Hannen Lake is a 39-acre highly nutrient-loaded hypereutrophic lake in Benton County, Iowa. Water quality is graded F based on 2 sampling years through 2024. Overall water quality is declining over recent sampling years. Compared to the 1 other monitored lakes in Benton County, Hannen Lake ranks #1 for water quality.
Source: EPA Water Quality Portal sampling records, Iowa DNR, last sampled 2024-10-23. Grade methodology: LakeGrade methodology.
Watch Hannen Lake water quality
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Swimming Safety
Based on E. coli testing, Hannen Lake frequently exceeded the EPA safe-swimming threshold (235 MPN/100mL) — on 20% of 10 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: F, Very Poor
Very murky, less than 1.2 ft of visibility. Chlorophyll-a: 119.9 µg/L. Trophic State Index: 76.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 1.2 ft | F |
| Phosphorus | No data | |
| Chlorophyll-a (Algae) | 119.9 µg/L | F |
| Trophic State Index (TSI) | 76 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 39 acres |
Where to stay near Hannen Lake
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Fishing Hannen Lake
Water Quality Trend: ↓ Declining
Based on 5 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↓ Declining | -0.011 m/yr | 5 |
| Chlorophyll-a | ↓ Declining | +14.98 µg/L/yr | 5 |
Water Temperature
Hannen Lake averages about 76°F at the surface in summer, usually warmest in Aug (about 78°F) and coldest in Sep (about 68°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 Hannen Lake water temperature breakdown →
Location & Depth Map
Free interactive depth-contour map of Hannen Lake, reaching a maximum surveyed depth of 18 ft. Toggle Depth contours to overlay the bathymetric survey lines. Contour data from Iowa DNR.
County Ranking
Ranked #1 of 2 lakes in Benton County
Cleaner Lakes Within 30 Miles
Hannen Lake holds Grade F. One nearby lake has a higher grade.
See full comparison →Nearby Lakes in Benton County
Explore Hannen Lake across LakeGrade
More Iowa Lake Rankings
Other Grade F Lakes in Iowa
Hypereutrophic Lakes in Iowa
More records, surveys & data sources
Reservoir Info (USACE NID)
Hannen Lake is a man-made reservoir impounded by the Hannen Lake Dam (completed 1959), built primarily for recreation on the TR-COON CREEK; earth-type dam, 35 ft tall and 600 ft long.
- Surface area
- 39 ac
- Normal storage
- 390 ac-ft
- Max storage
- 886 ac-ft
- Drainage area
- 0.96 sq mi
- Hazard class
- Significant
- Owner
- Benton County Conservation Board
Source: USACE National Inventory of Dams, NID ID IA00128 · Operator website
EPA Impairment Status
Hannen Lake is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 4A).
Causes of impairment
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit IA 02-IOW-704 · Official waterbody report
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth is a modeled estimate.
Regulated Pollution Sources Nearby
2 EPA-permitted facilities operate within about 5 miles of Hannen Lake under Clean Water Act (NPDES) discharge permits. 1 facility has a reported Clean Water Act violation in the past three years.
- Blairstown City Of StpBlairstown · 3.7 mi away · NPDES IA0030660Recent violation
- Xethanol Biofuels, LlcBlairstown · 2.9 mi away · NPDES IA0076830
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
Most recent sample: 2024-10-23
Monitoring stations: 1