Hannen Lake
Benton County, Iowa39 acresView mapDirections
Nutrients and algae run well above the regional average.
- Hypereutrophic
- Impaired · Algal growth
- #1 of 2 in the county
- 32 ft max depth
Hannen Lake in Benton County, Iowa has a water quality grade of F (Algae-prone). Water clarity: 1.2 ft visibility. Phosphorus: 141 µg/L. Chlorophyll-a (algae): 119.9 µg/L. Surface area: 39 acres.
- Clarity
- 1.2 ft
- Typical in September
- 68°F
- Max depth
- ~32 ft est.
- Area
- 39 acres
- Type
- Hypereutrophic
- Trend
- ↓ Declining
Stays via Stay22
Stays near Hannen Lake
Compare lakeside cabins, hotels, and vacation rentals near Hannen Lake in Benton 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 Hannen Lake are listed below.
Top-rated places to stay near Hannen Lake
Prices for Sep 11–13, 4 adults. Change dates on the map below.
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Watch Hannen Lake’s letter
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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.
Hannen Lake is a 39-acre highly nutrient-loaded hypereutrophic lake in Benton County, Iowa with an estimated average depth of about 32 feet. 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: LakeQuality methodology.
Swimming Safety
Grade onlyBacteria D
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.
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 (Algae-prone)
Very murky, less than 1.2 ft of visibility. Phosphorus 141 µg/L. Chlorophyll-a 119.9 µg/L. TSI 76. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 1.2 ft | Not graded |
| Phosphorus | 141 µg/L | Poor |
| Chlorophyll-a (Algae) | 119.9 µg/L | Poor |
| Trophic State Index (TSI) | 76 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Water Temperature
Hannen Lake typically sits around 68°F at the surface in September. Summer average is about 76°F. 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 →
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | ~32 ft est. |
| Average Depth | ~14 ft est. |
| Surface Area | 39 acres |
Depth marked “est.” is a modeled maximum from GLOBathy (Khazaei et al. 2022, CC0), not a measured survey. Iowa has not published bathymetry for Hannen 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.
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 |
| Phosphorus | → Stable | +1.05 µg/L/yr | 4 |
| Chlorophyll-a | ↓ Declining | +14.98 µg/L/yr | 5 |
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
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.
County Ranking
Ranked #1 of 2 lakes in Benton County
Nearby Lakes With Higher Grades
Hannen 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 Benton County
Frequently asked questions about Hannen Lake
Where is Hannen Lake?
- Hannen Lake is a lake in Benton County, Iowa. LakeQuality grades it F for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Hannen Lake?
- Hannen Lake in Benton County, Iowa has a water quality grade of F (Algae-prone). Water clarity: 1.2 ft visibility. Phosphorus: 141 µg/L. Chlorophyll-a (algae): 119.9 µg/L. Surface area: 39 acres.
Is Hannen Lake safe to swim in?
- 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. That works out to a bacteria swim-safety grade of D — a historical summary of past testing, not a current reading. Hannen Lake has a water quality grade of F (Algae-prone). Bacteria levels can spike quickly after heavy rain, so always check Iowa's current beach and health-department advisories before you swim.
What is the water temperature of Hannen Lake?
- Hannen Lake typically sits around 68°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 Hannen Lake?
- No published bathymetric survey covers Hannen Lake, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 32 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 39 acres.
Explore Hannen Lake across LakeQuality
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Where this data comes from
77 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_WQX77 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)