Maryland impoundment · state assessment record
Triadelphia Reservoir
Assessment decisions reported by Maryland to EPA under Clean Water Act sections 303(d) and 305(b), last assessed in the 2022 reporting cycle.
- 570
- Assessed acres
- Impoundment
- Water type
This waterbody in one line
Triadelphia Reservoir, an impoundment in Maryland, 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. Maryland assesses fishing as supported; it reported no verdict on fish consumption itself.
- Not Supporting
- overall status
- P91
- 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.
Reading Triadelphia Reservoir's filed record
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 Triadelphia Reservoir nationally is assessed size: 569.7 acres, larger than 91% of the 38,628 lake-type units with an area on file.
Mixed use determinations
This record splits: Not Supporting on aquatic life, Fully Supporting on fish consumption. The divergence is inside the designated-use matrix, not a single all-or-nothing label.
Triadelphia Reservoir: Category 4A reading
Category 4A means a TMDL (or equivalent) is already approved for the impairment on Triadelphia Reservoir. The remaining work is implementation and tracking, read the cleanup-standing and cause rows for what the state already committed to address.
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 Triadelphia Reservoir 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, Maryland carries 63 filed lake-type waterbodies in the August 2026 extract, and this page renders the assessment record EPA assigned to Triadelphia Reservoir 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 | Not Supporting |
| Fishing | Fish-consumption or fishery | Fully 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 |
|---|---|---|---|
| Phosphorus, Total | Aquatic Life and Wildlife | 1998 | TMDL approved |
| Sedimentation/Siltation | Aquatic Life and Wildlife | 1998 | 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.
- Phosphorus, Total
- Crop Production (Crop Land or Dry Land) (suspected)
- Sedimentation/Siltation
- Crop Production (Crop Land or Dry Land) (suspected)
Reading the record
Triadelphia Reservoir: 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
Triadelphia Reservoir 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 570 assessed acres it ranks 3,412th 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 3,411 lakes with a larger one.
Still unfiled
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 Triadelphia Reservoir 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
570 Top 9% larger than 91% 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 Triadelphia Reservoir. 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, Triadelphia Reservoir ranks 6th by listed-cause count (2 listed causes) and 8th by assessed size among the 37 with an area filed; it sits in the not-supporting set (32 statewide) . Its listed causes include Phosphorus, Total; and Sedimentation/Siltation, which also appear 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 Triadelphia Reservoir's size or cause load
Similar assessed acres
Nearest filed extents (570 acres here).
- Twin Falls Flowage · 570 acres · WI
- Lake Sheen · 570 acres · FL
- Mud Hen Lake · 569 acres · WI
- Coyote Reservoir · 570 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
- Assessment verdicts are cycle-based, not real-time. For fish-consumption decisions, your state advisory program is the binding source. 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-021311080966-Triadelphia_Reservoir. 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.