Hawthorn Lake is a 160-acre productive, nutrient-rich eutrophic lake in Mahaska County, Iowa. Water quality is graded D based on 2 sampling years through 2024. Overall water quality is improving over recent sampling years. Limited fish-species data available. Public access is available with 2 public boat landings. Compared to the 2 other monitored lakes in Mahaska County, Hawthorn Lake ranks #1 for water quality.
Source: EPA Water Quality Portal sampling records, Iowa DNR, last sampled 2024-08-28. Grade methodology: LakeGrade methodology.
Watch Hawthorn Lake water quality
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Swimming Safety
Swimming not recommended, poor water quality with high algae risk
Water Quality Grade: D, Poor
Very murky, less than 2.4 ft of visibility. Chlorophyll-a: 27.5 µg/L. Trophic State Index: 64.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 2.4 ft | F |
| Phosphorus | No data | |
| Chlorophyll-a (Algae) | 27.5 µg/L | D |
| Trophic State Index (TSI) | 64 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 160 acres |
| Public Access | Yes |
Where to stay near Hawthorn Lake
Compare lakeside cabins, hotels, and vacation rentals near Hawthorn Lake in Mahaska 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.
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Fishing Hawthorn Lake
Water Quality Trend: ↑ Improving
Based on 5 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↑ Improving | +0.027 m/yr | 5 |
| Chlorophyll-a | → Stable | -0.27 µg/L/yr | 5 |
Recreation & Access
Water Temperature
Hawthorn Lake averages about 79°F at the surface in summer, usually warmest in Jul (about 82°F) and coldest in Apr (about 55°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 51 field readings (2011–2023) via the EPA Water Quality Portal. See the full Hawthorn Lake water temperature breakdown →
Location & Depth Map
Free interactive depth-contour map of Hawthorn Lake, reaching a maximum surveyed depth of 28 ft. Toggle Depth contours to overlay the bathymetric survey lines. Contour data from Iowa DNR.
County Ranking
Ranked #1 of 3 lakes in Mahaska County
Cleaner Lakes Within 30 Miles
Hawthorn Lake holds Grade D. One nearby lake has a higher grade.
See full comparison →Nearby Lakes in Mahaska County
Explore Hawthorn Lake across LakeGrade
More Iowa Lake Rankings
Other Grade D Lakes in Iowa
Eutrophic Lakes in Iowa
More records, surveys & data sources
Reservoir Info (USACE NID)
Hawthorn Lake is a man-made reservoir impounded by the Hawthorne Lake Dam (completed 1979), built primarily for recreation on the PLEASANT CREEK; earth-type dam, 45 ft tall and 900 ft long.
- Surface area
- 160 ac
- Normal storage
- 1,890 ac-ft
- Max storage
- 4,100 ac-ft
- Drainage area
- 5.2 sq mi
- Hazard class
- Significant
- Owner
- Iowa Department of Natural Resources
Source: USACE National Inventory of Dams, NID ID IA01874 · Operator website · Matched by proximity (0.14 km)
EPA Impairment Status
Hawthorn Lake is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 5).
Causes of impairment
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit IA 03-NSK-862 · 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 Hawthorn Lake under Clean Water Act (NPDES) discharge permits.
- Barnes City, City Of StpBarnes City · 2.5 mi away · NPDES IA0081329
- S + S FarmGibson · 4.4 mi away · NPDES IA0077887
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
Public boat-access points from the USGS National Boat Ramps inventory
Most recent sample: 2024-08-28
Monitoring stations: 1