Mark Twain Lake is a 5,900-acre productive, nutrient-rich eutrophic lake in Ralls County, Missouri. Water quality is graded D based on 3 sampling years through 2020. Overall water quality is improving over recent sampling years. Limited fish-species data available. Public access is available with 1 public boat landing. Compared to the 3 other monitored lakes in Ralls County, Mark Twain Lake ranks #3 for water quality.
Source: EPA Water Quality Portal sampling records, Missouri DNR, last sampled 2020-09-19. Grade methodology: LakeGrade methodology.
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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 2.5 ft of visibility. Phosphorus level: 60.8 µg/L. Chlorophyll-a: 18.3 µg/L. Trophic State Index: 62.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 2.5 ft | F |
| Phosphorus | 60.8 µg/L | D |
| Chlorophyll-a (Algae) | 18.3 µg/L | C |
| Trophic State Index (TSI) | 62 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 5,900 acres |
| Public Access | Yes |
Where to stay near Mark Twain Lake
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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 | +0.197 m/yr | 5 |
| Phosphorus | ↑ Improving | -4.9 µg/L/yr | 5 |
| Chlorophyll-a | ↑ Improving | -1.15 µg/L/yr | 5 |
Recreation & Access
Water Temperature
Mark Twain Lake averages about 79°F at the surface in summer, usually warmest in Jul (about 82°F) and coldest in Apr (about 54°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 106 field readings (2010–2020) via the EPA Water Quality Portal. See the full Mark Twain Lake water temperature breakdown →
Location
County Ranking
Ranked #3 of 4 lakes in Ralls County
Cleaner Lakes Within 30 Miles
Mark Twain Lake holds Grade D. 4 nearby lakes hold higher grades.
- CMark Twain Lake by Hwy JRalls County · 0.1 mi · Higher grade (D → C)
- CMark Twain Lake Indian Creek ArmRalls County · 2.8 mi · Higher grade (D → C)
- CMark Twain LakeMonroe County · 6.5 mi · Higher grade (D → C)
- DHunnewell LakeShelby County · 17.1 mi · Higher overall water quality score
Nearby Lakes in Ralls County
Explore Mark Twain Lake across LakeGrade
More Missouri Lake Rankings
Other Grade D Lakes in Missouri
Eutrophic Lakes in Missouri
Compare Mark Twain Lake With Nearby Lakes
More records, surveys & data sources
Reservoir Info (USACE NID)
Mark Twain Lake is a man-made reservoir impounded by the Clarence Cannon Dam (completed 1983), built primarily for flood risk reduction on the SALT RIVER; concrete-type dam, 188 ft tall and 1,940 ft long.
- Surface area
- 5,900 ac
- Normal storage
- 520,661 ac-ft
- Max storage
- 1,862,000 ac-ft
- Drainage area
- 2,318 sq mi
- Hazard class
- High
- Owner
- USACE - St. Louis District
All listed purposes: Flood Risk Reduction;Navigation;Fish and Wildlife Pond;Recreation;Hydroelectric;Water Supply.
Source: USACE National Inventory of Dams, NID ID MO82201 · Operator website
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. 1 facility has a reported Clean Water Act violation in the past three years.
- Harbortown Rv ParkMonroe City · 2.8 mi away · NPDES MO0139467Recent violation
- J. F. Spalding Land Treatment SystemMonroe City · 1.9 mi away · NPDES MO0127183
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
Public boat-access points from the USGS National Boat Ramps inventory
Most recent sample: 2020-09-19
Monitoring stations: 1