Willow Lake is a 19-acre productive, nutrient-rich eutrophic lake in Harrison County, Iowa. Water quality is graded C based on 2 sampling years through 2024. Overall water quality is declining over recent sampling years. Limited fish-species data available. Public access is available. Compared to the 1 other monitored lakes in Harrison County, Willow Lake ranks #1 for water quality.
Source: EPA Water Quality Portal sampling records, Iowa DNR, last sampled 2024-09-03. Grade methodology: LakeGrade methodology.
Watch Willow Lake water quality
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Swimming Safety
Based on E. coli testing, Willow Lake never exceeded the EPA safe-swimming threshold (235 MPN/100mL) — on 0% of 7 samples across 2 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: C, Fair
Murky, only visible to about 4.8 ft. Chlorophyll-a: 9.5 µg/L. Trophic State Index: 54.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 4.8 ft | D |
| Phosphorus | No data | |
| Chlorophyll-a (Algae) | 9.5 µg/L | B |
| Trophic State Index (TSI) | 54 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 19 acres |
| Public Access | Yes |
Where to stay near Willow Lake
Compare lakeside cabins, hotels, and vacation rentals near Willow Lake in Harrison 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 Willow Lake
Water Quality Trend: ↓ Declining
Based on 4 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↓ Declining | -0.59 m/yr | 4 |
| Chlorophyll-a | ↓ Declining | +2.09 µg/L/yr | 4 |
Water Temperature
Willow Lake averages about 78°F at the surface in summer, usually warmest in Jul (about 79°F) and coldest in May (about 65°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 26 field readings (2010–2023) via the EPA Water Quality Portal. See the full Willow Lake water temperature breakdown →
Location & Depth Map
Free interactive depth-contour map of Willow Lake, reaching a maximum surveyed depth of 22 ft. Toggle Depth contours to overlay the bathymetric survey lines. Contour data from Iowa DNR.
County Ranking
Ranked #1 of 2 lakes in Harrison County
Nearby Lakes in Harrison County
Explore Willow Lake across LakeGrade
More Iowa Lake Rankings
Other Grade C Lakes in Iowa
Eutrophic Lakes in Iowa
More records, surveys & data sources
Reservoir Info (USACE NID)
Willow Lake is a man-made reservoir impounded by the Willow Lake Dam (completed 1976), built primarily for debris control on the TR-WILLOW CREEK; earth-type dam, 31 ft tall and 390 ft long.
- Surface area
- 19 ac
- Normal storage
- 21 ac-ft
- Max storage
- 44 ac-ft
- Drainage area
- 0.21 sq mi
- Hazard class
- Low
- Owner
- Harrison County Conservation Board
All listed purposes: Debris Control;Recreation.
Source: USACE National Inventory of Dams, NID ID IA01566 · Operator website
Regulated Pollution Sources Nearby
1 EPA-permitted facility operate within about 5 miles of Willow Lake under Clean Water Act (NPDES) discharge permits.
- Crossroads Cattle CompanyWoodbine · 3.4 mi away · NPDES IA0080667
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
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Grading methodology based on Metropolitan Council standards
Most recent sample: 2024-09-03
Monitoring stations: 1