Montana lake · state assessment record
Flathead Lake
Assessment decisions reported by Montana to EPA under Clean Water Act sections 303(d) and 305(b), last assessed in the 2016 reporting cycle.
- 57,305
- Assessed acres
- Lake, Freshwater
- Water type
- Oligotrophic
- Reported trophic class
- 2,893.0 ft
- Water Surface Elevation (provisional)
This waterbody in one line
Flathead Lake, a lake in Montana, is assessed as not supporting one or more designated uses, with 4 listed impairment causes in the state's most recent Integrated Report cycle (2016). 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 (4 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
- P100
- Assessed size nationally
- P3
- 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.
Where Flathead Lake stands in 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 Flathead Lake nationally is assessed size: 57,305 acres, larger than 100% of the 38,628 lake-type units with an area on file.
Longer than most cause lists
4 listed causes puts this unit above 95% 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.
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.
Flathead Lake: Category 5, TMDL still owed
Category 5 is the Integrated Report band where a TMDL (or equivalent) is still expected for Flathead Lake. 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 Flathead Lake 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 Flathead 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. For the definitive verdicts on this record, Montana filed the assessment basis as Monitored & Extrapolated Data. Those are EPA's own words, not PlainLakes' summary of them; where a verdict is filed as extrapolated, the state did not reach it from monitoring alone.
| 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 |
|---|---|---|---|
| Nitrogen, Total | Aquatic Life | 1996 | TMDL approved |
| Phosphorus, Total | Aquatic Life | 1996 | TMDL approved |
| Mercury | Aquatic Life | 2000 | TMDL priority: Low |
| Polychlorinated Biphenyls (PCBs) | Aquatic Life | 2000 | TMDL priority: Low |
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.
- Nitrogen, Total
- Atmospheric Deposition - Nitrogen (suspected) Municipal Point Source Discharges (suspected) Unspecified Urban Stormwater (confirmed)
- Phosphorus, Total
- Atmospheric Deposition - Nitrogen (suspected) Municipal Point Source Discharges (suspected) Unspecified Urban Stormwater (confirmed)
- Mercury
- Atmospheric Deposition - Nitrogen (suspected) Dam or Impoundment (suspected) Impacts From Hydrostructure Flow Regulation/Modification (suspected) Municipal Point Source Discharges (suspected) Silviculture Harvesting (suspected) Unspecified Urban Stormwater (confirmed)
- Polychlorinated Biphenyls (PCBs)
- Atmospheric Deposition - Nitrogen (suspected) Dam or Impoundment (suspected) Impacts From Hydrostructure Flow Regulation/Modification (suspected) Municipal Point Source Discharges (suspected) Silviculture Harvesting (suspected) Unspecified Urban Stormwater (confirmed)
Montana also recorded the source of at least one listing here as unknown, including for Mercury and Polychlorinated Biphenyls (PCBs).
Reading the record
Assessment basis on Flathead Lake
1 filed assessment basis on this record
For the definitive verdicts on this record, Montana filed the assessment basis as Monitored & Extrapolated Data. Those are EPA's own words, not PlainLakes' summary of them; where a verdict is filed as extrapolated, the state did not reach it from monitoring alone.
The cycle is when the state reported; the monitoring year, where filed, is when it measured. How to read assessment currency.
Scale and rank
Flathead Lake 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 57,305 assessed acres it ranks 48th 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 47 lakes with a larger one.
Water level
A USGS gauge tracks this lake's water surface elevation daily, and the latest reading is 0.01 ft below the reading 30 days earlier, against a 365-day observed range of 6.8 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
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.
Water level: 365-day record
Daily water surface elevation values, feet above the Somers datum, a zero set for this project rather than sea level, from USGS gauge 12371550 (Flathead Lake at Polson Mt), shown weekly. Readings are daily-batch snapshots, not real-time conditions.
- Latest reading
- 2,893.0 ft 0.009999999999763531ft vs 30 days earlier
- 2026-08-15 · Provisional, subject to USGS revision
Water level: 365-day record
Subscribe to water level updates for Flathead Lake (RSS)
USGS files this series under its water surface elevation parameter and names that zero itself, in the record it publishes with the daily values, so the LMSL code its site inventory files does not describe these readings. The figure tracks this reservoir’s own stage and is comparable with its own history alone, never with another lake’s reading.
363 of the 365 daily values in this window (99%) are provisional and subject to USGS revision; USGS has approved this gauge's record through 2025-08-17.
Of the 365 daily values in this record, 4 were estimated by USGS rather than measured. USGS marks these individually; approval and estimation are separate judgements and a value can carry both.
Full gauge record: USGS monitoring location 12371550
In context
Where Flathead 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
57,305 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
4 Longer than most more listed causes than 95% 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 Flathead Lake. 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, Flathead Lake ranks 6th by listed-cause count (4 listed causes) and 2nd by assessed size among the 52 with an area filed; it sits in the not-supporting set (28 statewide) . Its listed causes include Phosphorus, Total; and Mercury, which also appear among Montana's most-filed impairment causes (led by Flow Regime Modification). It also lists Nitrogen, Total; and Polychlorinated Biphenyls (PCBs), outside that top-five list.
| Other large assessed lakes in Montana | Assessed acres |
|---|---|
| Fort Peck Reservoir Not Supporting | 233,296 |
| Canyon Ferry Reservoir Not Supporting | 32,810 |
| Lake Koocanusa Not Supporting | 28,875 |
| Hungry Horse Reservoir Fully Supporting | 23,596 |
| Lake Elwell (Tiber Reservoir) Fully Supporting | 15,041 |
| Hebgen Lake Not Assessed | 12,407 |
Peers near Flathead Lake's size or cause load
Similar assessed acres
Nearest filed extents (57,305 acres here).
- Champ-Northeast Arm · 57,022 acres · VT
- Kenai Lake · 57,618 acres · AK
- White Lake · 56,487 acres · LA
- Hartwell Lake · 55,950 acres · GA
Similar cause load
Nearest listed-cause counts (4 here). Peers already in the acreage column are omitted.
- Agency Lake · 4 causes · OR
- Alachua Sink · 4 causes · FL
- Aldrich Pond · 4 causes · MA
- American Falls Reservoir (Snake River) · 4 causes · ID
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 MT76O003_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.