Maryland impoundment · state assessment record
St. Mary's Lake
Assessment decisions reported by Maryland to EPA under Clean Water Act sections 303(d) and 305(b), last assessed in the 2010 reporting cycle.
- 231
- Assessed acres
- Impoundment
- Water type
This waterbody in one line
St. Mary's Lake, an impoundment in Maryland, is assessed as not supporting one or more designated uses, with one listed impairment cause in the state's most recent Integrated Report cycle (2010). By assessed acreage it sits in the upper quartile by assessed acreage nationally (38,628 lakes); its impairment list is among the longest nationally (one listed cause vs 23,610 assessed lakes). Aquatic life is assessed as fully supporting. Maryland assesses fishing as not supported; it reported no verdict on fish consumption itself.
- Not Supporting
- overall status
- P83
- Assessed size nationally
- P20
- Shorter cause list
- Fully 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.
How St. Mary's Lake reads against 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 St. Mary's Lake nationally is assessed size: 231.4 acres, larger than 83% of the 38,628 lake-type units with an area on file.
A split ruling across uses
This record splits: Not Supporting on fish consumption, Fully Supporting on aquatic life. The divergence is inside the designated-use matrix, not a single all-or-nothing label.
St. Mary's Lake: Category 4A reading
Category 4A means a TMDL (or equivalent) is already approved for the impairment on St. Mary's Lake. The remaining work is implementation and tracking, read the cleanup-standing and cause rows for what the state already committed to address.
Overall status: not supporting
The filer's overall status for St. Mary's Lake is Not Supporting. That tile summarizes the assessment unit; the designated-use rows and cause list are the evidence behind it.
Typical cause-list length
1 listed cause is a middle-tercile list length among assessed lake-type units. Look to IR category and use outcomes for the distinctive reading.
According to the U.S. Environmental Protection Agency's ATTAINS program, Maryland carries 63 filed lake-type waterbodies in the August 2026 extract, and this page renders the assessment record EPA assigned to St. Mary's 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 and Wildlife | Aquatic-life or related | Fully Supporting |
| Fishing | Fish-consumption or fishery | Not Supporting |
The record
Listed impairment causes
Causes the state listed as impairing designated uses of this impoundment, with the cycle each was first listed and the Clean Water Act action recorded against it.
| Cause | Affected uses | First listed | Cleanup standing |
|---|---|---|---|
| Mercury in Fish Tissue | Fishing | 2002 | TMDL approved |
Probable sources named by the state
Each source below is listed under the cause the state filed it against, and carries the word the state filed with it — confirmed, or suspected.
- Mercury in Fish Tissue
- Atmospheric Deposition - Toxics (suspected)
Reading the record
St. Mary's Lake: Category 4A reading
4A Category 4A: impaired, TMDL completed
Maryland filed this assessment unit as Category 4A: impaired, with a pollutant-budget TMDL already completed under 303(d). The listing remains an impairment finding; the plan is on file rather than still awaiting write-up.
Scale and rank
St. Mary's Lake is one of 63 lake-type waterbodies reported to EPA from Maryland, and one of 37 whose reported record carries enough populated assessment or gauge data for this site to list and index it. At 231 assessed acres it ranks 6,464th 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 6,463 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.
In context
Where St. Mary's 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
231 Top 17% larger than 83% 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
1 Longer than most more listed causes than 49% 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 St. Mary's Lake. Hover or tap any bar for its full count and share.
In context
Maryland context
ATTAINS carries 63 lake-type waterbodies in Maryland, 32 assessed as not supporting at least one use and 18 fully supporting. Among the 37 indexed units there, St. Mary's Lake ranks 21st by listed-cause count (one listed cause) and 12th by assessed size among the 37 with an area filed; it sits in the not-supporting set (32 statewide) . Its listed causes include Mercury in Fish Tissue, which also appears among Maryland's most-filed impairment causes (led by Phosphorus, Total).
| Other large assessed lakes in Maryland | Assessed acres |
|---|---|
| Deep Creek Lake Not Supporting | 3,887 |
| Conowingo Reservoir Not Supporting | 3,070 |
| Liberty Reservoir Not Supporting | 2,933 |
| Loch Raven Reservoir Not Supporting | 2,236 |
| Prettyboy Reservoir Not Supporting | 1,472 |
| Jennings Randolph Reservoir Fully Supporting | 941 |
Peers near St. Mary's Lake's size or cause load
Similar assessed acres
Nearest filed extents (231 acres here).
- English Lake · 232 acres · WI
- Mills · 231 acres · MN
- Ore Lake · 232 acres · MI
- Tooke Lake · 231 acres · FL
Similar cause load
Nearest listed-cause counts (1 here). Peers already in the acreage column are omitted.
- Aaron · 1 causes · MN
- Aaron River Reservoir · 1 causes · MA
- Abbotts Creek (including Lexington-Thomasville Water Supply Reservoir at normal reservoir elevation, Tom-A-Lex Lake) · 1 causes · NC
- Abeniki Lake · 1 causes · NH
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 impoundment. 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 MD-021401030718-ST_MARYS_LAKE. 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.