Three Mile Lake
Union County, Iowa880 acresView mapDirections
Nutrients and algae run above the regional average.
- Eutrophic
- #1 of 6 in the county
- 46 ft max depth
- Public access
Three Mile Lake in Union County, Iowa has a water quality grade of D (Nutrient-rich). Water clarity: 2.7 ft visibility. Phosphorus: 43 µg/L. Chlorophyll-a (algae): 28.4 µg/L. Surface area: 880 acres.
- Clarity
- 2.7 ft
- Typical in September
- 70°F
- Max depth
- ~46 ft est.
- Area
- 880 acres
- Type
- Eutrophic
- Trend
- ↑ Improving
- Public access
- Yes
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Stays near Three Mile Lake
Compare lakeside cabins, hotels, and vacation rentals near Three Mile Lake in Union County, Iowa — live availability and prices for your trip. Switch the map to Satellite to see the lake and how close each stay is to the water. The water-quality grade measures nutrients and algae, not lodging — cabins and rentals around Three Mile Lake are listed below.
Top-rated places to stay near Three Mile Lake
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Free interactive depth-contour map of Three Mile Lake, reaching a maximum surveyed depth of 48 ft. Toggle Depth contours to overlay the bathymetric survey lines. Contour data from Iowa DNR.
Three Mile Lake is a 880-acre productive, nutrient-rich eutrophic lake in Union County, Iowa with an estimated average depth of about 46 feet. Water quality is graded D based on 2 sampling years through 2024. Overall water quality is improving over recent sampling years. Limited fish-species data available. Public access is available. Compared to the 5 other monitored lakes in Union County, Three Mile Lake ranks #1 for water quality.
Source: EPA Water Quality Portal sampling records, Iowa DNR, last sampled 2024-06-26. Grade methodology: LakeQuality methodology.
Swimming Safety
Grade onlyBacteria D
Based on clarity and algae grades from past sampling — we do not hold live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
Based on E. coli testing, Three Mile Lake frequently exceeded the EPA safe-swimming threshold (235 MPN/100mL) — on 11.1% of 9 samples across 3 seasons.
This is a historical summary of past testing, not a current condition — bacteria levels change quickly, especially after rain. Always check Iowa’s current beach and health-department advisories before swimming.
Water Quality Grade: D (Nutrient-rich)
Very murky, less than 2.7 ft of visibility. Phosphorus 43 µg/L. Chlorophyll-a 28.4 µg/L. TSI 62. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 2.7 ft | Not graded |
| Phosphorus | 43 µg/L | Fair |
| Chlorophyll-a (Algae) | 28.4 µg/L | Poor |
| Trophic State Index (TSI) | 62 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Water Temperature
Three Mile Lake typically sits around 70°F at the surface in September. Summer average is about 77°F. Usually warmest in Jul (about 81°F) and coldest in Sep (about 70°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 28 field readings (2010–2023) via the EPA Water Quality Portal. See the full Three Mile Lake water temperature breakdown →
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | ~46 ft est. |
| Surface Area | 880 acres |
| Public Access | Yes |
Depth marked “est.” is a modeled maximum from GLOBathy (Khazaei et al. 2022, CC0), not a measured survey. Iowa has not published bathymetry for Three Mile Lake.
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth here is modelled, not surveyed — HydroLAKES v1.0 estimates lake depth from surrounding terrain rather than from soundings. Treat it as an approximation, and prefer a state bathymetric survey where one exists.
Fishing Three Mile Lake
Water Quality Trend: ↑ Improving
Based on 5 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↓ Declining | -0.032 m/yr | 5 |
| Phosphorus | ↑ Improving | -1.35 µg/L/yr | 4 |
| Chlorophyll-a | ↑ Improving | -3.06 µg/L/yr | 5 |
Reservoir Info (USACE NID)
Three Mile Lake is a man-made reservoir impounded by the Three Mile Reservoir Dam (completed 1995), built primarily for flood risk reduction on the THREE MILE CREEK; earth-type dam, 63 ft tall and 1,850 ft long.
- Surface area
- 880 ac
- Normal storage
- 14,091 ac-ft
- Max storage
- 29,950 ac-ft
- Drainage area
- 36.3 sq mi
- Hazard class
- Significant
- Owner
- Iowa Department of Natural Resources
All listed purposes: Flood Risk Reduction;Recreation;Water Supply.
Source: USACE National Inventory of Dams, NID ID IA02944 · Operator website
Regulated Pollution Sources Nearby
3 EPA-permitted facilities operate within about 5 miles of Three Mile Lake under Clean Water Act (NPDES) discharge permits. 2 facilities have a reported Clean Water Act violation in the past three years.
- Three Mile Lake Campground-StpAfton · 0.6 mi away · NPDES IA0074594Recent violation
- Afton City Park SplashpadAfton · 3.1 mi away · NPDES IA0052182Recent violation
- Afton City Of StpAfton · 4 mi away · NPDES IA0041831
Facilities permitted to discharge under the Clean Water Act within ~5 miles; proximity does not by itself mean a facility affects this lake. Source: EPA ECHO (ICIS-NPDES), as of 2026-07.
County Ranking
Ranked #1 of 6 lakes in Union County
Nearby Lakes in Union County
Frequently asked questions about Three Mile Lake
Where is Three Mile Lake?
- Three Mile Lake is a lake in Union County, Iowa. LakeQuality grades it D for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Three Mile Lake?
- Three Mile Lake in Union County, Iowa has a water quality grade of D (Nutrient-rich). Water clarity: 2.7 ft visibility. Phosphorus: 43 µg/L. Chlorophyll-a (algae): 28.4 µg/L. Surface area: 880 acres.
Is Three Mile Lake safe to swim in?
- Based on E. coli testing, Three Mile Lake frequently exceeded the EPA safe-swimming threshold (235 MPN/100mL) — on 11.1% of 9 samples across 3 seasons. That works out to a bacteria swim-safety grade of D — a historical summary of past testing, not a current reading. Three Mile Lake has a water quality grade of D (Nutrient-rich). Bacteria levels can spike quickly after heavy rain, so always check Iowa's current beach and health-department advisories before you swim.
What is the water temperature of Three Mile Lake?
- Three Mile Lake typically sits around 70°F at the surface in September, from 28 EPA Water Quality Portal field readings (2010–2023). That is a typical value for the month, not a live reading.
How deep is Three Mile Lake?
- No published bathymetric survey covers Three Mile Lake, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 46 feet, but that is a modelled estimate derived from surrounding terrain rather than a measurement on the water, so treat it as approximate. The lake covers 880 acres.
Explore Three Mile Lake across LakeQuality
Depth, room, public access, and the state wake-sport rules
Species activity outlook by water temperature and season
Where to stay at Iowa's top lakes, each with its grade
High nutrients, frequent algae, reduced clarity
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Where this data comes from
51 measurements from 1 monitoring station, 2020–2024. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Iowa DNR Surface Water Monitoring Data
21IOWA_WQX51 samples · 2020–2024
Analytical methods
- Unknown
- Phosphorus by Colorimetry
- 445.0 ~ EPA; Chlorophyll and Pheophytin in Algae by Fluorescence
- In-Vitro Determination of Chlorophyll
- ISU modification of APHA 2130-B
Monitoring station identifiers (1)
Grade thresholds from the EPA National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See how grades are calculated and data sources.
Other records on this page
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Lake size & watershed from HydroLAKES (HydroSHEDS)