Some of these decisions are not final. 371 of these 371 records carry a verdict Iowa decided in its 2024 Integrated Report, which carries an interim EPA action, so they can change before EPA takes final action on the cycle. They are counted in every figure on this page.
State water-quality picture · #23 by reported lake-type waterbodies
Iowa lakes and reservoirs
Every figure on this page is computed from the assessment decisions Iowa reported to the U.S. Environmental Protection Agency under Clean Water Act sections 303(d) and 305(b) (reporting cycles 2024), plus daily water-level observations from the U.S. Geological Survey.
- 371
- Reported waterbodies
- 137
- Not supporting a use
- Not reported
- Fish use assessments
- 0
- Gauged lakes
Iowa in one line
371 lake-type waterbodies in Iowa are reported to EPA ATTAINS across reporting cycles 2024: 137 assessed as not supporting at least one designated use, 66 fully supporting, and 168 without a definitive overall verdict. The most-listed impairment cause is Chlorophyll-a, on 66 waterbodies.
- 371
- reported waterbodies
- 137
- not supporting a use
- 66
- fully supporting
- 67%
- of definitive verdicts impaired
An unassessed waterbody is neither confirmed clean nor confirmed impaired; states assess on multi-year cycles.
Iowa's mid-pack EPA ATTAINS lake-type picture
371 reported units and a mid-third ruled-impairment footing (67% not supporting) - neither inventory extreme nor load extreme on this extract.
Data updated 2026-08-17
According to the U.S. Environmental Protection Agency's ATTAINS program, Iowa reported 371 lake-type waterbodies in Iowa in the August 2026 extract. See our methodology for filer attribution and multi-year reporting cycles.
Overall-status mix on Iowa's mid-size inventory
Every reported lake-type unit in Iowa falls into exactly one overall-status bucket. This bar is the inventory partition; the cause chart and national gauge below answer different questions.
Most-listed impairment causes in Iowa
Waterbodies carrying each listed cause, most first
- Chlorophyll-a
Chlorophyll-a
66 waterbodies
- Turbidity
Turbidity
46 waterbodies
- pH
pH
31 waterbodies
- Secchi Disk Transparency
Secchi Disk Transparency
28 waterbodies
- Escherichia coli (E. coli) 26
Escherichia coli (E. coli)
26 waterbodies
What this shows Chlorophyll-a is the most-listed impairment cause in Iowa, on 66 of the lake-type waterbodies reported there.
Computed share from EPA ATTAINS filed records, not an official EPA rating or safety grade.
Iowa: 67% of ruled verdicts are not-supporting, against a national share of 53%, 1.3× the national rate.
Reporting practice
Iowa's definitive-verdict share, by the numbers
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.
Iowa holds the 23rd-largest reported lake-type inventory in the country, 371 assessment units, carrying a definitive verdict on 54.7% of it. The counts above describe the 203 units carrying a verdict, not all 371 reported. Iowa has also retired 329 lake-type assessment units, superseded filings that are kept on the site but not counted here.
The states reporting at the nearest rates are Michigan (54.3%) and Rhode Island (60.1%).
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.
Iowa against every comparable state
Share of reported lake-type waterbodies carrying a definitive verdict
54.7% 30th of 47 higher than 17 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 Iowa
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 | Red Rock Reservoir Not Supporting | 19,000 |
| 2 | Rathbun Reservoir - Main Basin Fully Supporting | 6,330 |
| 3 | Saylorville Reservoir Not Supporting | 5,950 |
| 4 | Coralville Reservoir Not Supporting | 4,900 |
| 5 | Big Spirit Lake Fully Supporting | 4,169 |
| 6 | Clear Lake Not Supporting | 3,684 |
| 7 | Storm Lake Not Supporting | 3,147 |
| 8 | Tuttle Lake (a.k.a. Okamanpeedan Lake) Not Supporting | 2,360 |
| 9 | West Okoboji Lake - Main Basin South Fully Supporting | 1,885 |
| 10 | Rathbun Reservoir - Chariton Arm Not Supporting | 1,860 |
| 11 | East Okoboji Lake Not Supporting | 1,835 |
| 12 | Rathbun Reservoir - South Fork Arm Not Supporting | 1,660 |
By cause count
Most-listed waterbodies in Iowa
Waterbodies carrying the most listed impairment causes among the lake-type units reported by Iowa.
| # | Waterbody | Listed causes |
|---|---|---|
| 1 | Carter Lake Not Supporting | 4 |
| 1 | Green Valley Lake Not Supporting | 4 |
| 1 | Iowa Lake Not Supporting | 4 |
| 1 | Lake Darling Not Supporting | 4 |
| 1 | Lake Keomah Not Supporting | 4 |
| 1 | Lake Of Three Fires Not Supporting | 4 |
| 1 | Union Grove Lake Not Supporting | 4 |
| 8 | Avenue Of The Saints Lake Not Supporting | 3 |
| 8 | Big Hollow Lake Not Supporting | 3 |
| 8 | Black Hawk Lake Not Supporting | 3 |
Every row in this table ties with at least one other on the ranked value: 7 at 4 (rank 1), 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.
States with similar inventory scale or impairment load
Two EPA ATTAINS peer sets outside Iowa. The reporting-practice chart above stays on definitive-verdict share; these neighborhoods are cross-state nearest neighbors on register size and not-supporting counts.
Similar reported inventory
Nearest states by lake-type units on the register (371 here).
- Colorado · 344 units · 27 off
- North Dakota · 340 units · 31 off
- Maine · 339 units · 32 off
- Kansas · 328 units · 43 off
Comparable not-supporting counts
Nearest states by units assessed not supporting a use (137 here). Inventory peers above are omitted.
- Pennsylvania · 135 not supporting · 2 off
- Alabama · 143 not supporting · 6 off
- Indiana · 145 not supporting · 8 off
- South Dakota · 145 not supporting · 8 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: 54.7% of the inventory reported from Iowa carries a definitive verdict, 30th 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 Iowa 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 Iowa 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 Iowa, 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.