Willow Lake
Harrison County, Iowa19 acresView mapDirections
Clarity, phosphorus, and algae sit in the clean band for this region.
↑ up from C at the last regrade
- Eutrophic
- #1 of 2 in the county
Willow Lake in Harrison County, Iowa has a water quality grade of B (Good). Water clarity: 4.8 ft visibility. Phosphorus: 35 µg/L. Chlorophyll-a (algae): 9.5 µg/L. Surface area: 19 acres.
- Clarity
- 4.8 ft
- Typical in August
- 79°F
- Area
- 19 acres
- Type
- Eutrophic
- Phosphorus
- 35 µg/L
- Chlorophyll-a
- 9.5 µg/L
- Trend
- ↓ Declining
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Stays near Willow Lake
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Watch Willow Lake’s letter
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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.
Willow Lake is a 19-acre productive, nutrient-rich eutrophic lake in Harrison County, Iowa. Water quality is graded B based on 2 sampling years through 2024. Overall water quality is declining over recent sampling years. 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: LakeQuality methodology.
Swimming Safety
Grade onlyBacteria A
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, 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: B (Good)
Murky, only visible to about 4.8 ft. Phosphorus 35 µg/L. Chlorophyll-a 9.5 µg/L. TSI 54. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 4.8 ft | Not graded |
| Phosphorus | 35 µg/L | Good |
| Chlorophyll-a (Algae) | 9.5 µg/L | Good |
| Trophic State Index (TSI) | 54 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Water Temperature
Willow Lake typically sits around 79°F at the surface in August. Summer average is about 78°F. 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 →
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 19 acres |
No bathymetric survey has been published for Willow Lake, so its depth is not documented here.
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 |
| Phosphorus | ↓ Declining | +14.75 µg/L/yr | 4 |
| Chlorophyll-a | ↓ Declining | +2.09 µg/L/yr | 4 |
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.
County Ranking
Ranked #1 of 2 lakes in Harrison County
Nearby Lakes in Harrison County
Frequently asked questions about Willow Lake
Where is Willow Lake?
- Willow Lake is a lake in Harrison County, Iowa. LakeQuality grades it B for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Willow Lake?
- Willow Lake in Harrison County, Iowa has a water quality grade of B (Good). Water clarity: 4.8 ft visibility. Phosphorus: 35 µg/L. Chlorophyll-a (algae): 9.5 µg/L. Surface area: 19 acres.
Is Willow Lake safe to swim in?
- 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. That works out to a bacteria swim-safety grade of A — a historical summary of past testing, not a current reading. Willow Lake has a water quality grade of B (Good). 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 Willow Lake?
- Willow Lake typically sits around 79°F at the surface in August, from 26 EPA Water Quality Portal field readings (2010–2023). That is a typical value for the month, not a live reading.
Explore Willow Lake across LakeQuality
More Iowa Lake Rankings
Other Grade B Lakes in Iowa
Eutrophic Lakes in Iowa
Where this data comes from
54 measurements from 1 monitoring station, 2021–2024. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Iowa DNR Surface Water Monitoring Data
21IOWA_WQX54 samples · 2021–2024
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
Regulated discharger data from EPA ECHO (ICIS-NPDES)