South Dakota reservoir · state assessment record
Lynn Lake
Day County. Assessment decisions reported by South Dakota to EPA under Clean Water Act sections 303(d) and 305(b), last assessed in the 2014 reporting cycle.
- 57
- Assessed acres
- Lake / Reservoir / Pond
- Water type
This waterbody in one line
Lynn Lake, a reservoir in South Dakota, is assessed as not supporting one or more designated uses, with one listed impairment cause in the state's most recent Integrated Report cycle (2014). By assessed acreage it sits above the national median by assessed acreage (38,628 lakes); its impairment list is among the longest nationally (one listed cause vs 23,610 assessed lakes). Aquatic life is assessed as not supporting.
- Not Supporting
- overall status
- P58
- Assessed size nationally
- P20
- Shorter cause list
- Not Supporting
- aquatic life
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.
Lynn Lake against the ATTAINS 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: in-state cause rank
Inside South Dakota, cause-list rank 124 of 176 is mid-pack, the state peer table is not driven by an extreme list length here.
Every ruled-on use is not supporting
On the uses this filer ruled on, aquatic life, every determination is Not Supporting. That is a use-by-use reading of the filed record, not a single overall lake grade.
Lynn Lake: Category 4A reading
Category 4A means a TMDL (or equivalent) is already approved for the impairment on Lynn Lake. The remaining work is implementation and tracking, read the cleanup-standing and cause rows for what the state already committed to address.
Overall status: not supporting
The filer's overall status for Lynn Lake is Not Supporting. That tile summarizes the assessment unit; the designated-use rows and cause list are the evidence behind it.
Small within South Dakota
State size rank 137 of 176 puts Lynn Lake toward the small end of South Dakota's indexed lake-type units with an area.
According to the U.S. Environmental Protection Agency's ATTAINS program, South Dakota carries 195 filed lake-type waterbodies in the August 2026 extract, and this page renders the assessment record EPA assigned to Lynn Lake 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. South Dakota filed its own confidence on the 2 definitive verdicts it rated: Fair. 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 |
|---|---|---|
| Fish and Wildlife Propagation, Recreation, and Stock Watering Waters | Aquatic-life or related | Not Supporting Filer confidence: Fair |
| Warmwater Marginal Fish Life Propagation Waters | Aquatic-life or related | Not Supporting Filer confidence: Fair |
| Immersion Recreation Waters | Recreation | Not Assessed |
| Limited Contact Recreation Waters | Recreation | Not Assessed |
The record
Listed impairment causes
Causes the state listed as impairing designated uses of this reservoir, with the cycle each was first listed and the Clean Water Act action recorded against it.
| Cause | Affected uses | First listed | Cleanup standing |
|---|---|---|---|
| Mercury in Fish Tissue | Fish and Wildlife Propagation, Recreation, and Stock Watering Waters; Warmwater Marginal Fish Life Propagation Waters | 2016 | TMDL approved |
Probable sources named by the state
Each source below is listed under the cause the state filed it against, and carries the word the state filed with it — confirmed, or suspected.
- Mercury in Fish Tissue
- Atmospheric Deposition (confirmed)
Reading the record
Lynn Lake: Category 4A reading
4A Category 4A: impaired, TMDL completed
South Dakota filed this assessment unit as Category 4A: impaired, with a pollutant-budget TMDL already completed under 303(d). The listing remains an impairment finding; the plan is on file rather than still awaiting write-up.
Scale and rank
Lynn Lake is one of 195 lake-type waterbodies reported to EPA from South Dakota, and one of 176 whose reported record carries enough populated assessment or gauge data for this site to list and index it. At 57 assessed acres it ranks 16,403rd by assessed size among the 38,628 lakes the dataset carries an area for, counting a state’s repeated filings of one water once, behind 16,402 lakes with a larger one.
Awaiting a filing
The state has not yet filed a definitive fish-consumption verdict and a definitive recreation verdict for this record; those fields populate when it reports them in a future Integrated Report cycle.
Reported trophic class
South Dakota carried the trophic field for this assessment unit and filed it as unclassified, which records that no class was determined rather than that the water is unenriched. What the trophic scale measures.
In context
Where Lynn Lake sits nationally
Assessed size among U.S. lakes
Distribution of assessed acreage across ATTAINS lakes filed with an area, counting a state’s repeated filings of one water once
57 Top 42% larger than 58% of 38,628 lakes
lakes the dataset carries an area for, counting a state’s repeated filings of one water once, bucketed by value
Source EPA ATTAINS · Reporting cycles 1987–2026
Listed causes vs waterbodies carrying a verdict
How many impairment causes U.S. waterbodies with a reported verdict carry
1 Longer than most more listed causes than 49% 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 Lynn Lake. Hover or tap any bar for its full count and share.
In context
South Dakota context
ATTAINS carries 195 lake-type waterbodies in South Dakota, 145 assessed as not supporting at least one use and 41 fully supporting. Among the 176 indexed units there, Lynn Lake ranks 124th by listed-cause count (one listed cause) and 137th by assessed size among the 176 with an area filed; it sits in the not-supporting set (145 statewide) . Its listed causes include Mercury in Fish Tissue, which also appears among South Dakota's most-filed impairment causes (led by Mercury in Fish Tissue).
| Other large assessed lakes in South Dakota | Assessed acres |
|---|---|
| Lake Thompson Not Supporting | 13,888 |
| Lake Poinsett Not Supporting | 9,795 |
| Waubay Lake Not Supporting | 9,531 |
| Bitter Lake Not Supporting | 7,865 |
| Dry Lake Number 2 Not Supporting | 6,442 |
| Orman Dam (Belle Fourche Reservoir) Not Supporting | 6,106 |
Peers near Lynn Lake's size or cause load
Similar assessed acres
Nearest filed extents (57 acres here).
- Gulden · 57 acres · MN
- Paris Twin West · 57 acres · IL
- Rock Pond · 57 acres · NY
- Sprague Pond · 57 acres · NY
Similar cause load
Nearest listed-cause counts (1 here). Peers already in the acreage column are omitted.
- Aaron · 1 causes · MN
- Aaron River Reservoir · 1 causes · MA
- Abbotts Creek (including Lexington-Thomasville Water Supply Reservoir at normal reservoir elevation, Tom-A-Lex Lake) · 1 causes · NC
- Abeniki Lake · 1 causes · NH
What to do with this record
- Assessment verdicts are cycle-based, not real-time. For fish-consumption decisions, your state advisory program is the binding source. 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 SD-JA-L-LYNN_01. 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.