Montana lake · state assessment record
Holter Lake
Assessment decisions reported by Montana to EPA under Clean Water Act sections 303(d) and 305(b), last assessed in the 2000 reporting cycle.
- 4,358
- Assessed acres
- Lake, Freshwater
- Water type
- Oligotrophic
- Reported trophic class
This waterbody in one line
Holter Lake, a lake in Montana, is assessed as not supporting one or more designated uses, with one listed impairment cause in the state's most recent Integrated Report cycle (2000). By assessed acreage it sits among the largest 5% by assessed acreage nationally (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
- P98
- 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.
Holter 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: assessed size
The number that most separates Holter Lake nationally is assessed size: 4,358 acres, larger than 98% of the 38,628 lake-type units with an area on file.
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.
Holter Lake: Category 5, TMDL still owed
Category 5 is the Integrated Report band where a TMDL (or equivalent) is still expected for Holter 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 Holter Lake is Not Supporting. That tile summarizes the assessment unit; the designated-use rows and cause list are the evidence behind it.
Typical cause-list length
1 listed cause is a middle-tercile list length among assessed lake-type units. Look to IR category and use outcomes for the distinctive reading.
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 Holter 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 one definitive verdict 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 |
|---|---|---|
| Aquatic Life | Aquatic-life or related | Not Supporting |
| Agricultural | Other | Not Assessed |
| Drinking Water | Drinking-water supply designation | Not Assessed |
| Primary Contact Recreation | Recreation | 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 |
|---|---|---|---|
| Mercury | 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.
- Mercury
- Historic Bottom Deposits (Not Sediment) (suspected) Illegal Dumps or Other Inappropriate Waste Disposal (suspected) Impacts From Abandoned Mine Lands (Inactive) (suspected) Atmospheric Deposition - Toxics (suspected) Placer Mining (suspected)
Montana also recorded the source of at least one listing here as unknown, including for Mercury.
Reading the record
Assessment basis on Holter Lake
1 filed assessment basis on this record
For the one definitive verdict 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
Holter 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 4,358 assessed acres it ranks 708th 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 707 lakes with a larger one.
Still unfiled
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
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
Holter Lake 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 Holter 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
4,358 Top 2% larger than 98% 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 Holter 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, Holter Lake ranks 24th by listed-cause count (one listed cause) and 13th by assessed size among the 52 with an area filed; it sits in the not-supporting set (28 statewide) . Its listed causes include Mercury, which also appears 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 |
| Lake Koocanusa Not Supporting | 28,875 |
| Hungry Horse Reservoir Fully Supporting | 23,596 |
| Lake Elwell (Tiber Reservoir) Fully Supporting | 15,041 |
Peers near Holter Lake's size or cause load
Similar assessed acres
Nearest filed extents (4,358 acres here).
- Pleasant River · 4,354 acres · ME
- O. C. Fisher Lake · 4,353 acres · TX
- Sheepscot River · 4,366 acres · ME
- Star · 4,379 acres · MN
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 MT41I007_020. 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.