Twelve Mile Lake
Union County, Iowa633 acresView mapDirections
Iowa DNR publishes the water-quality measurements for Twelve Mile Lake. Here is what has been measured.
- Hypereutrophic
- Impaired · Algal growth
- 55 ft max depth
- Public access
Twelve Mile Lake in Union County, Iowa has water-quality measurements published by Iowa DNR, shown here as reported. Water clarity: 2.4 ft visibility. Phosphorus: 79 µg/L. Chlorophyll-a (algae): 45 µg/L. Surface area: 633 acres.
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.
Twelve Mile Lake is a 633-acre highly nutrient-loaded hypereutrophic lake in Union County, Iowa with an estimated average depth of about 55 feet. Phosphorus level is declining over recent sampling years. Limited fish-species data available. Public access is available with 2 public boat landings. Compared to the 5 other monitored lakes in Union County, Twelve Mile Lake ranks #2 for water quality.
Source: EPA Water Quality Portal sampling records, Iowa DNR, last sampled 2024-09-30. Methodology: how these readings are compared.
Twelve Mile Lake covers about 630 acres in Union County. It has 2 public boat landings, so a lakefront rental is not the only way to get a boat on the water. If you are planning to stay, you can search lake houses and cabins around Twelve Mile Lake on Vrbo, or look for a hotel room near the lake on Booking.com or Expedia.
What Twelve Mile Lake is good for
From access and monitoring records, not editorial picks.
- Boat launch2 documented ramps
Swimming conditions
Readings onlyRecreation use not supported
Based on clarity and algae readings from past sampling — there is no live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
Nutrient and algae readings are not a swimming clearance. Twelve Mile Lake is listed impaired under Clean Water Act §303(d) (IR 5) for algal growth, turbidity. Check local advisories before you swim.
Whether Twelve Mile Lake is safe to swim in today is a call for the Iowa Department of Natural Resources and the county health department, who post current advisories. LakeQuality does not issue swimming clearances.
Water quality measurements for Twelve Mile Lake
These are the measurements Iowa DNR publishes for Twelve Mile Lake, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Very murky, less than 2.4 ft of visibility. Phosphorus 79 µg/L. Chlorophyll-a 45 µg/L. TSI 67.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 2.4ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 79µg/L | Poor GoodFairPoor |
| Chlorophyll-a (algae) | 45µg/L | Poor GoodFairPoor |
| Trophic state (TSI)Carlson's 0–100 index of how much algae a lake can support, from clarity, phosphorus and chlorophyll-a. | 67 | Hypereutrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Very high nutrients, dense algae, poor clarity
Ask about Twelve Mile Lake
Answers come straight from the public measurements and survey records on this page.
Water Temperature
Twelve Mile Lake typically sits around 70°F at the surface in September. Summer average is about 77°F. Usually warmest in Jul (about 80°F) and coldest in Sep (about 70°F). These are typical values by month — not a live reading.
Column height runs from this lake’s coldest month (70°F) to its warmest (80°F).
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 →
Lake Details
Max depth
~55 ft est.
Surface area
633 acres
Public access
Yes
Listed by Iowa DNR
Depth marked “est.” is a modeled maximum from GLOBathy (Khazaei et al. 2022, CC0), not a measured survey. Iowa has not published bathymetry for Twelve Mile Lake.
Lake Size & Watershed
Watershed (drainage) area
25 sq mi
HydroLAKES / HydroSHEDS
Shoreline
7.6 mi
HydroLAKES / HydroSHEDS
Elevation
1,168 ft
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.
Recreation & Access
- Twelve Mile Lake - UnionPublic
- Twelve Mile Lake - UnionPublic
Named access points in the USGS National Boat Ramps inventory, within about 3 miles. More lakes with boat ramps
Stays via Stay22
Stays 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. The water-quality readings cover nutrients and algae, not lodging. Cabins and rentals around Twelve Mile Lake are listed below.
Top-rated places to stay near Twelve Mile Lake
Prices for Oct 23–25, 4 adults. Change dates on the map below.
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? Cabins and lodging at Twelve Mile Lake → · more Iowa lake trips
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Fishing Twelve Mile Lake
Water Quality Trend: Stable
Based on 5 years of monitoring (2020–2024).
Water clarity
Stable5 years
Phosphorus
Worsening+7.5 µg/L/yr4 years
Chlorophyll-a
Improving-4.01 µg/L/yr5 years
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
Designated uses not supporting: Recreation.
Source: EPA ATTAINS assessment unit IA 05-GRA-1367 · Official waterbody report
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.
Nearby Lakes in Union County
Frequently asked questions about Twelve Mile Lake
Where is Twelve Mile Lake?
- Twelve Mile Lake is a lake in Union County, Iowa. Its water-quality measurements are published by Iowa DNR and shown on this page as reported.
What is the water quality of Twelve Mile Lake?
- Twelve Mile Lake in Union County, Iowa has water-quality measurements published by Iowa DNR, shown here as reported. Water clarity: 2.4 ft visibility. Phosphorus: 79 µg/L. Chlorophyll-a (algae): 45 µg/L. Surface area: 633 acres.
Is Twelve Mile Lake safe to swim in?
- Nutrient and algae readings are not a swimming clearance. Twelve Mile Lake is listed impaired under Clean Water Act §303(d) (IR 5) for algal growth, turbidity. Check local advisories before you swim.
What is the water temperature of Twelve Mile Lake?
- Twelve Mile Lake typically sits around 70°F at the surface in September, from 29 EPA Water Quality Portal field readings (2010–2023). That is a typical value for the month, not a live reading.
How deep is Twelve Mile Lake?
- No published bathymetric survey covers Twelve Mile Lake, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 55 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 633 acres.
Are there boat ramps on Twelve Mile Lake?
- Twelve Mile Lake has 2 documented boat landings in the USGS National Boat Ramps inventory. Named access points include Twelve Mile Lake - Union, Twelve Mile Lake - Union. Public access is listed as yes.
Can I fish at Twelve Mile Lake?
- Twelve Mile Lake has 2 documented boat landings in the USGS National Boat Ramps inventory. Seasons, size and bag limits, and whether you need a license are set by Iowa's fish and wildlife agency, so check its current regulations before you go.
Explore Twelve 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 readings
Iowa's cleanest lakes, ranked on agency measurements
More lakes in Iowa
Other Lakes in Iowa
Hypereutrophic Lakes in Iowa
Compare Twelve Mile Lake With Nearby Lakes
Where this data comes from
62 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_WQX62 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)
EPA condition classes from the National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See where the data comes from and data sources.
Other records on this page
Impairment status from EPA ATTAINS 303(d) database
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Lake size & watershed from HydroLAKES (HydroSHEDS)
Named boat-access points from the USGS National Boat Ramps inventory