | 67 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Where to stay near Mallard Lake
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Fishing Mallard Lake
Water Quality Trend: ↓ Declining
Based on 5 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | → Stable | +0.01 m/yr | 5 |
| Phosphorus | ↓ Declining | +7.75 µg/L/yr | 5 |
| Chlorophyll-a | ↓ Declining | +3.24 µg/L/yr | 5 |
Water Temperature
Mallard Lake averages about 84°F at the surface in summer, usually warmest in Jul (about 86°F) and coldest in Apr (about 63°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 78 field readings (2015–2024) via the EPA Water Quality Portal. See the full Mallard Lake water temperature breakdown →
Location
County Ranking
Ranked #3 of 9 lakes in St. Louis County
Cleaner Lakes Within 30 Miles
Mallard Lake holds Grade F. 3 nearby lakes hold higher grades.
- DSimpson Park LakeSt. Louis County · 10.1 mi · Higher grade (F → D)
- DSimpson Park LakeSt. Louis County · 10.1 mi · Higher grade (F → D)
- DForest Park PondSt. Louis city County · 12.1 mi · Higher grade (F → D)
Nearby Lakes in St. Louis County
Explore Mallard Lake across LakeGrade
More Missouri Lake Rankings
Other Grade F Lakes in Missouri
Jefferson County · Clarity: 1.3 ft
St. Charles County · Clarity: 2.6 ft
St. Charles County · Clarity: 2.3 ft
St. Francois County · Clarity: 1.1 ft
Audrain County · Clarity: 1.8 ft
Ralls County · Very Poor
Eutrophic Lakes in Missouri
More records, surveys & data sources
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth is a modeled estimate.
Regulated Pollution Sources Nearby
5 EPA-permitted facilities operate within about 5 miles of Mallard Lake under Clean Water Act (NPDES) discharge permits, including 1 classified by the EPA as major discharger. 2 facilities have a reported Clean Water Act violation in the past three years.
- Dcsd Treatment Plant 1St. Charles · 3.2 mi away · NPDES MO0085472MajorRecent violation
- Chesterfield Central DwtpChesterfield · 2 mi away · NPDES MO0003727Recent violation
- St Louis, Howard Bend WtpChesterfield · 3 mi away · NPDES MO0004928
- Pfizer IncChesterfield · 4.4 mi away · NPDES MONX00725
- Bayer Research And Development ServicesChesterfield · 4.5 mi away · NPDES MONX00426
Facilities permitted to discharge under the Clean Water Act within ~5 miles; proximity does not by itself mean a facility affects this lake. Source: EPA ECHO (ICIS-NPDES), as of 2026-07.
Data Sources
Water quality data from the EPA Water Quality Portal
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Lake size & watershed from HydroLAKES (HydroSHEDS)
Grading methodology based on Metropolitan Council standards
Most recent sample: 2024-09-19
Monitoring stations: 1