State water-quality picture · #13 by reported lake-type waterbodies
Idaho lakes and reservoirs
Every figure on this page is computed from the assessment decisions Idaho reported to the U.S. Environmental Protection Agency under Clean Water Act sections 303(d) and 305(b) (reporting cycles 2002–2024), plus daily water-level observations from the U.S. Geological Survey.
- 630
- Reported waterbodies
- 51
- Not supporting a use
- Not reported
- Fish use assessments
- 4
- Gauged lakes
Idaho in one line
630 lake-type waterbodies in Idaho are reported to EPA ATTAINS across reporting cycles 2002–2024: 51 assessed as not supporting at least one designated use, 198 fully supporting, and 381 without a definitive overall verdict. The most-listed impairment cause is Phosphorus, Total, on 23 waterbodies.
- 630
- reported waterbodies
- 51
- not supporting a use
- 198
- fully supporting
- 22%
- of definitive verdicts impaired
An unassessed waterbody is neither confirmed clean nor confirmed impaired; states assess on multi-year cycles. 12 of those 198 fully supporting verdicts (6%) rest on designated uses this site does not report (irrigation, navigation and industrial water supply), with Idaho recording no verdict on fish consumption, recreation, aquatic life and drinking water. They are counted above because the state filed them, and excluded from the impaired share below because that figure compares states.
A large Idaho inventory with a lighter ruled-impairment share
630 reported lake-type units put Idaho in the largest third of state registers, while its ruled not-supporting share (22%) sits in the lowest third. Scale and load pull apart on this page.
Data updated 2026-08-17
According to the U.S. Environmental Protection Agency's ATTAINS program, Idaho reported 630 lake-type waterbodies in Idaho in the August 2026 extract. See our methodology for filer attribution and multi-year reporting cycles.
How Idaho's large inventory splits across overall status
Each of the 630 lake-type units reported by Idaho lands in exactly one overall-status bucket. On a register this large, the bar is an inventory partition, the cause chart and national gauge below answer different questions.
Most-listed impairment causes in Idaho
Waterbodies carrying each listed cause, most first
- Phosphorus, Total
Phosphorus, Total
23 waterbodies
- Mercury
Mercury
21 waterbodies
- Flow Regime Modification
Flow Regime Modification
10 waterbodies
- Sedimentation/Siltation 9
Sedimentation/Siltation
9 waterbodies
- Dissolved Oxygen 7
Dissolved Oxygen
7 waterbodies
What this shows Phosphorus, Total is the most-listed impairment cause in Idaho, on 23 of the lake-type waterbodies reported there.
Computed share from EPA ATTAINS filed records, not an official EPA rating or safety grade.
Idaho: 22% of ruled verdicts are not-supporting, against a national share of 53%, 2.4× below the national rate.
Reporting practice
Most of Idaho's inventory still lacks a definitive verdict
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.
Idaho holds the 13th-largest reported lake-type inventory in the country, 630 assessment units, carrying a definitive verdict on 37.6% of it. The counts above describe the 237 units carrying a verdict, not all 630 reported. Idaho has also retired 10 lake-type assessment units, superseded filings that are kept on the site but not counted here.
The states reporting at the nearest rates are California (36.9%) and New Mexico (42%).
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.
Idaho against every comparable state
Share of reported lake-type waterbodies carrying a definitive verdict
37.6% 39th of 47 higher than 8 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 Idaho
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 | Pend Oreille Lake Not Supporting | 80,829 |
| 2 | Bear Lake Not Assessed | 34,454 |
| 3 | American Falls Reservoir (Snake River) Not Supporting | 31,724 |
| 4 | Cascade Reservoir Not Supporting | 25,040 |
| 5 | American Falls Reservoir (Snake River) Tribal Waters Not Assessed | 23,785 |
| 6 | Grays Lake Not Assessed | 23,678 |
| 7 | Priest Lake Not Assessed | 23,342 |
| 8 | Coeur d’Alene Lake Not Supporting | 22,977 |
| 9 | Blackfoot Reservoir Not Assessed | 17,457 |
| 10 | Palisades Reservoir Not Assessed | 15,433 |
| 11 | Dworshak Reservoir Not Assessed | 13,973 |
| 12 | Brownlee Reservoir, Lower (Porters Flat to Brownlee Dam) Not Supporting | 13,194 |
By cause count
Most-listed waterbodies in Idaho
Waterbodies carrying the most listed impairment causes among the lake-type units reported by Idaho.
| # | Waterbody | Listed causes |
|---|---|---|
| 1 | Brownlee Reservoir, Lower (Porters Flat to Brownlee Dam) Not Supporting | 9 |
| 1 | Oxbow Reservoir Not Supporting | 9 |
| 3 | Brownlee Reservoir, Upper (Weiser to Porters Flat) Not Supporting | 8 |
| 4 | Pioneer Reservoir Not Supporting | 5 |
| 5 | American Falls Reservoir (Snake River) Not Supporting | 4 |
| 5 | Coeur d’Alene Lake Not Supporting | 4 |
| 5 | Salmon Falls Creek Reservoir Not Supporting | 4 |
| 8 | Cedar Creek Reservoir Not Supporting | 3 |
| 8 | Hawkins Reservoir Not Supporting | 3 |
| 8 | Hells Canyon Reservoir Not Supporting | 3 |
8 of the 10 rows in this table tie on the ranked value: 2 at 9 (rank 1), 3 at 4 (rank 5), 3 at 3 (rank 8). 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 Idaho
Lakes with a linked USGS gauge and a daily water-level record. Readings are daily-batch snapshots, not real-time.
States near Idaho's lighter not-supporting load
Two EPA ATTAINS peer sets outside Idaho. The reporting-practice chart above stays on definitive-verdict share; these neighborhoods highlight register-size and not-supporting peers while Idaho's ruled impairment share sits in the lowest third.
Similar reported inventory
Nearest states by lake-type units on the register (630 here).
- Missouri · 623 units · 7 off
- New Jersey · 611 units · 19 off
- South Carolina · 611 units · 19 off
- California · 607 units · 23 off
States with a similar not-supporting load
Nearest states by units assessed not supporting a use (51 here). Inventory peers above are omitted.
- North Dakota · 52 not supporting · 1 off
- Georgia · 47 not supporting · 4 off
- New Mexico · 46 not supporting · 5 off
- Connecticut · 44 not supporting · 7 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: 37.6% of the inventory reported from Idaho carries a definitive verdict, 39th 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 Idaho 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 Idaho 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 Idaho, 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.