Diamond Lake
Poweshiek County, Iowa93 acresView mapDirections
Nutrients and algae run well above the regional average.
- Hypereutrophic
- Impaired · Algal growth
- #1 of 2 in the county
- 29 ft max depth
Diamond Lake in Poweshiek County, Iowa has a water quality grade of F (Algae-prone). Water clarity: 1.6 ft visibility. Phosphorus: 82.5 µg/L. Chlorophyll-a (algae): 44.6 µg/L. Surface area: 93 acres.
- Clarity
- 1.6 ft
- Typical in September
- 68°F
- Max depth
- ~29 ft est.
- Area
- 93 acres
- Type
- Hypereutrophic
- Trend
- ↓ Declining
Stays via Stay22
Stays near Diamond Lake
Compare lakeside cabins, hotels, and vacation rentals near Diamond Lake in Poweshiek County, Iowa — live availability and prices for your trip. Switch the map to Satellite to see the lake and how close each stay is to the water. The water-quality grade measures nutrients and algae, not lodging — cabins and rentals around Diamond Lake are listed below.
Top-rated places to stay near Diamond Lake
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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.
Diamond Lake is a 93-acre highly nutrient-loaded hypereutrophic lake in Poweshiek County, Iowa with an estimated average depth of about 29 feet. Water quality is graded F based on 2 sampling years through 2024. Overall water quality is declining over recent sampling years. 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: LakeQuality methodology.
Swimming Safety
Grade onlyAlgae risk
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 F is nutrients and algae, not a swimming clearance. Diamond Lake is listed impaired under Clean Water Act §303(d) (IR 5) for algal growth. Check local advisories before you swim.
Water Quality Grade: F (Algae-prone)
Very murky, less than 1.6 ft of visibility. Phosphorus 82.5 µg/L. Chlorophyll-a 44.6 µg/L. TSI 69. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 1.6 ft | Not graded |
| Phosphorus | 82.5 µg/L | Poor |
| Chlorophyll-a (Algae) | 44.6 µg/L | Poor |
| Trophic State Index (TSI) | 69 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Water Temperature
Diamond Lake typically sits around 68°F at the surface in September. Summer average is about 77°F. 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 →
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | ~29 ft est. |
| Average Depth | ~13 ft est. |
| Surface Area | 93 acres |
Depth marked “est.” is a modeled maximum from GLOBathy (Khazaei et al. 2022, CC0), not a measured survey. Iowa has not published bathymetry for Diamond Lake.
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth here is modelled, not surveyed — HydroLAKES v1.0 estimates lake depth from surrounding terrain rather than from soundings. Treat it as an approximation, and prefer a state bathymetric survey where one exists.
Fishing Diamond Lake
Water Quality Trend: ↓ Declining
Based on 5 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↑ Improving | +0.057 m/yr | 5 |
| Phosphorus | ↓ Declining | +48.3 µg/L/yr | 4 |
| Chlorophyll-a | ↓ Declining | +10.59 µg/L/yr | 5 |
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
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.
County Ranking
Ranked #1 of 2 lakes in Poweshiek County
Nearby Lakes With Higher Grades
Diamond Lake holds Grade F. One nearby lake has a higher nutrient-and-algae grade — useful for a swim day, not a reason to skip lodging here.
See full comparison →Nearby Lakes in Poweshiek County
Frequently asked questions about Diamond Lake
Where is Diamond Lake?
- Diamond Lake is a lake in Poweshiek County, Iowa. LakeQuality grades it F for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Diamond Lake?
- Diamond Lake in Poweshiek County, Iowa has a water quality grade of F (Algae-prone). Water clarity: 1.6 ft visibility. Phosphorus: 82.5 µg/L. Chlorophyll-a (algae): 44.6 µg/L. Surface area: 93 acres.
Is Diamond Lake safe to swim in?
- The F is nutrients and algae, not a swimming clearance. Diamond Lake is listed impaired under Clean Water Act §303(d) (IR 5) for algal growth. Check local advisories before you swim.
What is the water temperature of Diamond Lake?
- Diamond Lake typically sits around 68°F at the surface in September, from 29 EPA Water Quality Portal field readings (2010–2023). That is a typical value for the month, not a live reading.
How deep is Diamond Lake?
- No published bathymetric survey covers Diamond Lake, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 29 feet, but that is a modelled estimate derived from surrounding terrain rather than a measurement on the water, so treat it as approximate. The lake covers 93 acres.
Explore Diamond Lake across LakeQuality
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Where this data comes from
62 measurements from 1 monitoring station, 2020–2024. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Iowa DNR Surface Water Monitoring Data
21IOWA_WQX62 samples · 2020–2024
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
Impairment status from EPA ATTAINS 303(d) database
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Lake size & watershed from HydroLAKES (HydroSHEDS)