Diamond Lake is a 93-acre productive, nutrient-rich eutrophic lake in Poweshiek County, Iowa. Water quality is graded F based on 2 sampling years through 2024. Water clarity is improving over recent sampling years. Limited fish-species data available. Public access is available. Compared to the 1 other monitored lakes in Poweshiek County, Diamond Lake ranks #1 for water quality.
Source: EPA Water Quality Portal sampling records, Iowa DNR, last sampled 2024-08-28. Grade methodology: LakeGrade methodology.
Watch Diamond Lake water quality
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Swimming Safety
Avoid swimming, very poor water quality, potential algae toxins
Water Quality Grade: F, Very Poor
Very murky, less than 1.6 ft of visibility. Chlorophyll-a: 44.6 µg/L. Trophic State Index: 69.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 1.6 ft | F |
| Phosphorus | No data | |
| Chlorophyll-a (Algae) | 44.6 µg/L | F |
| Trophic State Index (TSI) | 69 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 93 acres |
| Public Access | Yes |
Where to stay near Diamond Lake
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Fishing Diamond Lake
Water Quality Trend: → Stable
Based on 5 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↑ Improving | +0.057 m/yr | 5 |
| Chlorophyll-a | ↓ Declining | +10.59 µg/L/yr | 5 |
Water Temperature
Diamond Lake averages about 77°F at the surface in summer, usually warmest in Aug (about 79°F) and coldest in Sep (about 68°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 29 field readings (2010–2023) via the EPA Water Quality Portal. See the full Diamond Lake water temperature breakdown →
Location & Depth Map
Free interactive depth-contour map of Diamond Lake, reaching a maximum surveyed depth of 16 ft. Toggle Depth contours to overlay the bathymetric survey lines. Contour data from Iowa DNR.
County Ranking
Ranked #1 of 2 lakes in Poweshiek County
Cleaner Lakes Within 30 Miles
Diamond Lake holds Grade F. One nearby lake has a higher grade.
See full comparison →Nearby Lakes in Poweshiek County
Explore Diamond Lake across LakeGrade
More Iowa Lake Rankings
Other Grade F Lakes in Iowa
Eutrophic Lakes in Iowa
More records, surveys & data sources
Reservoir Info (USACE NID)
Diamond Lake is a man-made reservoir impounded by the Diamond Lake Dam (Poweshiek Co) (completed 1952), built primarily for water supply on the TR-MOON CREEK; earth-type dam, 40 ft tall and 900 ft long.
- Surface area
- 93 ac
- Normal storage
- 780 ac-ft
- Max storage
- 1,100 ac-ft
- Drainage area
- 4.42 sq mi
- Hazard class
- Significant
- Owner
- Montezuma Water Works
All listed purposes: Water Supply;Recreation.
Source: USACE National Inventory of Dams, NID ID IA01368 · Operator website · Matched by proximity (0.21 km)
EPA Impairment Status
Diamond Lake is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 5).
Causes of impairment
Source: EPA ATTAINS assessment unit IA 03-NSK-861 · Official waterbody report
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth is a modeled estimate.
Regulated Pollution Sources Nearby
2 EPA-permitted facilities operate within about 5 miles of Diamond Lake under Clean Water Act (NPDES) discharge permits, including 1 classified by the EPA as major discharger. 2 facilities have a reported Clean Water Act violation in the past three years.
- Montezuma City Of StpMontezuma · 1.5 mi away · NPDES IA0036935MajorRecent violation
- Montezuma, City Of WtpMontezuma · 0.3 mi away · NPDES IA0084310Recent 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
Impairment status from EPA ATTAINS 303(d) database
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Lake size & watershed from HydroLAKES (HydroSHEDS)
Grading methodology based on Metropolitan Council standards
Most recent sample: 2024-08-28
Monitoring stations: 1