Wisconsin lake · state assessment record
Lake Koshkonong
In Dane, Jefferson and Rock counties. Assessment decisions reported by Wisconsin to EPA under Clean Water Act sections 303(d) and 305(b), last assessed in the 2026 reporting cycle.
- 10,596
- Assessed acres
- Lake, Freshwater
- Water type
- Eutrophic
- Reported trophic class
- 776.6 ft
- Water Surface Elevation (provisional)
This waterbody in one line
Lake Koshkonong, a lake in Wisconsin, is assessed as not supporting one or more designated uses, with 2 listed impairment causes in the state's most recent Integrated Report cycle (2026). By assessed acreage it sits among the largest 1% of assessed lakes nationally (38,628 lakes with a filed area); its impairment list is among the longest nationally (2 listed causes vs 23,610 assessed lakes). Aquatic life is assessed as not supporting. A USGS gauge tracks its water surface elevation daily.
- Not Supporting
- overall status
- P99
- Assessed size nationally
- P10
- 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.
Lake Koshkonong 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: assessed size
The number that most separates Lake Koshkonong nationally is assessed size: 10,596 acres, larger than 99% of the 38,628 lake-type units with an area on file.
No ruled-on use clears the bar
On the uses this filer ruled on, recreation, aquatic life, every determination is Not Supporting. That is a use-by-use reading of the filed record, not a single overall lake grade.
Lake Koshkonong: Category 4A reading
Category 4A means a TMDL (or equivalent) is already approved for the impairment on Lake Koshkonong. The remaining work is implementation and tracking, read the cleanup-standing and cause rows for what the state already committed to address.
Longer than most cause lists
2 listed causes puts this unit above 80% of assessed lake-type records by list length. Treat that as a coding-and-sampling signal: states that split pollutants finely mint longer lists without proving worse water.
Overall status: not supporting
The filer's overall status for Lake Koshkonong 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, Wisconsin carries 19,871 filed lake-type waterbodies in the August 2026 extract, and this page renders the assessment record EPA assigned to Lake Koshkonong 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. The latest monitoring Wisconsin filed against this record runs through 2026. For the definitive verdicts on this record, Wisconsin filed the assessment basis as Monitored Data. Those are EPA's own words, not PlainLakes' summary of them. Wisconsin filed its own confidence on the 2 definitive verdicts it rated: Low, Good and Excellent. 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 Aquatic Life | Aquatic-life or related | Not Supporting Filer confidence: Low on biological; Good on physical/chemical |
| Recreation | Recreation | Not Supporting Filer confidence: Excellent |
| Fish Consumption | Fish-consumption or fishery | Not Assessed |
| Public Health and Welfare | Recreation | Not Assessed |
| Wildlife | 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 |
|---|---|---|---|
| Phosphorus, Total | Fish and Aquatic Life; Recreation | 2002 | TMDL approved |
| Total Suspended Solids (TSS) | Fish and Aquatic Life | 2002 | 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.
- Total Suspended Solids (TSS)
- Crop Production (Non-Irrigated) (suspected) Non-Point Source (confirmed) Discharges From Municipal Separate Storm Sewer Systems (MS4) (suspected)
Reading the record
Lake Koshkonong: Category 4A reading
4A Category 4A: impaired, TMDL completed
Wisconsin 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
Lake Koshkonong is one of 19,871 lake-type waterbodies reported to EPA from Wisconsin, and one of 1,219 whose reported record carries enough populated assessment or gauge data for this site to list and index it. At 10,596 assessed acres it ranks 298th 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 297 lakes with a larger one.
Water level
A USGS gauge tracks this lake's water surface elevation daily, and the latest reading is 0.22 ft above the reading 30 days earlier, against a 365-day observed range of 5.5 ft. The state has not yet filed a definitive fish-consumption verdict for this record; that field populates when it reports it in a future Integrated Report cycle.
Reported trophic class
Eutrophic is the trophic class the filing agency reported to EPA ATTAINS for this assessment unit. It describes nutrient enrichment, not whether the water is safe, and it is not a PlainLakes inference. What the trophic scale measures.
Water level: 365-day record
Daily water surface elevation values, feet above the North American Vertical Datum of 1988 (NAVD 88), from USGS gauge 05427235 (Lake Koshkonong Near Newville, Wi), shown weekly. Readings are daily-batch snapshots, not real-time conditions.
- Latest reading
- 776.6 ft 0.22000000000002728ft vs 30 days earlier
- 2026-08-16 · Provisional, subject to USGS revision
Water level: 365-day record
Subscribe to water level updates for Lake Koshkonong (RSS)
Every daily value in this window is provisional and subject to USGS revision: USGS has not yet approved any value in the trailing 365 days of this record.
Full gauge record: USGS monitoring location 05427235
In context
Where Lake Koshkonong 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
10,596 Top 1% larger than 99% 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
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 Lake Koshkonong. Hover or tap any bar for its full count and share.
In context
Wisconsin context
ATTAINS carries 19,871 lake-type waterbodies in Wisconsin, 493 assessed as not supporting at least one use and 4,225 fully supporting. Among the 1,219 indexed units there, Lake Koshkonong ranks 15th by listed-cause count (2 listed causes) and 8th by assessed size among the 1,170 with an area filed; it sits in the not-supporting set (493 statewide) . Its listed causes include Phosphorus, Total, which also appears among Wisconsin's most-filed impairment causes (led by Phosphorus, Total). It also lists Total Suspended Solids (TSS), outside that top-five list.
| Other large assessed lakes in Wisconsin | Assessed acres |
|---|---|
| Lake Winnebago Not Supporting | 131,942 |
| Lake Pepin Not Supporting | 25,503 |
| Petenwell Lake Not Supporting | 23,001 |
| Lake Poygan Not Supporting | 14,024 |
| Lake Chippewa Fully Supporting | 13,616 |
| Turtle Flambeau Flowage Not Supporting | 12,943 |
Peers near Lake Koshkonong's size or cause load
Similar assessed acres
Nearest filed extents (10,596 acres here).
- Tims Ford Reservoir · 10,596 acres · TN
- Coosa River (Lay Lake) · 10,559 acres · AL
- St Johns River Above Tocoi · 10,641 acres · FL
- Toledo Bend Reservoir · 10,530 acres · TX
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
- Assessment verdicts are cycle-based, not real-time. For fish-consumption decisions, your state advisory program is the binding source. Consumption status guide
- Water levels shown are daily snapshots. Seasonal drawdown and datum conventions matter when comparing levels. How to read level data
- 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 WI10001493. 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.