Three Mile Lake is a 880-acre productive, nutrient-rich eutrophic lake in Union County, Iowa. Water quality is graded D based on 2 sampling years through 2024. Water clarity is declining over recent sampling years. Limited fish-species data available. Public access is available with 3 public boat landings. 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: LakeGrade methodology.
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Swimming Safety
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, Poor
Very murky, less than 2.7 ft of visibility. Chlorophyll-a: 28.4 µg/L. Trophic State Index: 63.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 2.7 ft | F |
| Phosphorus | No data | |
| Chlorophyll-a (Algae) | 28.4 µg/L | D |
| Trophic State Index (TSI) | 63 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 880 acres |
| Public Access | Yes |
Where to stay 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.
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Fishing Three Mile Lake
Water Quality Trend: → Stable
Based on 5 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↓ Declining | -0.032 m/yr | 5 |
| Chlorophyll-a | ↑ Improving | -3.06 µg/L/yr | 5 |
Recreation & Access
Water Temperature
Three Mile Lake averages about 77°F at the surface in summer, 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 →
Location & Depth Map
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.
County Ranking
Ranked #1 of 6 lakes in Union County
Nearby Lakes in Union County
Explore Three Mile Lake across LakeGrade
More Iowa Lake Rankings
Other Grade D Lakes in Iowa
Eutrophic Lakes in Iowa
Compare Three Mile Lake With Nearby Lakes
More records, surveys & data sources
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
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth is a modeled estimate.
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.
Data Sources
Water quality data from the EPA Water Quality Portal
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Lake size & watershed from HydroLAKES (HydroSHEDS)
Grading methodology based on Metropolitan Council standards
Public boat-access points from the USGS National Boat Ramps inventory
Most recent sample: 2024-06-26
Monitoring stations: 1