South Dakota reservoir · state assessment record
Lake Isabel
Dewey County. Assessment decisions reported by South Dakota to EPA under Clean Water Act sections 303(d) and 305(b), last assessed in the 2026 reporting cycle.
- 108
- Assessed acres
- Lake / Reservoir / Pond
- Water type
- Mesotrophic
- Reported trophic class
This waterbody in one line
Lake Isabel, a reservoir in South Dakota, is assessed as not supporting one or more designated uses, with one listed impairment cause in the state's most recent Integrated Report cycle (2026). By assessed acreage it sits above the national median by assessed acreage (38,628 lakes); its impairment list is among the longest nationally (one listed cause vs 23,610 assessed lakes). Aquatic life is assessed as not supporting.
- Not Supporting
- overall status
- P71
- Assessed size nationally
- P20
- 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.
Lake Isabel 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 Lake Isabel nationally is assessed size: 107.9 acres, larger than 71% 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 recreation. The divergence is inside the designated-use matrix, not a single all-or-nothing label.
Lake Isabel: Category 5, TMDL still owed
Category 5 is the Integrated Report band where a TMDL (or equivalent) is still expected for Lake Isabel. Read the listed causes and any cleanup actions filed beside them before treating the overall status tile as a finished restoration story.
Overall status: not supporting
The filer's overall status for Lake Isabel 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, South Dakota carries 195 filed lake-type waterbodies in the August 2026 extract, and this page renders the assessment record EPA assigned to Lake Isabel 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. South Dakota filed 2025 as the year this waterbody was last monitored. South Dakota filed its own confidence on the 5 definitive verdicts it rated: Good and Excellent. That rating is the filer's, in EPA's own field, and EPA publishes no definition of the four words it offers.
| Designated use | Group | Verdict |
|---|---|---|
| Domestic Water Supply Waters | Drinking-water supply designation | Fully Supporting Filer confidence: Good |
| Fish and Wildlife Propagation, Recreation, and Stock Watering Waters | Aquatic-life or related | Not Supporting Filer confidence: Good |
| Immersion Recreation Waters | Recreation | Fully Supporting Filer confidence: Excellent |
| Limited Contact Recreation Waters | Recreation | Fully Supporting Filer confidence: Excellent |
| Warmwater Permanent Fish Life Propagation Waters | Aquatic-life or related | Not Supporting Filer confidence: Good |
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 |
|---|---|---|---|
| Mercury in Fish Tissue | Warmwater Permanent Fish Life Propagation Waters; Fish and Wildlife Propagation, Recreation, and Stock Watering Waters | 2016 | TMDL priority: High |
Reading the record
Cleanup queue for Lake Isabel
1 listed cause ranked High in the TMDL queue, still without a plan
South Dakota ranked Mercury in Fish Tissue high in its TMDL development queue, and no cleanup plan is on file for that listing 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
Lake Isabel is one of 195 lake-type waterbodies reported to EPA from South Dakota, and one of 176 whose reported record carries enough populated assessment or gauge data for this site to list and index it. At 108 assessed acres it ranks 11,134th 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 11,133 lakes with a larger one.
Not yet on record
The state has not yet filed a definitive fish-consumption verdict for this record; that field populates when it reports it in a future Integrated Report cycle.
Reported trophic class
Mesotrophic is the trophic class the filing agency reported to EPA ATTAINS for this assessment unit. It describes nutrient enrichment, not whether the water is safe, and it is not a PlainLakes inference. What the trophic scale measures.
Why no water-level record
Lake Isabel 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 Lake Isabel 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
108 Top 29% larger than 71% 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 Lake Isabel. Hover or tap any bar for its full count and share.
In context
South Dakota context
ATTAINS carries 195 lake-type waterbodies in South Dakota, 145 assessed as not supporting at least one use and 41 fully supporting. Among the 176 indexed units there, Lake Isabel ranks 116th by listed-cause count (one listed cause) and 114th by assessed size among the 176 with an area filed; it sits in the not-supporting set (145 statewide) . Its listed causes include Mercury in Fish Tissue, which also appears among South Dakota's most-filed impairment causes (led by Mercury in Fish Tissue).
| Other large assessed lakes in South Dakota | Assessed acres |
|---|---|
| Lake Thompson Not Supporting | 13,888 |
| Lake Poinsett Not Supporting | 9,795 |
| Waubay Lake Not Supporting | 9,531 |
| Bitter Lake Not Supporting | 7,865 |
| Dry Lake Number 2 Not Supporting | 6,442 |
| Orman Dam (Belle Fourche Reservoir) Not Supporting | 6,106 |
Peers near Lake Isabel's size or cause load
Similar assessed acres
Nearest filed extents (108 acres here).
- Leland Lake · 108 acres · WA
- Chambers Lake · 108 acres · WA
- Campbell Slough · 108 acres · MN
- Hystad · 108 acres · MN
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
- 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 SD-GR-L-ISABEL_01. 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.