Michigan lake · state assessment record
Clarks Marsh (Iosco Co.)
Assessment decisions reported by Michigan to EPA under Clean Water Act sections 303(d) and 305(b), last assessed in the 2018 reporting cycle.
- Lake, Freshwater
- Water type
This waterbody in one line
Clarks Marsh (Iosco Co.), a lake in Michigan, is assessed as not supporting one or more designated uses, with 2 listed impairment causes in the state's most recent Integrated Report cycle (2018). Its impairment list is among the longest nationally (2 listed causes vs 23,610 assessed lakes). Fish consumption is assessed as not supporting.
- Not Supporting
- overall status
- P10
- Shorter cause list
- Not Supporting
- fish consumption
Assessment verdicts are the state agency's decisions as reported to EPA ATTAINS; PlainLakes renders them without modification.
A composite of this record's EPA ATTAINS designated-use verdicts, filed impairment count, and assessment currency, weighted and benchmarked against the national corpus of assessed active waterbodies. This is a water-quality standing, not a human-recreation or property-value judgment, and dimensions with no filed verdict drop out rather than count against the score.
How Clarks Marsh (Iosco Co.) reads against the filed register
Paragraphs are selected by this unit's percentiles and use matrix: different lakes lead with different dimensions. Figures are ATTAINS / portal register arithmetic, not advice.
The signature stat: cause-list length
The standout filed fact is cause-list length: 2 listed impairment causes, a longer list than 80% of assessed lake-type units (n=23,610). List length tracks how states sample and code, not a severity score.
Every ruled-on use is not supporting
On the uses this filer ruled on, fish consumption, every determination is Not Supporting. That is a use-by-use reading of the filed record, not a single overall lake grade.
Clarks Marsh (Iosco Co.): Category 5, TMDL still owed
Category 5 is the Integrated Report band where a TMDL (or equivalent) is still expected for Clarks Marsh (Iosco Co.). Read the listed causes and any cleanup actions filed beside them before treating the overall status tile as a finished restoration story.
Overall status: not supporting
The filer's overall status for Clarks Marsh (Iosco Co.) is Not Supporting. That tile summarizes the assessment unit; the designated-use rows and cause list are the evidence behind it.
According to the U.S. Environmental Protection Agency's ATTAINS program, Michigan carries 4,307 filed lake-type waterbodies in the August 2026 extract, and this page renders the assessment record EPA assigned to Clarks Marsh (Iosco Co.) without modification. See our methodology for reporting-unit limits and gauge linking rules.
The record
Designated-use assessments
Fully Supporting and Not Supporting are the state's definitive verdicts; other entries mean the use was not conclusively assessed in this cycle. For the definitive verdicts on this record, Michigan filed the assessment basis as Monitored Data. Those are EPA's own words, not PlainLakes' summary of them. Michigan filed its own confidence on the one definitive verdict it rated: Good. That rating is the filer's, in EPA's own field, and EPA publishes no definition of the four words it offers.
| Designated use | Group | Verdict |
|---|---|---|
| Agriculture | Other | Fully Supporting |
| Fish Consumption | Fish-consumption or fishery | Not Supporting Filer confidence: Good |
| Industrial Water Supply | Other | Fully Supporting |
| Navigation | Other | Fully Supporting |
| Cold Water Fishery | Aquatic-life or related | Not Assessed |
| Other Indigenous Aquatic Life and Wildlife | Aquatic-life or related | Not Assessed |
| Partial Body Contact Recreation | Recreation | Not Assessed |
| Total Body Contact Recreation | Recreation | Not Assessed |
| Warm Water Fishery | Aquatic-life or related | Not Assessed |
The record
Listed impairment causes
Causes the state listed as impairing designated uses of this lake, with the cycle each was first listed and the Clean Water Act action recorded against it.
| Cause | Affected uses | First listed | Cleanup standing |
|---|---|---|---|
| Perfluorooctane Sulfonate (PFOS) in Fish Tissue | Fish Consumption | 2024 | TMDL priority: Low |
| Mercury in Fish Tissue | Fish Consumption | 2026 | TMDL priority: Low |
Probable sources
Michigan listed the source of the impairment as unknown. That is the state's own filed value, not a gap in this record: the cause has been identified, the origin has not.
