Hannen Lake
Benton County, Iowa39 acresView mapDirections
Iowa DNR publishes the water-quality measurements for Hannen Lake. Here is what has been measured.
- Hypereutrophic
- Impaired · Algal growth
- 32 ft max depth
Hannen Lake in Benton County, Iowa has water-quality measurements published by Iowa DNR, shown here as reported. Water clarity: 1.2 ft visibility. Phosphorus: 141 µg/L. Chlorophyll-a (algae): 119.9 µg/L. Surface area: 39 acres.
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. 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. Methodology: how these readings are compared.
Hannen Lake covers about 39 acres in Benton County. If you are planning to stay, you can search lake houses and cabins around Hannen Lake on Vrbo, or look for a hotel room near the lake on Booking.com or Expedia.
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.
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.
Whether Hannen 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 Hannen Lake
These are the measurements Iowa DNR publishes for Hannen 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 1.2 ft of visibility. Phosphorus 141 µg/L. Chlorophyll-a 119.9 µg/L. TSI 76.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 1.2ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 141µg/L | Poor GoodFairPoor |
| Chlorophyll-a (algae) | 120µg/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. | 76 | Hypereutrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Very high nutrients, dense algae, poor clarity
Ask about Hannen Lake
Answers come straight from the public measurements and survey records on this page.
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.
Column height runs from this lake’s coldest month (68°F) to its warmest (78°F).
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
Max depth
~32 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
Watershed (drainage) area
1 sq mi
HydroLAKES / HydroSHEDS
Shoreline
1.3 mi
HydroLAKES / HydroSHEDS
Elevation
833 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.
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 readings cover nutrients and algae, not lodging. Cabins and rentals around Hannen Lake are listed below.
Top-rated places to stay near Hannen 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 whole trip? See where to stay at Iowa’s top lakes →
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Fishing Hannen Lake
Water Quality Trend: Worsening
Based on 5 years of monitoring (2020–2024).
Water clarity
Worsening5 years
Phosphorus
Stable+1.05 µg/L/yr4 years
Chlorophyll-a
Worsening+14.98 µg/L/yr5 years
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
Designated uses not supporting: Recreation.
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.
Nearby Lakes in Benton County
Frequently asked questions about Hannen Lake
Where is Hannen Lake?
- Hannen Lake is a lake in Benton County, Iowa. Its water-quality measurements are published by Iowa DNR and shown on this page as reported.
What is the water quality of Hannen Lake?
- Hannen Lake in Benton County, Iowa has water-quality measurements published by Iowa DNR, shown here as reported. 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 is a historical summary of past bacteria testing, not a current reading and not a swimming clearance. Bacteria levels can spike quickly after heavy rain, so whether Hannen Lake is safe to swim in today is a call for Iowa's water agency and the county health department; check their current beach 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)
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)