Colorado reservoir · state assessment record
Horseshoe Lake (Lake Meriam)
Assessment decisions reported by Colorado to EPA under Clean Water Act sections 303(d) and 305(b), last assessed in the 2018 reporting cycle.
- 157
- Assessed acres
- Lake / Reservoir / Pond
- Water type
This waterbody in one line
Horseshoe Lake (Lake Meriam), a reservoir in Colorado, is assessed as not supporting one or more designated uses, with 3 listed impairment causes in the state's most recent Integrated Report cycle (2018). By assessed acreage it sits in the upper quartile 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 not supporting.
- Not Supporting
- overall status
- P78
- Assessed size nationally
- P5
- 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.
How Horseshoe Lake (Lake Meriam) 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 Colorado, cause-list rank is the local separator: Horseshoe Lake (Lake Meriam) is 7 of 122 indexed units by listed-cause count, toward the top of that state's filed lists.
Not one verdict, several
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.
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.
Horseshoe Lake (Lake Meriam): Category 5, TMDL still owed
Category 5 is the Integrated Report band where a TMDL (or equivalent) is still expected for Horseshoe Lake (Lake Meriam). 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 Horseshoe Lake (Lake Meriam) 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, Colorado carries 344 filed lake-type waterbodies in the August 2026 extract, and this page renders the assessment record EPA assigned to Horseshoe Lake (Lake Meriam) 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. Colorado filed 2015 as the year this waterbody was last monitored.
| Designated use | Group | Verdict |
|---|---|---|
| Agriculture | Other | Fully Supporting |
| Aquatic Life Cold Water-Class 1 | Aquatic-life or related | Not Supporting |
| Recreation | Recreation | Fully Supporting |
| Water Supply | Drinking-water supply designation | Not Supporting |
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 |
|---|---|---|---|
| Arsenic | Water Supply | 2012 | TMDL priority: High |
| Mercury in Fish Tissue | Aquatic Life Cold Water-Class 1 | 2012 | TMDL priority: High |
| Temperature | Aquatic Life Cold Water-Class 1 | 2012 | TMDL priority: High |
Reading the record
Cleanup queue for Horseshoe Lake (Lake Meriam)
3 listed causes ranked High in the TMDL queue, still without a plan
Colorado ranked 3 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
Horseshoe Lake (Lake Meriam) is one of 344 lake-type waterbodies reported to EPA from Colorado, and one of 122 whose reported record carries enough populated assessment or gauge data for this site to list and index it. At 157 assessed acres it ranks 8,635th 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 8,634 lakes with a larger one.
Awaiting a filing
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.
In context
Where Horseshoe Lake (Lake Meriam) 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
157 Top 22% larger than 78% 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 Horseshoe Lake (Lake Meriam). Hover or tap any bar for its full count and share.
In context
Colorado context
ATTAINS carries 344 lake-type waterbodies in Colorado, 89 assessed as not supporting at least one use and 74 fully supporting. Among the 122 indexed units there, Horseshoe Lake (Lake Meriam) ranks 7th by listed-cause count (3 listed causes) and 75th by assessed size among the 122 with an area filed; it sits in the not-supporting set (89 statewide) . Its listed causes include Arsenic and Mercury in Fish Tissue, which also appear among Colorado's most-filed impairment causes (led by Arsenic). It also lists Temperature, outside that top-five list.
| Other large assessed lakes in Colorado | Assessed acres |
|---|---|
| John Martin Reservoir. Not Supporting | 17,147 |
| Blue Mesa Reservoir Fully Supporting | 8,957 |
| Lake Granby. Not Supporting | 7,036 |
| Two Buttes Reservoir, Two Buttes Pond, Hasty Lake, Holbrook Reservoir, Burchfield Lake, Nee-Skah (Queens) Reservoir, Neeso Pah Reservoir, Nee Noshe Reservoir. Fully Supporting | 6,123 |
| Lake and reserviors tributary to the South Platte River Not Assessed | 5,770 |
| Lake Meredith. Not Supporting | 5,531 |
Peers near Horseshoe Lake (Lake Meriam)'s size or cause load
Similar assessed acres
Nearest filed extents (157 acres here).
- Marble · 157 acres · MN
- Twin Bear Lake · 157 acres · WI
- Sparkling Lake · 157 acres · WI
- Bloomington Creek - Source to Mouth · 157 acres · ID
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
- 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 COARMA26_B. 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.