Mallard Lake
St. Louis County, MissouriView mapDirections
Nutrients and algae run well above the regional average.
- Hypereutrophic
- #2 of 5 in the county
- 20 ft max depth
Mallard Lake in St. Louis County, Missouri has a water quality grade of F (Algae-prone). Water clarity: 1.9 ft visibility. Phosphorus: 84 µg/L. Chlorophyll-a (algae): 32.2 µg/L.
- Clarity
- 1.9 ft
- Typical in September
- 79°F
- Max depth
- ~20 ft est.
- Type
- Hypereutrophic
- Phosphorus
- 84 µg/L
- Chlorophyll-a
- 32.2 µg/L
- Trend
- ↓ Declining
Stays via Stay22
Stays near Mallard Lake
Compare lakeside cabins, hotels, and vacation rentals near Mallard Lake in St. Louis 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. The water-quality grade measures nutrients and algae, not lodging — cabins and rentals around Mallard Lake are listed below.
Top-rated places to stay near Mallard Lake
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Watch Mallard Lake’s letter
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Mallard Lake is a highly nutrient-loaded hypereutrophic lake in St. Louis County, Missouri with an estimated average depth of about 20 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 4 other monitored lakes in St. Louis County, Mallard Lake ranks #2 for water quality.
Source: EPA Water Quality Portal sampling records, Missouri DNR, last sampled 2024-09-19. Grade methodology: LakeQuality methodology.
Swimming Safety
Grade onlyAlgae risk
Based on clarity and algae grades from past sampling — we do not hold live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
The F is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
Water Quality Grade: F (Algae-prone)
Very murky, less than 1.9 ft of visibility. Phosphorus 84 µg/L. Chlorophyll-a 32.2 µg/L. TSI 67. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 1.9 ft | Not graded |
| Phosphorus | 84 µg/L | Poor |
| Chlorophyll-a (Algae) | 32.2 µg/L | Poor |
| Trophic State Index (TSI) | 67 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Water Temperature
Mallard Lake typically sits around 79°F at the surface in September. Summer average is about 84°F. 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 →
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | ~20 ft est. |
| Average Depth | ~12 ft est. |
Depth marked “est.” is a modeled maximum from GLOBathy (Khazaei et al. 2022, CC0), not a measured survey. Missouri has not published bathymetry for Mallard Lake.
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.
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 |
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.
County Ranking
Ranked #2 of 5 lakes in St. Louis County
Nearby Lakes With Higher Grades
Mallard Lake holds Grade F. One nearby lake has a higher nutrient-and-algae grade — useful for a swim day, not a reason to skip lodging here.
See full comparison →Nearby Lakes in St. Louis County
Frequently asked questions about Mallard Lake
Where is Mallard Lake?
- Mallard Lake is a lake in St. Louis County, Missouri. LakeQuality grades it F for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Mallard Lake?
- Mallard Lake in St. Louis County, Missouri has a water quality grade of F (Algae-prone). Water clarity: 1.9 ft visibility. Phosphorus: 84 µg/L. Chlorophyll-a (algae): 32.2 µg/L.
Is Mallard Lake safe to swim in?
- The F is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
What is the water temperature of Mallard Lake?
- Mallard Lake typically sits around 79°F at the surface in September, from 78 EPA Water Quality Portal field readings (2015–2024). That is a typical value for the month, not a live reading.
How deep is Mallard Lake?
- No published bathymetric survey covers Mallard Lake, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 20 feet, but that is a modelled estimate derived from surrounding terrain rather than a measurement on the water, so treat it as approximate.
Explore Mallard Lake across LakeQuality
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?
Mallard Lake is about 17 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
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 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
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Lake size & watershed from HydroLAKES (HydroSHEDS)