Green Castle Lake
Marshall County, Iowa17 acresView mapDirections
Nutrients and algae sit mid-range for this region.
- Mesotrophic
- #2 of 2 in the county
Green Castle Lake in Marshall County, Iowa has a water quality grade of C (Fair). Water clarity: 4.6 ft visibility. Phosphorus: 59 µg/L. Chlorophyll-a (algae): 6.9 µg/L. Surface area: 17 acres.
- Clarity
- 4.6 ft
- Typical in September
- 66°F
- Area
- 17 acres
- Type
- Mesotrophic
- Trend
- ↓ Declining
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Watch Green Castle Lake’s letter
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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.
Green Castle Lake is a 17-acre mesotrophic 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: LakeQuality methodology.
Swimming Safety
Grade onlyModerate
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.
The C is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
Water Quality Grade: C (Fair)
Murky, only visible to about 4.6 ft. Phosphorus 59 µg/L. Chlorophyll-a 6.9 µg/L. TSI 56. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 4.6 ft | Not graded |
| Phosphorus | 59 µg/L | Poor |
| Chlorophyll-a (Algae) | 6.9 µg/L | Good |
| Trophic State Index (TSI) | 56 | Mesotrophic |
Moderate nutrients, good water quality
Water Temperature
Green Castle Lake typically sits around 66°F at the surface in September. Summer average is about 76°F. 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 →
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 17 acres |
No bathymetric survey has been published for Green Castle Lake, so its depth is not documented here.
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 |
| Phosphorus | ↓ Declining | +39.85 µg/L/yr | 4 |
| Chlorophyll-a | ↓ Declining | +8.22 µg/L/yr | 4 |
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.
County Ranking
Ranked #2 of 2 lakes in Marshall County
Nearby Lakes With Higher Grades
Green Castle Lake holds Grade C. 2 nearby lakes hold higher nutrient-and-algae grades — useful for a swim day, not a reason to skip lodging here.
See full comparison →Nearby Lakes in Marshall County
Frequently asked questions about Green Castle Lake
Where is Green Castle Lake?
- Green Castle Lake is a lake in Marshall County, Iowa. LakeQuality grades it C for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Green Castle Lake?
- Green Castle Lake in Marshall County, Iowa has a water quality grade of C (Fair). Water clarity: 4.6 ft visibility. Phosphorus: 59 µg/L. Chlorophyll-a (algae): 6.9 µg/L. Surface area: 17 acres.
Is Green Castle Lake safe to swim in?
- The C is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
What is the water temperature of Green Castle Lake?
- Green Castle Lake typically sits around 66°F at the surface in September, from 26 EPA Water Quality Portal field readings (2010–2023). That is a typical value for the month, not a live reading.
Explore Green Castle Lake across LakeQuality
More Iowa Lake Rankings
Other Grade C Lakes in Iowa
Mesotrophic 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)