Washington reservoir · state assessment record
Mason Lake
Assessment decisions reported by Washington to EPA under Clean Water Act sections 303(d) and 305(b), last assessed in the 2022 reporting cycle.
- 977
- Assessed acres
- Lake / Reservoir / Pond
- Water type
This waterbody in one line
Mason Lake, a reservoir in Washington, is assessed as not supporting one or more designated uses, with 2 listed impairment causes in the state's most recent Integrated Report cycle (2022). By assessed acreage it sits among the largest 10% by assessed acreage nationally (38,628 lakes); its impairment list is among the longest nationally (2 listed causes vs 23,610 assessed lakes). Aquatic life is assessed as not supporting. Washington assesses harvesting as not supported; it reported no verdict on fish consumption itself.
- Not Supporting
- overall status
- P94
- Assessed size nationally
- P10
- Shorter cause list
- Not Supporting
- aquatic life
Assessment verdicts are the state agency's decisions as reported to EPA ATTAINS; PlainLakes renders them without modification.
A composite of this record's EPA ATTAINS designated-use verdicts, filed impairment count, and assessment currency, weighted and benchmarked against the national corpus of assessed active waterbodies. This is a water-quality standing, not a human-recreation or property-value judgment, and dimensions with no filed verdict drop out rather than count against the score.
Where Mason Lake stands in the filed register
Paragraphs are selected by this unit's percentiles and use matrix: different lakes lead with different dimensions. Figures are ATTAINS / portal register arithmetic, not advice.
The signature stat: assessed size
The number that most separates Mason Lake nationally is assessed size: 977.2 acres, larger than 94% of the 38,628 lake-type units with an area on file.
Every ruled-on use is not supporting
On the uses this filer ruled on, fish consumption, aquatic life, every determination is Not Supporting. That is a use-by-use reading of the filed record, not a single overall lake grade.
Mason Lake: Category 5, TMDL still owed
Category 5 is the Integrated Report band where a TMDL (or equivalent) is still expected for Mason Lake. Read the listed causes and any cleanup actions filed beside them before treating the overall status tile as a finished restoration story.
Longer than most cause lists
2 listed causes puts this unit above 80% of assessed lake-type records by list length. Treat that as a coding-and-sampling signal: states that split pollutants finely mint longer lists without proving worse water.
Overall status: not supporting
The filer's overall status for Mason Lake is Not Supporting. That tile summarizes the assessment unit; the designated-use rows and cause list are the evidence behind it.
According to the U.S. Environmental Protection Agency's ATTAINS program, Washington carries 832 filed lake-type waterbodies in the August 2026 extract, and this page renders the assessment record EPA assigned to Mason Lake without modification. See our methodology for reporting-unit limits and gauge linking rules.
The record
Designated-use assessments
Fully Supporting and Not Supporting are the state's definitive verdicts; other entries mean the use was not conclusively assessed in this cycle.
| Designated use | Group | Verdict |
|---|---|---|
| Aquatic Life - General | Aquatic-life or related | Not Supporting |
| Harvesting | Fish-consumption or fishery | Not Supporting |
| Recreation - Primary Contact | Recreation | Insufficient Information |
The record
Listed impairment causes
Causes the state listed as impairing designated uses of this reservoir, with the cycle each was first listed and the Clean Water Act action recorded against it.
| Cause | Affected uses | First listed | Cleanup standing |
|---|---|---|---|
| Non-Native Aquatic Plants | Aquatic Life - General | 2004 | None reported |
| Methylmercury in Fish Tissue | Harvesting | 2018 | TMDL priority: Low |
Reading the record
Where Mason Lake sits among peers
89 of 89 Washington lakes assessed this deeply also had no use supported (every lake judged to this depth, so the outcome does not set this record apart)
Washington ruled definitively on 2 designated uses at Mason Lake and found none of them supported. The dataset shows 89 Washington lakes judged to that same depth.
Washington's assessed lakes are most often judged on 1 use; this one was judged on 2.
| 1 | 2 | 3 | 4 | 5+ | |
|---|---|---|---|---|---|
| All supported | 0 | 0 | 0 | 0 | 0 |
| Mixed | Not possible | 0 | 0 | 0 | 0 |
| None supported | 272 | 89 (this record) | 22 | 8 | 2 |
Ringed cell: Mason Lake. Source: EPA ATTAINS, Reporting cycles 1987–2026.
