Maryland impoundment · state assessment record
Conowingo Reservoir
Assessment decisions reported by Maryland to EPA under Clean Water Act sections 303(d) and 305(b), last assessed in the 2024 reporting cycle.
- 3,070
- Assessed acres
- Impoundment
- Water type
This waterbody in one line
Conowingo Reservoir, an impoundment in Maryland, is assessed as not supporting one or more designated uses, with 3 listed impairment causes in the state's most recent Integrated Report cycle (2024). By assessed acreage it sits among the largest 5% by assessed acreage nationally (38,628 lakes); its impairment list is among the longest nationally (3 listed causes 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
- P98
- Assessed size nationally
- P5
- 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 Conowingo Reservoir 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: in-state cause rank
Inside Maryland, cause-list rank is the local separator: Conowingo Reservoir is 1 of 37 indexed units by listed-cause count, toward the top of that state's filed lists.
Large assessed unit
At 3,070 acres, Conowingo Reservoir clears the upper tercile of assessed lake-type units with an area on file (978 still larger). Size comparisons use the portal's deduplicated lake ranking population, not raw ATTAINS row counts.
A split ruling across uses
This record splits: Not Supporting on fish consumption, Fully Supporting on recreation, aquatic life. The divergence is inside the designated-use matrix, not a single all-or-nothing label.
Longer than most cause lists
3 listed causes puts this unit above 90% 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.
Conowingo Reservoir: Category 5, TMDL still owed
Category 5 is the Integrated Report band where a TMDL (or equivalent) is still expected for Conowingo Reservoir. Read the listed causes and any cleanup actions filed beside them before treating the overall status tile as a finished restoration story.
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 Conowingo 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 | Fully Supporting |
| Fishing | Fish-consumption or fishery | Not Supporting |
| Public Water Supply | Drinking-water supply designation | Not Supporting |
| Water Contact Sports | Recreation | 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 |
|---|---|---|---|
| PCBs in Fish Tissue | Fishing | 2008 | TMDL priority: High |
| Chlorophyll-a | Public Water Supply | 2018 | TMDL priority: Low |
| Perfluorooctane Sulfonate (PFOS) in Fish Tissue | Fishing | 2024 | TMDL priority: High |
Probable sources
Maryland listed the source of the impairment as unknown. That is the state's own filed value, not a gap in this record: the cause has been identified, the origin has not.
Reading the record
Cleanup queue for Conowingo Reservoir
2 listed causes ranked High in the TMDL queue, still without a plan
Maryland ranked 2 listed causes high in its TMDL development queue, and no cleanup plan is on file for those listings yet. That queue rank is the state's own scheduling signal, not an EPA national rule.
A High ranking names a position in the state's plan-writing queue; it is not a measure of how impaired the water is. How listing and cleanup work.
Scale and rank
Conowingo 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 3,070 assessed acres it ranks 979th 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 978 lakes with a larger one.
In context
Where Conowingo 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
3,070 Top 2% larger than 98% 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
3 Longer than most more listed causes than 90% 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 Conowingo 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, Conowingo Reservoir ranks 1st by listed-cause count (3 listed causes) and 2nd by assessed size among the 37 with an area filed; it sits in the not-supporting set (32 statewide) . Its listed causes include PCBs in Fish Tissue and Chlorophyll-a, which also appear among Maryland's most-filed impairment causes (led by Phosphorus, Total). It also lists Perfluorooctane Sulfonate (PFOS) in Fish Tissue, outside that top-five list.
| Other large assessed lakes in Maryland | Assessed acres |
|---|---|
| Deep Creek Lake Not Supporting | 3,887 |
| 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 |
| Youghiogheny River Lake Not Supporting | 788 |
Peers near Conowingo Reservoir's size or cause load
Similar assessed acres
Nearest filed extents (3,070 acres here).
- Lake Gaston · 3,072 acres · VA
- Decatur · 3,072 acres · IL
- Lake Ray Hubbard · 3,069 acres · TX
- Dead Lakes · 3,073 acres · FL
Similar cause load
Nearest listed-cause counts (3 here). Peers already in the acreage column are omitted.
- 67th St. Beach · 3 causes · IL
- Abiquiu Reservoir · 3 causes · NM
- Absecon Creek (AC Reserviors) (gage to SB) · 3 causes · NJ
- Absecon Creek (below gage) · 3 causes · NJ
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-02120204-Conowingo_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.