Some of these decisions are not final. 508 of these 1,191 records carry a verdict Massachusetts decided in its 2026 Integrated Report, which is open for public comment, so they can change before the cycle is submitted to EPA. They are counted in every figure on this page.
State water-quality picture · #9 by reported lake-type waterbodies
Massachusetts lakes and reservoirs
Every figure on this page is computed from the assessment decisions Massachusetts reported to the U.S. Environmental Protection Agency under Clean Water Act sections 303(d) and 305(b) (reporting cycles 2016–2026), plus daily water-level observations from the U.S. Geological Survey.
- 1,191
- Reported waterbodies
- 678
- Not supporting a use
- 241
- Fish use not supporting
- 1
- Gauged lake
Massachusetts in one line
1,191 lake-type waterbodies in Massachusetts are reported to EPA ATTAINS across reporting cycles 2016–2026: 678 assessed as not supporting at least one designated use, 40 fully supporting, and 473 without a definitive overall verdict. The most-listed impairment cause is Mercury in Fish Tissue, on 185 waterbodies.
- 1,191
- reported waterbodies
- 678
- not supporting a use
- 40
- fully supporting
- 94%
- of definitive verdicts impaired
An unassessed waterbody is neither confirmed clean nor confirmed impaired; states assess on multi-year cycles.
Massachusetts's large register still carries a heavy not-supporting share
Among the largest third of state lake-type registers (1,191 units), Massachusetts also sits in the upper third for ruled not-supporting share (94%). The size and the load move together here, not every large inventory does.
Data updated 2026-08-17
According to the U.S. Environmental Protection Agency's ATTAINS program, Massachusetts reported 1,191 lake-type waterbodies in Massachusetts in the August 2026 extract. See our methodology for filer attribution and multi-year reporting cycles.
How Massachusetts's large inventory splits across overall status
Each of the 1,191 lake-type units reported by Massachusetts 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 Massachusetts
Waterbodies carrying each listed cause, most first
- Mercury in Fish Tissue
Mercury in Fish Tissue
185 waterbodies
- Non-Native Aquatic Plants
Non-Native Aquatic Plants
184 waterbodies
- Fanwort
Fanwort
146 waterbodies
- Aquatic Plants (Macrophytes)
Aquatic Plants (Macrophytes)
115 waterbodies
- Dissolved Oxygen
Dissolved Oxygen
103 waterbodies
What this shows Mercury in Fish Tissue is the most-listed impairment cause in Massachusetts, on 185 of the lake-type waterbodies reported there.
Computed share from EPA ATTAINS filed records, not an official EPA rating or safety grade.
Massachusetts: 94% of ruled verdicts are not-supporting, against a national share of 53%, 1.8× the national rate.
Reporting practice
Massachusetts'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.
Massachusetts holds the 9th-largest reported lake-type inventory in the country, 1,191 assessment units, carrying a definitive verdict on 60.3% of it. The counts above describe the 718 units carrying a verdict, not all 1,191 reported. Massachusetts has also retired 39 lake-type assessment units, superseded filings that are kept on the site but not counted here.
The states reporting at the nearest rates are Rhode Island (60.1%) and Nebraska (63.5%).
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.
Massachusetts against every comparable state
Share of reported lake-type waterbodies carrying a definitive verdict
60.3% 28th of 47 higher than 19 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 Massachusetts
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 | Quabbin Reservoir Not Supporting | 24,010 |
| 2 | Wachusett Reservoir Not Supporting | 3,962 |
| 3 | Assawompset Pond Not Assessed | 2,034 |
| 4 | North Watuppa Pond Not Supporting | 1,728 |
| 4 | Long Pond Lakeville/Freetown. Not Supporting | 1,728 |
| 6 | South Watuppa Pond Not Supporting | 1,473 |
| 7 | Webster Lake Not Supporting | 1,275 |
| 8 | Sudbury Reservoir Not Supporting | 1,181 |
| 9 | Great Quittacas Pond Not Assessed | 1,125 |
| 10 | Cobble Mountain Reservoir Not Assessed | 1,034 |
| 11 | Otis Reservoir Not Supporting | 989 |
| 12 | Long Pond Brewster/Harwich. Not Supporting | 715 |
2 of the 12 rows in this table tie on the ranked value: 2 at 1,728 (rank 4). Tied rows share a rank and are listed in no meaningful order.
By cause count
Most-listed waterbodies in Massachusetts
Waterbodies carrying the most listed impairment causes among the lake-type units reported by Massachusetts.
| # | Waterbody | Listed causes |
|---|---|---|
| 1 | Grove Pond Not Supporting | 10 |
| 1 | Lake Quinsigamond Not Supporting | 10 |
| 3 | Pequot Pond Not Supporting | 9 |
| 4 | Assabet River Reservoir Not Supporting | 8 |
| 4 | Billington Sea Not Supporting | 8 |
| 4 | Central Pond Not Supporting | 8 |
| 4 | Hager Pond Not Supporting | 8 |
| 4 | Lake Cochituate Assessment unit MA82127 Not Supporting | 8 |
| 4 | Lake Cochituate Assessment unit MA82126 Not Supporting | 8 |
| 4 | Lake Shirley Not Supporting | 8 |
9 of the 10 rows in this table tie on the ranked value: 2 at 10 (rank 1), 7 at 8 (rank 4). 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 Massachusetts
Lakes with a linked USGS gauge and a daily water-level record. Readings are daily-batch snapshots, not real-time.
States near Massachusetts's heavier not-supporting load
Two EPA ATTAINS peer sets outside Massachusetts. The reporting-practice chart above stays on definitive-verdict share; the neighborhoods below emphasize cross-state nearest neighbors when the not-supporting count is in the upper third.
States with a similar reported inventory
Nearest states by lake-type units on the register (1,191 here).
- Oregon · 1,431 units · 240 off
- Florida · 1,475 units · 284 off
- Washington · 832 units · 359 off
- Indiana · 1,588 units · 397 off
Similar not-supporting load
Nearest states by units assessed not supporting a use (678 here). Inventory peers above are omitted.
- New Jersey · 583 not supporting · 95 off
- New York · 563 not supporting · 115 off
- Michigan · 807 not supporting · 129 off
- Wisconsin · 493 not supporting · 185 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: 60.3% of the inventory reported from Massachusetts carries a definitive verdict, 28th 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 Massachusetts 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 Massachusetts 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 Massachusetts, 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.