Montana lake · state assessment record
Lake Koocanusa
Assessment decisions reported by Montana to EPA under Clean Water Act sections 303(d) and 305(b), last assessed in the 2024 reporting cycle.
- 28,875
- Assessed acres
- Lake, Freshwater
- Water type
- Oligotrophic
- Reported trophic class
This waterbody in one line
Lake Koocanusa, a lake in Montana, is assessed as not supporting one or more designated uses, with 2 listed impairment causes in the state's most recent Integrated Report cycle (2024). 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.
- Not Supporting
- overall status
- P100
- 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.
How Lake Koocanusa 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: assessed size
The number that most separates Lake Koocanusa nationally is assessed size: 28,875 acres, larger than 100% of the 38,628 lake-type units with an area on file.
Mixed use determinations
This record splits: Not Supporting on aquatic life, Fully Supporting on recreation. The divergence is inside the designated-use matrix, not a single all-or-nothing label.
Lake Koocanusa: Category 5, TMDL still owed
Category 5 is the Integrated Report band where a TMDL (or equivalent) is still expected for Lake Koocanusa. Read the listed causes and any cleanup actions filed beside them before treating the overall status tile as a finished restoration story.
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 Koocanusa 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, Montana carries 73 filed lake-type waterbodies in the August 2026 extract, and this page renders the assessment record EPA assigned to Lake Koocanusa 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, Montana filed the assessment basis as Monitored Data. Those are EPA's own words, not PlainLakes' summary of them.
| Designated use | Group | Verdict |
|---|---|---|
| Agricultural | Other | Fully Supporting |
| Aquatic Life | Aquatic-life or related | Not Supporting |
| Drinking Water | Drinking-water supply designation | Fully Supporting |
| Primary Contact Recreation | Recreation | Fully Supporting |
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 |
|---|---|---|---|
| Selenium | Aquatic Life | 2012 | TMDL priority: Low |
| Flow Regime Modification | Aquatic Life | Not reported | None reported |
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.
- Selenium
- Sources Outside State Jurisdiction or Borders (confirmed)
- Flow Regime Modification
- Dam or Impoundment (suspected)
Reading the record
Where Lake Koocanusa sits among peers
3 of 19 Montana lakes assessed this deeply also came out mixed
Montana ruled definitively on 4 designated uses at Lake Koocanusa and found 1 of 4 not supported. The dataset shows 19 Montana lakes judged to that same depth.
Montana's assessed lakes are most often judged on 3 uses; this one was judged on 4.
| 1 | 2 | 3 | 4 | 5+ | |
|---|---|---|---|---|---|
| All supported | 2 | 4 | 12 | 13 | 0 |
| Mixed | Not possible | 2 | 5 | 3 (this record) | 0 |
| None supported | 5 | 2 | 5 | 3 | 0 |
Ringed cell: Lake Koocanusa. Source: EPA ATTAINS, Reporting cycles 1987–2026.
Scale and rank
Lake Koocanusa is one of 73 lake-type waterbodies reported to EPA from Montana, and one of 52 whose reported record carries enough populated assessment or gauge data for this site to list and index it. At 28,875 assessed acres it ranks 108th 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 107 lakes with a larger one.
Not yet on record
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
Oligotrophic 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.
Why no water-level record
Lake Koocanusa has no daily USGS water-level or reservoir-storage series on this page because NWIS lists a lake monitoring site named for the waterbody, but the site is not active, and PlainLakes ingests only the active inventory, so there is no series to attach. How PlainLakes links gauges.
In context
Where Lake Koocanusa 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
28,875 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 Koocanusa. Hover or tap any bar for its full count and share.
In context
Montana context
ATTAINS carries 73 lake-type waterbodies in Montana, 28 assessed as not supporting at least one use and 28 fully supporting. Among the 52 indexed units there, Lake Koocanusa ranks 16th by listed-cause count (2 listed causes) and 4th by assessed size among the 52 with an area filed; it sits in the not-supporting set (28 statewide) . Its listed causes include Selenium and Flow Regime Modification, which also appear among Montana's most-filed impairment causes (led by Flow Regime Modification).
| Other large assessed lakes in Montana | Assessed acres |
|---|---|
| Fort Peck Reservoir Not Supporting | 233,296 |
| Flathead Lake Not Supporting | 57,305 |
| Canyon Ferry Reservoir Not Supporting | 32,810 |
| Hungry Horse Reservoir Fully Supporting | 23,596 |
| Lake Elwell (Tiber Reservoir) Fully Supporting | 15,041 |
| Hebgen Lake Not Assessed | 12,407 |
Peers near Lake Koocanusa's size or cause load
Similar assessed acres
Nearest filed extents (28,875 acres here).
- Klamath River HU, Lost River HA, Clear Lake, Boles HSAs · 29,220 acres · CA
- Lake George · 28,523 acres · NY
- Oologah Lake · 29,460 acres · OK
- Millwood Lake · 29,500 acres · AR
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
- 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 MT76D003_010. 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.