Green Castle Lake is a 17-acre productive, nutrient-rich eutrophic lake in Marshall County, Iowa. Water quality is graded C based on 2 sampling years through 2024. Overall water quality is declining over recent sampling years. Compared to the 1 other monitored lakes in Marshall County, Green Castle Lake ranks #2 for water quality.
Source: EPA Water Quality Portal sampling records, Iowa DNR, last sampled 2024-08-25. Grade methodology: LakeGrade methodology.
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Swimming Safety
Generally safe for swimming, moderate water quality
Water Quality Grade: C, Fair
Murky, only visible to about 4.6 ft. Chlorophyll-a: 6.9 µg/L. Trophic State Index: 52.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 4.6 ft | D |
| Phosphorus | No data | |
| Chlorophyll-a (Algae) | 6.9 µg/L | B |
| Trophic State Index (TSI) | 52 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 17 acres |
Where to stay near Green Castle Lake
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Fishing Green Castle Lake
Water Quality Trend: ↓ Declining
Based on 4 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↓ Declining | -0.363 m/yr | 4 |
| Chlorophyll-a | ↓ Declining | +8.22 µg/L/yr | 4 |
Water Temperature
Green Castle Lake averages about 76°F at the surface in summer, usually warmest in Aug (about 78°F) and coldest in Sep (about 66°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 Green Castle Lake water temperature breakdown →
Location & Depth Map
Free interactive depth-contour map of Green Castle Lake, reaching a maximum surveyed depth of 23 ft. Toggle Depth contours to overlay the bathymetric survey lines. Contour data from Iowa DNR.
County Ranking
Ranked #2 of 2 lakes in Marshall County
Cleaner Lakes Within 30 Miles
Green Castle Lake holds Grade C. One nearby lake has a higher grade.
See full comparison →Nearby Lakes in Marshall County
Explore Green Castle Lake across LakeGrade
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Other Grade C Lakes in Iowa
Eutrophic Lakes in Iowa
More records, surveys & data sources
Reservoir Info (USACE NID)
Green Castle Lake is a man-made reservoir impounded by the Green Castle Dam (completed 1954), built primarily for recreation on the TR- BRUSH CREEK; earth-type dam, 30 ft tall and 650 ft long.
- Surface area
- 17 ac
- Normal storage
- 150 ac-ft
- Max storage
- 263 ac-ft
- Drainage area
- 0.44 sq mi
- Hazard class
- Significant
- Owner
- Marshall County Conservation Board
Source: USACE National Inventory of Dams, NID ID IA02418 · Operator website
Regulated Pollution Sources Nearby
3 EPA-permitted facilities operate within about 5 miles of Green Castle Lake under Clean Water Act (NPDES) discharge permits. 3 facilities have a reported Clean Water Act violation in the past three years.
- Laurel, City Of StpLaurel · 3.8 mi away · NPDES IA0072214Recent violation
- Marshalltown Estates MhpMarshalltown · 4 mi away · NPDES IA0068659Recent violation
- Gilman City Of StpGilman · 4.6 mi away · NPDES IA0029033Recent violation
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: 2024-08-25
Monitoring stations: 1