Montana lake · state assessment record
Lake Elwell (Tiber Reservoir)
Assessment decisions reported by Montana to EPA under Clean Water Act sections 303(d) and 305(b), last assessed in the 2006 reporting cycle.
- 15,041
- Assessed acres
- Lake, Freshwater
- Water type
- Oligotrophic
- Reported trophic class
This waterbody in one line
Lake Elwell (Tiber Reservoir), a lake in Montana, is assessed as fully supporting its designated uses, with no impairment causes listed in the state's most recent Integrated Report cycle (2006). By assessed acreage it sits among the largest 1% of assessed lakes nationally (38,628 lakes with a filed area); its impairment list is shorter than on half of assessed lakes (0 listed causes). Aquatic life is assessed as fully supporting.
- Fully Supporting
- overall status
- P99
- Assessed size nationally
- P51
- Shorter cause list
- Fully 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 Elwell (Tiber Reservoir) 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: cause-list length
The standout filed fact is a short cause list: 0 causes, shorter than 51% of assessed lake-type units. A short list can mean a focused impairment, or a shallow assessment.
Large assessed unit
At 15,041 acres, Lake Elwell (Tiber Reservoir) clears the upper tercile of assessed lake-type units with an area on file (209 still larger). Size comparisons use the portal's deduplicated lake ranking population, not raw ATTAINS row counts.
Every ruled-on use is fully supporting
The filed use determinations for recreation, aquatic life are all Fully Supporting. Where an overall IR category still flags the unit, look at the cause list and report standing rather than treating the use row as empty.
Large within Montana
Among Montana's 52 indexed units with an area, Lake Elwell (Tiber Reservoir) ranks 6 by assessed size, toward the large end of that state file.
IR category in the supporting band
EPA's Integrated Report category for Lake Elwell (Tiber Reservoir) is 1. In ATTAINS that band means the assessed uses are supporting (or supporting with other waters in the unit), so the narrative weight shifts to how deep the assessment went and whether monitoring is current, not to a restoration plan.
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 Elwell (Tiber Reservoir) 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 | Fully Supporting |
| Drinking Water | Drinking-water supply designation | Fully Supporting |
| Primary Contact Recreation | Recreation | Fully Supporting |
The record
Listed impairment causes
The state's reported assessment lists no impairment causes for this lake.
Reading the record
Lake Elwell (Tiber Reservoir): Category 1 reading
1 Category 1: all designated uses supported
Montana filed this assessment unit as Category 1: every designated use assessed here was reported as supported. That is the filer's Integrated Report classification, not a PlainLakes score.
Scale and rank
Lake Elwell (Tiber Reservoir) 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 15,041 assessed acres it ranks 210th 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 209 lakes with a larger one.
What this record holds
Montana completed an assessment here and listed no impairment cause, so the record holds a verdict and its supporting use rows but no pollutant to track over time. 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.
In context
Where Lake Elwell (Tiber Reservoir) 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
15,041 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
0 Among the shortest cause lists fewer listed causes than 51% 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 Elwell (Tiber Reservoir). 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 Elwell (Tiber Reservoir) ranks 30th by listed-cause count (no listed causes) and 6th by assessed size among the 52 with an area filed; it is one of the 28 fully-supporting indexed records in the state .
| 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 |
| Hebgen Lake Not Assessed | 12,407 |
Peers near Lake Elwell (Tiber Reservoir)'s size or cause load
Similar assessed acres
Nearest filed extents (15,041 acres here).
- Sutton Creek · 15,017 acres · OR
- Coosa River (Weiss Lake) · 15,066 acres · AL
- Lake Ft. Gibson · 14,900 acres · OK
- Toledo Bend Reservoir · 14,871 acres · TX
Similar cause load
Nearest listed-cause counts (0 here). Peers already in the acreage column are omitted.
- 31st Street Harbor · 0 causes · IL
- 42nd Avenue East Beach · 0 causes · MN
- 59th Street Harbor · 0 causes · IL
- 657 Dunleavy (Beach) · 0 causes · MI
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 MT41P003_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.