Scale and rank
Mason Lake is one of 832 lake-type waterbodies reported to EPA from Washington, and one of 385 whose reported record carries enough populated assessment or gauge data for this site to list and index it. At 977 assessed acres it ranks 2,303rd by assessed size among the 38,628 lakes the dataset carries an area for, counting a state’s repeated filings of one water once, behind 2,302 lakes with a larger one.
Awaiting a filing
The state has not yet filed a definitive recreation verdict for this record; that field populates when it reports it in a future Integrated Report cycle.
Why no water-level record
Mason Lake has no daily USGS water-level or reservoir-storage series on this page because NWIS lists a lake monitoring site named for the waterbody, but the site is not active, and PlainLakes ingests only the active inventory, so there is no series to attach. How PlainLakes links gauges.
In context
Where Mason Lake sits nationally
Assessed size among U.S. lakes
Distribution of assessed acreage across ATTAINS lakes filed with an area, counting a state’s repeated filings of one water once
977 Top 6% larger than 94% of 38,628 lakes
lakes the dataset carries an area for, counting a state’s repeated filings of one water once, bucketed by value
Source EPA ATTAINS · Reporting cycles 1987–2026
Listed causes vs waterbodies carrying a verdict
How many impairment causes U.S. waterbodies with a reported verdict carry
2 Longer than most more listed causes than 80% of 23,610 waterbodies carrying a verdict
named waterbodies carrying a verdict, bucketed by value
Source EPA ATTAINS · Reporting cycles 1987–2026
In both charts each bar is a band and taller bars hold more waterbodies; the dashed line and filled bar mark Mason Lake. Hover or tap any bar for its full count and share.
In context
Washington context
ATTAINS carries 832 lake-type waterbodies in Washington, 393 assessed as not supporting at least one use and 0 fully supporting. Among the 385 indexed units there, Mason Lake ranks 69th by listed-cause count (2 listed causes) and 7th by assessed size among the 385 with an area filed; it sits in the not-supporting set (393 statewide) . Its listed causes include Non-Native Aquatic Plants and Methylmercury in Fish Tissue, which also appear among Washington's most-filed impairment causes (led by Non-Native Aquatic Plants).
| Other large assessed lakes in Washington | Assessed acres |
|---|---|
| Bumping Lake Not Assessed | 1,354 |
| Sullivan Lake Not Supporting | 1,298 |
| Deer Lake Not Supporting | 1,146 |
| Loon Lake Not Supporting | 1,094 |
| American Lake Not Supporting | 1,092 |
| Newman Lake Not Supporting | 1,091 |
Peers near Mason Lake's size or cause load
Similar assessed acres
Nearest filed extents (977 acres here).
- Green Lake · 977 acres · ND
- Catfish Lake · 978 acres · WI
- Elwood Reservoir · 978 acres · NE
- Copco Lake · 977 acres · CA
Similar cause load
Nearest listed-cause counts (2 here). Peers already in the acreage column are omitted.
- 12th St. Beach · 2 causes · IL
- 31st St. Beach · 2 causes · IL
- 49th St. Beach · 2 causes · IL
- 57th St. Beach · 2 causes · IL
What to do with this record
- The listing here is on a fishery use, not on fish consumption, and this state reported no consumption verdict for this reservoir. Your state fish-advisory program is the source for whether the catch is safe to eat. Consumption status guide
- Impairment listings follow the Clean Water Act 303(d) process; a listing is a regulatory finding, not a closure notice. 303(d) explained
PlainLakes renders state assessment decisions and federal gauge observations without modification. It does not issue advisories.
Assessment decisions on this page are rendered directly from state Integrated Report records in EPA's ATTAINS system; gauge figures are daily-value observations from the USGS National Water Information System, no number is typed in by an editor. Overall status, use verdicts, and causes are the state's reported decisions for assessment unit WA17110019001157_001_001. Counts and percentiles are computed across all lake-type records PlainLakes ingests from ATTAINS. 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.