State water-quality picture · #33 by reported lake-type waterbodies
Connecticut lakes and reservoirs
Every figure on this page is computed from the assessment decisions Connecticut reported to the U.S. Environmental Protection Agency under Clean Water Act sections 303(d) and 305(b) (reporting cycles 2006–2024), plus daily water-level observations from the U.S. Geological Survey.
- 182
- Reported waterbodies
- 44
- Not supporting a use
- 12
- Fish use not supporting
- 0
- Gauged lakes
Connecticut in one line
182 lake-type waterbodies in Connecticut are reported to EPA ATTAINS across reporting cycles 2006–2024: 44 assessed as not supporting at least one designated use, 95 fully supporting, and 43 without a definitive overall verdict. The most-listed impairment cause is Algae, on 23 waterbodies.
- 182
- reported waterbodies
- 44
- not supporting a use
- 95
- fully supporting
- 32%
- of definitive verdicts impaired
An unassessed waterbody is neither confirmed clean nor confirmed impaired; states assess on multi-year cycles.
Connecticut's mid-size register with a lighter not-supporting share
Connecticut sits in the middle third by inventory size (182 units) and in the lowest third for ruled not-supporting share (32%).
Data updated 2026-08-17
According to the U.S. Environmental Protection Agency's ATTAINS program, Connecticut reported 182 lake-type waterbodies in Connecticut in the August 2026 extract. See our methodology for filer attribution and multi-year reporting cycles.
Overall-status mix on Connecticut's mid-size inventory
Every reported lake-type unit in Connecticut 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 Connecticut
Waterbodies carrying each listed cause, most first
- Algae
Algae
23 waterbodies
- Nutrients
Nutrients
18 waterbodies
- Chlorophyll-a
Chlorophyll-a
17 waterbodies
- Escherichia coli (E. coli)
Escherichia coli (E. coli)
12 waterbodies
- Polychlorinated Biphenyls (PCBs) 8
Polychlorinated Biphenyls (PCBs)
8 waterbodies
What this shows Algae is the most-listed impairment cause in Connecticut, on 23 of the lake-type waterbodies reported there.
Computed share from EPA ATTAINS filed records, not an official EPA rating or safety grade.
Connecticut: 32% of ruled verdicts are not-supporting, against a national share of 53%, 1.7× below the national rate.
Reporting practice
Connecticut'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.
Connecticut holds the 33rd-largest reported lake-type inventory in the country, 182 assessment units, carrying a definitive verdict on 76.4% of it. The counts above describe the 139 units carrying a verdict, not all 182 reported. Connecticut 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 Montana (76.7%) and Florida (68%).
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.
Connecticut against every comparable state
Share of reported lake-type waterbodies carrying a definitive verdict
76.4% 22nd of 47 higher than 25 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 Connecticut
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 | Candlewood, Lake (New Fairfield/Danbury/Sherman/New Milford) Fully Supporting | 5,086 |
| 2 | Lillinonah, Lake (Newtown/Southbury/Bridgewater/Brookfield) Not Supporting | 1,595 |
| 3 | Bantam Lake (Litchfield/Morris) Not Supporting | 955 |
| 4 | Colebrook River (Reservoir) Lake (Colebrook) Not Assessed | 852 |
| 5 | Pachaug Pond (Griswold) Fully Supporting | 837 |
| 6 | Saugatuck Reservoir (Weston/Easton/Redding) Fully Supporting | 823 |
| 7 | Waramaug, Lake (Kent/Warren/Washington) Not Supporting | 641 |
| 8 | Zoar, Lake (Monroe/Newtown/Oxford/Southbury) Not Supporting | 581 |
| 9 | Washining Lake (Twin Lakes, Eastern) (Salisbury) Fully Supporting | 565 |
| 10 | Gardner Lake (Salem/Montville/Bozrah) Fully Supporting | 527 |
| 11 | Shenipsit Lake (Tolland/Ellington/Vernon) Fully Supporting | 512 |
| 12 | Pocotopaug Lake (East Hampton) Not Supporting | 502 |
By cause count
Most-listed waterbodies in Connecticut
Waterbodies carrying the most listed impairment causes among the lake-type units reported by Connecticut.
| # | Waterbody | Listed causes |
|---|---|---|
| 1 | Browning Pond (Norwich Landfill)-01 Not Supporting | 6 |
| 1 | Cedar Pond (North Branford) Not Supporting | 6 |
| 1 | Hatch Pond (Kent) Not Supporting | 6 |
| 1 | Kenosia, Lake (Danbury) Not Supporting | 6 |
| 1 | Lillinonah, Lake (Newtown/Southbury/Bridgewater/Brookfield) Not Supporting | 6 |
| 1 | Linsley Pond (Branford/North Branford) Not Supporting | 6 |
| 7 | Batterson Park Pond (Farmington/New Britain) Not Supporting | 5 |
| 7 | Housatonic Lake (Shelton/Derby/Seymour/Oxford/Monroe) Not Supporting | 5 |
| 7 | Zoar, Lake (Monroe/Newtown/Oxford/Southbury) Not Supporting | 5 |
| 10 | Bantam Lake (Litchfield/Morris) Not Supporting | 4 |
9 of the 10 rows in this table tie on the ranked value: 6 at 6 (rank 1), 3 at 5 (rank 7). 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 near Connecticut's lighter not-supporting load
Two EPA ATTAINS peer sets outside Connecticut. The reporting-practice chart above stays on definitive-verdict share; these neighborhoods highlight register-size and not-supporting peers while Connecticut's ruled impairment share sits in the lowest third.
States with a similar reported inventory
Nearest states by lake-type units on the register (182 here).
- New Mexico · 176 units · 6 off
- South Dakota · 195 units · 13 off
- Arkansas · 144 units · 38 off
- Utah · 143 units · 39 off
Comparable not-supporting counts
Nearest states by units assessed not supporting a use (44 here). Inventory peers above are omitted.
- West Virginia · 44 not supporting · 0 off
- Georgia · 47 not supporting · 3 off
- Delaware · 40 not supporting · 4 off
- Idaho · 51 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: 76.4% of the inventory reported from Connecticut carries a definitive verdict, 22nd 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 Connecticut 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 Connecticut 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 Connecticut, 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.