Mark Twain Lake is a productive, nutrient-rich eutrophic lake in Monroe County, Missouri. Water quality is graded C based on 3 sampling years through 2024. Overall water quality is improving over recent sampling years. Limited fish-species data available. Public access is available with 8 public boat landings. Compared to the 2 other monitored lakes in Monroe County, Mark Twain Lake ranks #2 for water quality.
Source: EPA Water Quality Portal sampling records, Missouri DNR, last sampled 2024-08-31. Grade methodology: LakeGrade methodology.
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Swimming Safety
Based on E. coli testing, Mark Twain Lake rarely exceeded the EPA safe-swimming threshold (235 MPN/100mL) — on 4.1% of 222 samples across 7 seasons.
This is a historical summary of past testing, not a current condition — bacteria levels change quickly, especially after rain. Always check Missouri’s current beach and health-department advisories before swimming.
Water Quality Grade: C, Fair
Murky, only visible to about 3.8 ft. Phosphorus level: 36 µg/L. Chlorophyll-a: 11.9 µg/L. Trophic State Index: 56.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 3.8 ft | D |
| Phosphorus | 36 µg/L | C |
| Chlorophyll-a (Algae) | 11.9 µg/L | C |
| Trophic State Index (TSI) | 56 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Where to stay near Mark Twain Lake
Compare lakeside cabins, hotels, and vacation rentals near Mark Twain Lake in Monroe 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.
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Fishing Mark Twain Lake
Invasive & Introduced Species
Red chips are ecologically harmful invasive species. Gray chips are stocked or introduced fish that aren't a current ecological concern.
Source: USGS Nonindigenous Aquatic Species (NAS) Database. Hover a chip to see scientific name, ecological context, status, and first-observed year.
Water Quality Trend: ↑ Improving
Based on 5 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↑ Improving | +0.133 m/yr | 5 |
| Phosphorus | ↑ Improving | -5.61 µg/L/yr | 5 |
| Chlorophyll-a | ↑ Improving | -0.93 µg/L/yr | 5 |
Recreation & Access
Water Temperature
Mark Twain Lake averages about 77°F at the surface in summer, usually warmest in Jul (about 80°F) and coldest in Apr (about 55°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 96 field readings (2010–2024) via the EPA Water Quality Portal. See the full Mark Twain Lake water temperature breakdown →
Location
County Ranking
Ranked #2 of 3 lakes in Monroe County
Cleaner Lakes Within 30 Miles
Mark Twain Lake holds Grade C. One nearby lake has a higher grade.
See full comparison →Nearby Lakes in Monroe County
Explore Mark Twain Lake across LakeGrade
More Missouri Lake Rankings
Other Grade C Lakes in Missouri
Eutrophic Lakes in Missouri
More records, surveys & data sources
EPA Impairment Status
Mark Twain Lake 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_7033.00 · Official waterbody report
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth is a modeled estimate.
Regulated Pollution Sources Nearby
2 EPA-permitted facilities operate within about 5 miles of Mark Twain Lake under Clean Water Act (NPDES) discharge permits. 2 facilities have a reported Clean Water Act violation in the past three years.
- Mdnr, Mark Twain State Park WwtfStoutsville · 1.7 mi away · NPDES MO0114138Recent violation
- Stoutsville Resort And Rv Park LlcStoutsville · 4.4 mi away · NPDES MO0116084Recent violation
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
Impairment status from EPA ATTAINS 303(d) database
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Lake size & watershed from HydroLAKES (HydroSHEDS)
Grading methodology based on Metropolitan Council standards
Public boat-access points from the USGS National Boat Ramps inventory
Most recent sample: 2024-08-31
Monitoring stations: 1