Reading the record
Where Clarks Marsh (Iosco Co.) sits among peers
16% of Michigan's lakes assessed this deeply also came out mixed
Michigan ruled definitively on 4 designated uses at Clarks Marsh (Iosco Co.) and found 1 of 4 not supported. The dataset shows 973 Michigan lakes judged to that same depth.
Michigan's assessed lakes are most often judged on 3 uses; this one was judged on 4.
Computed share from EPA ATTAINS filed records, not an official EPA rating or safety grade.
| 1 | 2 | 3 | 4 | 5+ | |
|---|---|---|---|---|---|
| All supported | 142 | 85 | 1.9k | 822 | 481 |
| Mixed | Not possible | 38 | 0 | 151 (this record) | 610 |
| None supported | 2 | 6 | 0 | 0 | 0 |
Ringed cell: Clarks Marsh (Iosco Co.). Source: EPA ATTAINS, Reporting cycles 1987–2026.
Scale and rank
Clarks Marsh (Iosco Co.) is one of 4,307 lake-type waterbodies reported to EPA from Michigan, and one of 776 whose reported record carries enough populated assessment or gauge data for this site to list and index it. The state filed this assessment unit with a size of 0 acres, which is a figure rather than a measurement, so no size comparison is possible.
Awaiting a filing
The state has not yet filed a definitive recreation verdict and a definitive aquatic-life verdict for this record; those fields populate when it reports them in a future Integrated Report cycle.
In context
Where Clarks Marsh (Iosco Co.) sits nationally
Listed causes vs waterbodies carrying a verdict
How many impairment causes U.S. waterbodies with a reported verdict carry
2 Longer than most more listed causes than 80% of 23,610 waterbodies carrying a verdict
named waterbodies carrying a verdict, bucketed by value
Source EPA ATTAINS · Reporting cycles 1987–2026
In both charts each bar is a band and taller bars hold more waterbodies; the dashed line and filled bar mark Clarks Marsh (Iosco Co.). Hover or tap any bar for its full count and share.
In context
Michigan context
ATTAINS carries 4,307 lake-type waterbodies in Michigan, 807 assessed as not supporting at least one use and 3,457 fully supporting. Among the 776 indexed units there, Clarks Marsh (Iosco Co.) ranks 266th by listed-cause count (2 listed causes) ; it sits in the not-supporting set (807 statewide) . Its listed causes include Perfluorooctane Sulfonate (PFOS) in Fish Tissue and Mercury in Fish Tissue, which also appear among Michigan's most-filed impairment causes (led by Mercury in Fish Tissue).
| Other large assessed lakes in Michigan | Assessed acres |
|---|---|
| Lake Huron (Michigan Jurisdiction) Not Supporting | 5,815,360 |
| Lake Michigan South of Frankfort (Michigan Jurisdiction) Not Supporting | 5,665,920 |
| St. Marys River Not Supporting | 129,983 |
| Lake St. Clair (Michigan Jurisdiction) Not Supporting | 103,616 |
| Lake Erie (Michigan Jurisdiction) Not Supporting | 71,680 |
| Houghton Lake Not Supporting | 20,066 |
Lakes near Clarks Marsh (Iosco Co.)'s cause load
Similar cause load
Nearest listed-cause counts (2 here). Peers already in the acreage column are omitted.
- 12th St. Beach · 2 causes · IL
- 31st St. Beach · 2 causes · IL
- 49th St. Beach · 2 causes · IL
- 57th St. Beach · 2 causes · IL
What to do with this record
- The fish-consumption use is assessed as not supporting. Before eating fish from this lake, check your state fish-advisory program for species-specific meal limits. Consumption status guide
- Impairment listings follow the Clean Water Act 303(d) process; a listing is a regulatory finding, not a closure notice. 303(d) explained
PlainLakes renders state assessment decisions and federal gauge observations without modification. It does not issue advisories.
Assessment decisions on this page are rendered directly from state Integrated Report records in EPA's ATTAINS system; gauge figures are daily-value observations from the USGS National Water Information System, no number is typed in by an editor. Overall status, use verdicts, and causes are the state's reported decisions for assessment unit MI040700070709-04. Counts and percentiles are computed across all lake-type records PlainLakes ingests from ATTAINS. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error. Data current as of 2026-08-17. Primary sources: EPA ATTAINS and USGS National Water Information System.