State water-quality picture · #44 by reported lake-type waterbodies
Montana lakes and reservoirs
Every figure on this page is computed from the assessment decisions Montana and the Fort Belknap Indian Community of the Fort Belknap Reservation of Montana reported to the U.S. Environmental Protection Agency under Clean Water Act sections 303(d) and 305(b) (reporting cycles 1990–2024), plus daily water-level observations from the U.S. Geological Survey.
- 73
- Reported waterbodies
- 28
- Not supporting a use
- Not reported
- Fish use assessments
- 3
- Gauged lakes
Montana in one line
73 lake-type waterbodies in Montana are reported to EPA ATTAINS across reporting cycles 1990–2024: 28 assessed as not supporting at least one designated use, 28 fully supporting, and 17 without a definitive overall verdict. The most-listed impairment cause is Flow Regime Modification, on 8 waterbodies.
- 73
- reported waterbodies
- 28
- not supporting a use
- 28
- fully supporting
- 50%
- of definitive verdicts impaired
An unassessed waterbody is neither confirmed clean nor confirmed impaired; states assess on multi-year cycles.
Montana's smaller reported lake-type inventory
At 73 units, Montana is in the lowest third of state lake-type registers. The charts below describe that compact filing set, not a national-scale inventory.
Data updated 2026-08-17
According to the U.S. Environmental Protection Agency's ATTAINS program, Montana and the Fort Belknap Indian Community of the Fort Belknap Reservation of Montana reported 73 lake-type waterbodies in Montana in the August 2026 extract. See our methodology for filer attribution and multi-year reporting cycles.
Overall-status split on Montana's compact register
Every reported lake-type unit in this smaller Montana filing set falls into exactly one overall-status bucket. The bar partitions 73 units; cause and gauge panels below stay separate.
Most-listed impairment causes in Montana
Waterbodies carrying each listed cause, most first
- Flow Regime Modification
Flow Regime Modification
8 waterbodies
- Phosphorus, Total
Phosphorus, Total
7 waterbodies
- Mercury
Mercury
6 waterbodies
- Salinity
Salinity
6 waterbodies
- Selenium
Selenium
6 waterbodies
What this shows Flow Regime Modification is the most-listed impairment cause in Montana, on 8 of the lake-type waterbodies reported there.
Computed share from EPA ATTAINS filed records, not an official EPA rating or safety grade.
Montana: 50% of ruled verdicts are not-supporting, against a national share of 53%, 1.1× below the national rate.
Reporting practice
Where Montana stands on definitive-verdict coverage
Each reporting authority decides how much of the water it files to EPA it actually assesses in a cycle. That decision sets the denominator behind every count on this page.
Montana holds the 44th-largest reported lake-type inventory in the country, 73 assessment units, carrying a definitive verdict on 76.7% of it. The counts above describe the 56 units carrying a verdict, not all 73 reported. Montana has also retired 1 lake-type assessment unit, a superseded filing that is kept on the site but not counted here.
The states reporting at the nearest rates are Nevada (76.8%) and Connecticut (76.4%).
Assessing more of an inventory is a reporting choice, set by cycle length, monitoring budget and how finely a state splits its waters into units. It is not a measure of water quality, and it is why a raw count from one state does not sit beside a raw count from another. How assessment coverage varies.
Not every unit counted here was assessed by Montana. 2 of them were filed by a sovereign tribal nation reporting directly to EPA under Clean Water Act section 518, which lets a tribe set and apply its own water quality standards in the same manner as a state: Fort Belknap Indian Community of the Fort Belknap Reservation of Montana (2). Those decisions are theirs, not Montana's.
Montana against every comparable state
Share of reported lake-type waterbodies carrying a definitive verdict
76.7% 21st of 47 higher than 26 of 47 states
the 47 comparable states, by definitive-verdict share
Each bar is a band; taller bars hold more states. The dashed line + filled bar mark this entry. Hover or tap any bar for its full count and share, and where it sits relative to this entry.
Source EPA ATTAINS · Reporting cycles 1987–2026
Each bar counts states whose definitive-verdict share falls in that 10-point band. The 47 states shown are those reporting at least 25 lake-type units; a share off a smaller inventory is precision the data does not have.
