Green Castle Lake
Marshall County, Iowa17 acresView mapDirections
Iowa DNR publishes the water-quality measurements for Green Castle Lake. Here is what has been measured.
- Mesotrophic
Green Castle Lake in Marshall County, Iowa has water-quality measurements published by Iowa DNR, shown here as reported. Water clarity: 4.6 ft visibility. Phosphorus: 59 µg/L. Chlorophyll-a (algae): 6.9 µg/L. Surface area: 17 acres.
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. 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. Methodology: how these readings are compared.
Swimming conditions
Readings onlyModerate algae
Based on clarity and algae readings from past sampling — there is no live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
Nutrient and algae readings are not a swimming clearance. Check local advisories before you swim.
Whether Green Castle Lake is safe to swim in today is a call for the Iowa Department of Natural Resources and the county health department, who post current advisories. LakeQuality does not issue swimming clearances.
Water quality measurements for Green Castle Lake
These are the measurements Iowa DNR publishes for Green Castle Lake, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Murky, only visible to about 4.6 ft. Phosphorus 59 µg/L. Chlorophyll-a 6.9 µg/L. TSI 56.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 4.6ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 59µg/L | Poor GoodFairPoor |
| Chlorophyll-a (algae) | 6.9µg/L | Good GoodFairPoor |
| Trophic state (TSI)Carlson's 0–100 index of how much algae a lake can support, from clarity, phosphorus and chlorophyll-a. | 56 | Mesotrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Moderate nutrients, good water quality
Ask about Green Castle Lake
Answers come straight from the public measurements and survey records on this page.
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.
Column height runs from this lake’s coldest month (66°F) to its warmest (78°F).
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
Surface area
17 acres
No bathymetric survey has been published for Green Castle Lake, so its depth is not documented here.
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Stays near Green Castle Lake
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Fishing Green Castle Lake
Water Quality Trend: Worsening
Based on 4 years of monitoring (2021–2024).
Water clarity
Worsening4 years
Phosphorus
Worsening+39.85 µg/L/yr4 years
Chlorophyll-a
Worsening+8.22 µg/L/yr4 years
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.
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. Its water-quality measurements are published by Iowa DNR and shown on this page as reported.
What is the water quality of Green Castle Lake?
- Green Castle Lake in Marshall County, Iowa has water-quality measurements published by Iowa DNR, shown here as reported. 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?
- Nutrient and algae readings are 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 lakes in Iowa
Other 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)
EPA condition classes from the National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See where the data comes from and data sources.
Other records on this page
Regulated discharger data from EPA ECHO (ICIS-NPDES)