Twelve Mile Lake is a 633-acre productive, nutrient-rich eutrophic lake in Union County, Iowa. Water quality is graded F 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 with 3 public boat landings. Compared to the 5 other monitored lakes in Union County, Twelve Mile Lake ranks #3 for water quality.
Source: EPA Water Quality Portal sampling records, Iowa DNR, last sampled 2024-09-30. Grade methodology: LakeGrade methodology.
Watch Twelve Mile Lake water quality
Subscribe for LakeQuality updates by email. No spam, unsubscribe anytime.
Swimming Safety
Avoid swimming, very poor water quality, potential algae toxins
Water Quality Grade: F, Very Poor
Very murky, less than 2.4 ft of visibility. Chlorophyll-a: 45 µg/L. Trophic State Index: 66.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 2.4 ft | F |
| Phosphorus | No data | |
| Chlorophyll-a (Algae) | 45 µg/L | F |
| Trophic State Index (TSI) | 66 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 633 acres |
| Public Access | Yes |
Where to stay near Twelve Mile Lake
Compare lakeside cabins, hotels, and vacation rentals near Twelve 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.
Top-rated places to stay near Twelve Mile Lake
Availability and prices from Stay22 partners (Booking.com, Vrbo, Expedia). We may earn a commission from bookings, at no extra cost to you.
Planning a trip to Twelve Mile Lake?
Get lake water-quality report cards plus the best cabin & lodging deals near Twelve Mile Lake — straight to your inbox. No spam, unsubscribe anytime.
Planning a whole trip? See where to stay at Iowa’s top lakes →
Fishing Twelve Mile Lake
Water Quality Trend: ↑ Improving
Based on 5 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | → Stable | 0 m/yr | 5 |
| Chlorophyll-a | ↑ Improving | -4.01 µg/L/yr | 5 |
Recreation & Access
Water Temperature
Twelve Mile Lake averages about 77°F at the surface in summer, usually warmest in Jul (about 80°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 29 field readings (2010–2023) via the EPA Water Quality Portal. See the full Twelve Mile Lake water temperature breakdown →
Location & Depth Map
Free interactive depth-contour map of Twelve Mile Lake, reaching a maximum surveyed depth of 40 ft. Toggle Depth contours to overlay the bathymetric survey lines. Contour data from Iowa DNR.
County Ranking
Ranked #3 of 6 lakes in Union County
Cleaner Lakes Within 30 Miles
Twelve Mile Lake holds Grade F. 3 nearby lakes hold higher grades.
- DThree Mile LakeUnion County · 2.4 mi · Higher grade (F → D)
- DThayer LakeUnion County · 10.1 mi · Higher grade (F → D)
- DNodaway LakeAdair County · 20.5 mi · Higher grade (F → D)
Nearby Lakes in Union County
Explore Twelve Mile Lake across LakeGrade
More Iowa Lake Rankings
Other Grade F Lakes in Iowa
Eutrophic Lakes in Iowa
Compare Twelve Mile Lake With Nearby Lakes
More records, surveys & data sources
Reservoir Info (USACE NID)
Twelve Mile Lake is a man-made reservoir impounded by the Twelve Mile Creek Watershed Site M-1 (completed 1983), built primarily for flood risk reduction on the TWELVE MILE CREEK; earth-type dam, 66 ft tall and 2,100 ft long.
- Surface area
- 633 ac
- Normal storage
- 10,200 ac-ft
- Max storage
- 27,200 ac-ft
- Drainage area
- 23 sq mi
- Hazard class
- High
- Owner
- Creston Waterworks
All listed purposes: Flood Risk Reduction;Recreation;Water Supply.
Source: USACE National Inventory of Dams, NID ID IA02194 · Operator website · Matched by proximity (0.34 km)
EPA Impairment Status
Twelve Mile Lake is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 5).
Causes of impairment
Source: EPA ATTAINS assessment unit IA 05-GRA-1367 · Official waterbody report · Matched by proximity (2.25 km)
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth is a modeled estimate.
Regulated Pollution Sources Nearby
1 EPA-permitted facility operate within about 5 miles of Twelve Mile Lake under Clean Water Act (NPDES) discharge permits. 1 facility has a reported Clean Water Act violation in the past three years.
- Sirwa Water Treatment PlantCreston · 0.5 mi away · NPDES IA0053504Recent violation
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
Impairment status from EPA ATTAINS 303(d) database
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: 2024-09-30
Monitoring stations: 1