F

Mark Twain Lake

Monroe County, WisconsinEutrophic

Mark Twain Lake is a productive, nutrient-rich eutrophic lake in Monroe County, MO. Water quality is graded F based on 3 sampling years through 2021. Compared to the 2 other monitored lakes in Monroe County, Mark Twain Lake ranks #3 for water quality.

Source: EPA Water Quality Portal sampling records, local state DNR records, last sampled 2021-08-03. Grade methodology: LakeGrade methodology.

Swimming Safety

Avoid swimming, very poor water quality, potential algae toxins

Water Quality Grade: F, Very Poor

Very murky, less than 2.5 ft of visibility. Phosphorus level: 80 µg/L. Chlorophyll-a: 92.6 µg/L. Trophic State Index: 69.

Metric	Value	Grade
Water Clarity (Secchi Depth)	2.5 ft	F
Phosphorus	80 µg/L	D
Chlorophyll-a (Algae)	92.6 µg/L	F
Trophic State Index (TSI)	69	Eutrophic

High nutrients, frequent algae, reduced clarity

Location

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County Ranking

Ranked #3 of 3 lakes in Monroe County

Cleaner Lakes Within 30 Miles

Mark Twain Lake holds Grade F. 5 nearby lakes hold higher grades.

See full comparison →

Nearby Lakes in Monroe County

Monroe City Lake B Nr. Dam

Mark Twain Lake

Perry City Lake

Mark Twain Lake Indian Creek Arm

Mark Twain Lake by Hwy J

Mark Twain Lake

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Other Grade F Lakes in Wisconsin

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St. Francois County · Clarity: 1.1 ft

Blind Pony Lake

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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

Mercury

Source: EPA ATTAINS assessment unit MO_7033.00 · Official waterbody report

Data Sources

Water quality data from the EPA Water Quality Portal

Impairment status from EPA ATTAINS 303(d) database

Grading methodology based on Metropolitan Council standards

Most recent sample: 2021-08-03

Monitoring stations: 1