Oregon reservoir · state assessment record
Mercer Lake
Assessment decisions reported by Oregon to EPA under Clean Water Act sections 303(d) and 305(b), last assessed in the 2010 reporting cycle.
- 350
- Assessed acres
- Lake / Reservoir / Pond
- Water type
This waterbody in one line
Mercer Lake, a reservoir in Oregon, is assessed as not supporting one or more designated uses, with 2 listed impairment causes 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 (2 listed causes vs 23,610 assessed lakes). Oregon assesses fishing as not supported; it reported no verdict on fish consumption itself.
- Not Supporting
- overall status
- P88
- Assessed size nationally
- P10
- Shorter cause list
- Not Supporting
- fish-consumption or fishery
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.
Mercer Lake against the ATTAINS 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 Mercer Lake nationally is assessed size: 350.1 acres, larger than 88% of the 38,628 lake-type units with an area on file.
No ruled-on use clears the bar
On the uses this filer ruled on, fish consumption, recreation, every determination is Not Supporting. That is a use-by-use reading of the filed record, not a single overall lake grade.
Mercer Lake: Category 5, TMDL still owed
Category 5 is the Integrated Report band where a TMDL (or equivalent) is still expected for Mercer 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 Mercer 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, Oregon carries 1,431 filed lake-type waterbodies in the August 2026 extract, and this page renders the assessment record EPA assigned to Mercer 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 |
|---|---|---|
| Aesthetic Quality | Other | Not Supporting |
| Fishing | Fish-consumption or fishery | Not Supporting |
| Livestock Watering | Other | Not Supporting |
| Private Domestic Water Supply | Drinking-water supply designation | Not Supporting |
| Public Domestic Water Supply | Drinking-water supply designation | Not Supporting |
| Water Contact Recreation | Recreation | Not Supporting |
| Boating | Recreation | Not Assessed |
| Fish and Aquatic Life | Aquatic-life or related | Not Assessed |
| Hydropower | Other | Not Assessed |
| Industrial Water Supply | Other | Not Assessed |
| Irrigation | Other | Not Assessed |
| Wildlife and Hunting | Other | Not Assessed |
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 |
|---|---|---|---|
| Chlorophyll-a | Aesthetic Quality | 1998 | TMDL priority: Low |
| Harmful Algal Blooms | Water Contact Recreation, Fishing, Private Domestic Water Supply, Public Domestic Water Supply, Livestock Watering | 2010 | TMDL priority: Low |
Reading the record
Where Mercer Lake sits among peers
46% of Oregon's lakes assessed this deeply also had no use supported
Oregon ruled definitively on 6 designated uses at Mercer Lake and found none of them supported. The dataset shows 68 Oregon lakes judged to that same depth.
Oregon's assessed lakes are most often judged on 5 or more uses, as this one was.
Computed share from EPA ATTAINS filed records, not an official EPA rating or safety grade.
| 1 | 2 | 3 | 4 | 5+ | |
|---|---|---|---|---|---|
| All supported | 4 | 2 | 6 | 2 | 0 |
| Mixed | Not possible | 4 | 3 | 0 | 37 |
| None supported | 43 | 18 | 10 | 19 | 31 (this record) |
Ringed cell: Mercer Lake. Source: EPA ATTAINS, Reporting cycles 1987–2026.
Scale and rank
Mercer Lake is one of 1,431 lake-type waterbodies reported to EPA from Oregon, and one of 167 whose reported record carries enough populated assessment or gauge data for this site to list and index it. At 350 assessed acres it ranks 4,784th 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 4,783 lakes with a larger one.
Open in the register
The state has not yet filed a definitive aquatic-life verdict for this record; that field populates when it reports it in a future Integrated Report cycle.
In context
Where Mercer 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
350 Top 12% larger than 88% 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 Mercer Lake. Hover or tap any bar for its full count and share.
In context
Oregon context
ATTAINS carries 1,431 lake-type waterbodies in Oregon, 165 assessed as not supporting at least one use and 14 fully supporting. Among the 167 indexed units there, Mercer Lake ranks 76th by listed-cause count (2 listed causes) and 94th by assessed size among the 167 with an area filed; it sits in the not-supporting set (165 statewide) . Its listed causes include Chlorophyll-a and Harmful Algal Blooms, which also appear among Oregon's most-filed impairment causes (led by Temperature).
| Other large assessed lakes in Oregon | Assessed acres |
|---|---|
| Goose Lake Not reported | 93,152 |
| Malheur Slough Not Supporting | 73,544 |
| Upper Klamath Lake Not Supporting | 62,130 |
| Columbia River Not Supporting | 55,630 |
| Lake Abert Not Assessed | 44,751 |
| Summer Lake Not reported | 31,606 |
Peers near Mercer Lake's size or cause load
Similar assessed acres
Nearest filed extents (350 acres here).
- Swede's Bay · 350 acres · MN
- Elk · 350 acres · MN
- Cedar Creek Valley Lake · 350 acres · KS
- Cody Lake · 350 acres · NE
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 OR_LK_1710020507_02_100167. 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.