Twelve Mile Lake
Union County, Iowa633 acresView mapDirections
Nutrients and algae run well above the regional average.
- Hypereutrophic
- Impaired · Algal growth
- #2 of 6 in the county
- 55 ft max depth
- Public access
Twelve Mile Lake in Union County, Iowa has a water quality grade of F (Algae-prone). Water clarity: 2.4 ft visibility. Phosphorus: 79 µg/L. Chlorophyll-a (algae): 45 µg/L. Surface area: 633 acres.
- Clarity
- 2.4 ft
- Typical in September
- 70°F
- Max depth
- ~55 ft est.
- Area
- 633 acres
- Type
- Hypereutrophic
- Phosphorus
- 79 µg/L
- Chlorophyll-a
- 45 µg/L
- Trend
- → Stable
- Public access
- Yes
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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 grade measures nutrients and algae, not lodging — cabins and rentals around Twelve Mile Lake are listed below.
Top-rated places to stay near Twelve Mile Lake
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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. Water quality is graded F based on 2 sampling years through 2024. 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. Grade methodology: LakeQuality methodology.
What Twelve Mile Lake is good for
From access and monitoring records, not editorial picks.
- Boat launch2 documented ramps
Swimming Safety
Grade onlyAlgae risk
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.
The F is nutrients and algae, 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.
Water Quality Grade: F (Algae-prone)
Very murky, less than 2.4 ft of visibility. Phosphorus 79 µg/L. Chlorophyll-a 45 µg/L. TSI 67. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 2.4 ft | Not graded |
| Phosphorus | 79 µg/L | Poor |
| Chlorophyll-a (Algae) | 45 µg/L | Poor |
| Trophic State Index (TSI) | 67 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
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.
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
| Characteristic | Value |
|---|---|
| Maximum Depth | ~55 ft est. |
| Average Depth | ~55 ft est. |
| Surface Area | 633 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 Twelve 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.
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
Fishing Twelve Mile Lake
Water Quality Trend: → Stable
Based on 5 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | → Stable | 0 m/yr | 5 |
| Phosphorus | ↓ Declining | +7.5 µg/L/yr | 4 |
| Chlorophyll-a | ↑ Improving | -4.01 µg/L/yr | 5 |
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)
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.
County Ranking
Ranked #2 of 6 lakes in Union County
Nearby Lakes With Higher Grades
Twelve Mile Lake holds Grade F. One nearby lake has a higher nutrient-and-algae grade — useful for a swim day, not a reason to skip lodging here.
See full comparison →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. LakeQuality grades it F for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Twelve Mile Lake?
- Twelve Mile Lake in Union County, Iowa has a water quality grade of F (Algae-prone). 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?
- The F is nutrients and algae, 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.
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 grade
Very high nutrients, dense algae, poor clarity
Iowa's cleanest and most-polluted lakes, ranked
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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)
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
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