Impaired Lakes in Colorado
35 of 110 measured Colorado lakes (32%) are officially listed as impaired under Clean Water Act §303(d) in the most recent EPA ATTAINS reporting cycle. 19 have an approved TMDL restoration plan.
Top Causes of Impairment in Colorado
Reading the Colorado impairment landscape
Colorado reports 35 impaired lakes — 32% of the state's 110 tracked water bodies. That share is high enough to make impairment a meaningful planning concern: roughly one in four lakes carries at least one §303(d) listing for a pollutant exceeding standards. The list below isn't a niche concern — it's a significant chunk of the state's surface-water inventory.
Metals (other than mercury) leads the impairment causes in Colorado — a contaminant category that is most relevant for fish consumption rather than for swimming or boating. Mercury and PCB impairments typically come from atmospheric deposition or legacy industrial discharge; they bioaccumulate up the food chain, so larger and longer-lived fish carry the highest tissue concentrations. Check the state's fish-consumption advisory before keeping a catch from these waters.
Impaired-waters listings come from the state environmental agency's §303(d) submission to EPA, refreshed on a roughly 2-year cycle. A listed lake stays on the list until either a TMDL is established and the cleanup completes, or subsequent monitoring shows the pollutant concentrations no longer exceed water-quality standards. The list below reflects the most recent available Colorado submission to the EPA Assessment, Total Maximum Daily Load Tracking and Implementation System (ATTAINS) dataset.
All 35 EPA-Listed Impaired Lakes in Colorado
| Lake | County | Causes | TMDL |
|---|---|---|---|
| Pueblo Reservoir | Pueblo | Oxygen depletion (low DO) | — |
| Lake John | Jackson | Metals (other than mercury), pH / acidity | ✓ |
| Elkhead Reservoir | Moffat | Mercury | ✓ |
| Chatfield Reservoir | Jefferson | Metals (other than mercury) | — |
| Willow Creek Reservoir | Grand | Metals (other than mercury) | ✓ |
| Wolford Mountain Reservoir | Grand | Metals (other than mercury) | — |
| Grand Lake | Grand | Metals (other than mercury) | — |
| Pearl Lake | Routt | Metals (other than mercury) | — |
| Lake Nighthorse | La Plata | Metals (other than mercury) | — |
| Trinidad Reservoir | Las Animas | Mercury, Metals (other than mercury), Oxygen depletion (low DO) | ✓ |
| Shadow Mountain Lake | Grand | Metals (other than mercury) | ✓ |
| Deweese Reservoir | Custer | Metals (other than mercury), Nutrients (phosphorus, nitrogen), Oxygen depletion (low DO) | ✓ |
| Jackson Reservoir | Morgan | pH / acidity | ✓ |
| Eggleston Lake | Delta | Metals (other than mercury), pH / acidity | — |
| Vega Reservoir | Mesa | Metals (other than mercury) | — |
| Steamboat Lake | Routt | Metals (other than mercury) | — |
| Stagecoach Lake | Routt | Mercury, Metals (other than mercury) | — |
| Prospect Lake | Weld | pH / acidity | — |
| North Sterling Reservoir | Logan | Metals (other than mercury), Oxygen depletion (low DO) | ✓ |
| Mountain Home Reservoir | Costilla | Metals (other than mercury), Oxygen depletion (low DO) | — |
| Mcphee Reservoir | Montezuma | Mercury | ✓ |
| Barr Lake | Adams | Metals (other than mercury), Oxygen depletion (low DO), pH / acidity | ✓ |
| Carter Reservoir | Larimer | Mercury, Metals (other than mercury) | ✓ |
| Berkeley Lake | Denver | Mercury, Metals (other than mercury), Oxygen depletion (low DO) | ✓ |
| Rifle Gap Reservoir | Garfield | Mercury, Metals (other than mercury) | ✓ |
| Lake Meredith | Crowley | Metals (other than mercury) | ✓ |
| Miramonte Reservoir | San Miguel | Oxygen depletion (low DO) | — |
| Road Canyon Reservoir | Hinsdale | Metals (other than mercury) | — |
| Adobe Creek Reservoir | Bent | Metals (other than mercury) | ✓ |
| Lake Henry | Crowley | Metals (other than mercury) | ✓ |
| Milton Reservoir | Weld | Metals (other than mercury), Oxygen depletion (low DO), pH / acidity | ✓ |
| North Delaney Lake | Jackson | Metals (other than mercury) | ✓ |
| Ned Wilson Lake | Rio Blanco | Oxygen depletion (low DO) | — |
| Rio Blanco Lake | Rio Blanco | Metals (other than mercury), pH / acidity | ✓ |
| Harvey Gap Reservoir | Garfield | Metals (other than mercury), Temperature | — |
Frequently Asked Questions
How many lakes are impaired in Colorado?
35 of 110 measured Colorado lakes (32%) are listed as impaired in the most recent EPA ATTAINS reporting cycle. Of those, 34 are on the federal Clean Water Act §303(d) list and 19 have an approved Total Maximum Daily Load (TMDL) restoration plan.
What does Clean Water Act §303(d) mean?
Section 303(d) of the federal Clean Water Act requires every state to identify waters that don't meet water quality standards even after pollution control measures. Each listed water must then have a Total Maximum Daily Load (TMDL) developed — a plan that caps how much of each pollutant can enter the waterbody. The "303(d) list" is the impaired-waters list. The EPA reviews each state's list every two years.
What are the top causes of impairment in Colorado?
The most common causes flagged across Colorado's impaired lakes are: Metals (other than mercury) (28 lakes); Oxygen depletion (low DO) (10 lakes); pH / acidity (7 lakes); Mercury (7 lakes); Nutrients (phosphorus, nitrogen) (1 lakes); Temperature (1 lakes). Mercury is widespread in fish tissue across the upper Midwest from atmospheric deposition. Nutrient impairment (phosphorus, nitrogen) typically reflects agricultural runoff and urban stormwater.
Can I swim in an impaired lake?
It depends on the cause of impairment. A lake impaired for "mercury in fish tissue" is generally safe to swim in — the warning is about long-term consumption of fish. A lake impaired for "pathogens" (E. coli) or "algal growth" may pose a swimming risk and often has a separate beach advisory. Check the cause categories listed for each lake below, and consult your state health department's beach monitoring program for current advisories.
Where does this impairment data come from?
Every impairment record on this page comes from the EPA's ATTAINS (Assessment and TMDL Tracking and Implementation System) public dataset, the federal repository of state-reported water quality assessments. EPA ATTAINS is published as GeoParquet files on the EPA Office of Water S3 bucket and refreshed several times per year. Causes and use-support codes follow EPA's standard taxonomy.
More on impaired waters
What it means when a lake is listed as impaired, why mercury is the most common reason, and why a listed lake can still be clear water with favourable readings.
Why states issue fish-consumption advisories, which contaminants — mercury, PCBs, PFAS — drive them, and how to eat the fish you catch more safely.
How phosphorus and nutrient runoff drive algae growth and turn lakes green, where the nutrients come from, and what it means for a lake’s readings.
Which lakes are actually improving: multi-year EPA monitoring trends, the named recoveries still in progress, and what it takes to move the readings.
Data source
Impairment data from the EPA ATTAINS (Assessment and TMDL Tracking and Implementation System) public dataset. Assessment unit IDs are matched to LakeQuality lakes by name and proximity; some lakes may not appear here if their ATTAINS name doesn't match our naming convention. For an official query, consult Colorado's state water-quality agency.