Mallard Lake
St. Louis County, MissouriView mapDirections
Missouri DNR publishes the water-quality measurements for Mallard Lake. Here is what has been measured.
- Hypereutrophic
- 20 ft max depth
Mallard Lake in St. Louis County, Missouri has water-quality measurements published by Missouri DNR, shown here as reported. Water clarity: 1.9 ft visibility. Phosphorus: 84 µg/L. Chlorophyll-a (algae): 32.2 µg/L.
Mallard Lake is a highly nutrient-loaded hypereutrophic lake in St. Louis County, Missouri with an estimated average depth of about 20 feet. 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. Methodology: how these readings are compared.
Swimming conditions
Readings onlyAlgae risk
Based on clarity and algae readings from past sampling — there is no live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
Nutrient and algae readings are not a swimming clearance. Check local advisories before you swim.
Whether Mallard Lake 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 Mallard Lake
These are the measurements Missouri DNR publishes for Mallard Lake, 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 1.9 ft of visibility. Phosphorus 84 µg/L. Chlorophyll-a 32.2 µg/L. TSI 67.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 1.9ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 84µg/L | Poor GoodFairPoor |
| Chlorophyll-a (algae) | 32.2µg/L | Poor GoodFairPoor |
| Trophic state (TSI)Carlson's 0–100 index of how much algae a lake can support, from clarity, phosphorus and chlorophyll-a. | 67 | Hypereutrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Very high nutrients, dense algae, poor clarity
Ask about Mallard Lake
Answers come straight from the public measurements and survey records on this page.
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.
Column height runs from this lake’s coldest month (63°F) to its warmest (86°F).
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
Max depth
~20 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
Watershed (drainage) area
28 sq mi
HydroLAKES / HydroSHEDS
Surface area
77 acres
HydroLAKES / HydroSHEDS
Shoreline
1.8 mi
HydroLAKES / HydroSHEDS
Elevation
430 ft
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.
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 readings cover nutrients and algae, not lodging. Cabins and rentals around Mallard Lake are listed below.
Top-rated places to stay near Mallard Lake
Prices for Oct 30–Nov 1, 4 adults. Change dates on the map below.
Availability and prices from Stay22 partners (Booking.com, Vrbo, Expedia). We may earn a commission from bookings, at no extra cost to you.
Planning a whole trip? See where to stay at Missouri’s top lakes →
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Fishing Mallard Lake
Water Quality Trend: Worsening
Based on 5 years of monitoring (2020–2024).
Water clarity
Stable5 years
Phosphorus
Worsening+7.75 µg/L/yr5 years
Chlorophyll-a
Worsening+3.24 µg/L/yr5 years
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.
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. Its water-quality measurements are published by Missouri DNR and shown on this page as reported.
What is the water quality of Mallard Lake?
- Mallard Lake in St. Louis County, Missouri has water-quality measurements published by Missouri DNR, shown here as reported. Water clarity: 1.9 ft visibility. Phosphorus: 84 µg/L. Chlorophyll-a (algae): 32.2 µg/L.
Is Mallard Lake safe to swim in?
- Nutrient and algae readings are 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
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 lakes in Missouri
Other 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)
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
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Lake size & watershed from HydroLAKES (HydroSHEDS)