By size
Largest assessed lakes in Montana
By assessed acreage as reported in ATTAINS. Acreage is the assessed unit's size, which can differ from a lake's full surface area.
| # | Waterbody | Assessed acres |
|---|---|---|
| 1 | Fort Peck Reservoir Not Supporting | 233,296 |
| 2 | Flathead Lake Not Supporting | 57,305 |
| 3 | Canyon Ferry Reservoir Not Supporting | 32,810 |
| 4 | Lake Koocanusa Not Supporting | 28,875 |
| 5 | Hungry Horse Reservoir Fully Supporting | 23,596 |
| 6 | Lake Elwell (Tiber Reservoir) Fully Supporting | 15,041 |
| 7 | Hebgen Lake Not Assessed | 12,407 |
| 8 | Medicine Lake Not Supporting | 9,726 |
| 9 | Noxon Reservoir Fully Supporting | 7,848 |
| 10 | Benton Lake Not Supporting | 5,345 |
| 11 | Lake Frances (Francis) Fully Supporting | 5,252 |
| 12 | Fresno Reservoir Not Supporting | 5,007 |
By cause count
Most-listed waterbodies in Montana
Waterbodies carrying the most listed impairment causes among the lake-type units reported by Montana and by the Fort Belknap Indian Community of the Fort Belknap Reservation of Montana.
| # | Waterbody | Listed causes |
|---|---|---|
| 1 | Hauser Lake Not Supporting | 8 |
| 2 | Benton Lake Not Supporting | 5 |
| 2 | Canyon Ferry Reservoir Not Supporting | 5 |
| 2 | Freezeout Lake Not Supporting | 5 |
| 2 | Nevada Lake Not Supporting | 5 |
| 6 | Ennis Lake Not Supporting | 4 |
| 6 | Flathead Lake Not Supporting | 4 |
| 6 | Lake Helena Not Supporting | 4 |
| 6 | Priest Butte Lake Not Supporting | 4 |
| 6 | Swan Lake Not Supporting | 4 |
9 of the 10 rows in this table tie on the ranked value: 4 at 5 (rank 2), 5 at 4 (rank 6). Tied rows share a rank and are listed in no meaningful order; where a tie reaches the top the medal is shared, because nothing separates those rows.
Gauge network
Lakes with a daily level record in Montana
Lakes with a linked USGS gauge and a daily water-level record. Readings are daily-batch snapshots, not real-time.
States near Montana's lighter not-supporting load
Two EPA ATTAINS peer sets outside Montana. The reporting-practice chart above stays on definitive-verdict share; these neighborhoods highlight register-size and not-supporting peers while Montana's ruled impairment share sits in the lowest third.
Similar reported inventory
Nearest states by lake-type units on the register (73 here).
States with a similar not-supporting load
Nearest states by units assessed not supporting a use (28 here). Inventory peers above are omitted.
- Maine · 31 not supporting · 3 off
- Maryland · 32 not supporting · 4 off
- Delaware · 40 not supporting · 12 off
- Mississippi · 14 not supporting · 14 off
Peers are nearest-neighbor matches on EPA ATTAINS register size and not-supporting counts among states with at least 25 reported units. Impairment peers skip states already listed in the inventory column so the two link sets stay distinct.
Using this state picture
- Comparisons across states hold on shares, not counts: 76.7% of the inventory reported from Montana carries a definitive verdict, 21st of 47 comparable states. National rankings
- Fish-consumption verdicts here are assessment decisions. Species-level meal advice comes from the state fish-advisory program. Consumption guide
- Browse every named Montana waterbody with a populated record. Browse waterbodies
PlainLakes renders state assessment decisions and federal gauge observations without modification.
Figures on this page are computed from the assessment decisions Montana and the Fort Belknap Indian Community of the Fort Belknap Reservation of Montana reported to EPA's ATTAINS system and from USGS National Water Information System daily values, no number is typed in by an editor. State counts aggregate every lake-type assessment unit reported by Montana and by the Fort Belknap Indian Community of the Fort Belknap Reservation of Montana, including unnamed units that never receive their own page. 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.